THE NEW YORK ACADEMY OF SCIENCES SCIENTIFIC SURVEY OF PORTO RICO and the VIRGIN ISLANDS VOLUME XIX— Part 1 Meteorology of the Virgin Islands _D 3- LJ WHOI ^ □ 1-^ m □ a Robert G. Stone QK 230 N 45 NEW YORK Published bv the Academy PORTO RICO COMMITTEE 1942 Charles P. Berkey, Chairman John Hendley Barnhart Eunice Thomas Miner Charles M. Breder Roy Waldo Miner Barnum Brown Wilbur G. Valentine Howard A. Meyerhoff Horace W. Stunkard Copyright 1942 BY The New York Academy of Sciences THE NEW YORK ACADEMY OF SCIENCES SCIENTIFIC SURVEY OF PORTO RICO and the VIRGIN ISLANDS VOLUME XIX— Part 1 Meteorology of the Virgin Islands Robert G. Stone NEW YORK Published by the Academy Editor WILBUR G. VALENTINE Associate Editor ROY WALDO MINER SCIENTIFIC SURVEY OF PORTO RICO AND THE VIRGIN ISLANDS This natural history survey of Porto Rico and the Virgin Islands, conducted by The New York Academy of Sciences, zuas established in 1913. Continu- ous publication of the results of this survey is made possible through contri- butions from the Department of Agriculture and Commerce of Porto Rico, and the University of Porto Rico. METEOROLOGY OF THE VIRGIN ISLANDS By Robert G. Stone* CONTENTS PAGE Acknowledgments 5 Introduction 7 History of the Observations 7 Homogeneity of the Records 8 Tabulations 8 General Climatic Features 9 Vegetation and Climate 10 Atmospheric Pressure 15 Pressure Variations 17 Precipitation 19 Accuracy of the Measurements 19 General Distribution 20 Forests and Rainfall 21 Orographic Efifects 22 Year to Year Variation 24 Diurnal Variation 25 Intensity and Frequency 26 Evaporation 30 Thunderstorms, Squalls, and Hail 30 Chemistry of Rainwater 31 Water Supply and Irrigation 31 Temperature 31 Geographic Controls 31 Extremes : Cool Spells and Warm Spells 34 Character and Effects of Changes 36 Daily Cycle 36 Vertical Temperature Gradient 38 Humidity, Sunshine, and Clouds 39 Absolute Humidity 39 Relative Humidity 40 Sunshine and Radiation 40 Cloudiness 41 Storms 42 Wind 43 Surface Wind Velocity and Direction 43 Upper Winds 46 Winds and Weather Types (after Palgrave) 46 Visibility 47 * Harvard University, Blue Hill Observatory, Milton, Massachusetts; and Department of Me- teorology, College of Engineering, New York University. 2 SCIENTIFIC SURVEY OF PORTO RICO -1 PAGE Climate, Diseases, and Health 48 Climate and Sugar Cane in St. Croix (after Shaw) 49 The Upper Air and General Circulation in Relation to the Weather and Climate 53 Appendix A. Notes on the Organization, Instruction, and History of the Official D. W. I. Rainfall Stations, 1877-1917 67 Appendix B 70 List of Hurricanes Passing Over or Near the Virgin Islands 70 Accounts of Some Virgin Islands Hurricanes ' 70 Appendix Tables 82 Annotated Bibliography 132 Text Tables 1. Summary of Danish Weather Records at Christiansted, St. Croix, 1875-1916 11 2. Climatological Summary, Charlotte Amalie, St. Thomas, 1917-1936 12 3. Sea-Level Mean Pressures at St. Thomas 18 4. Frequency of Monthly Rainfall Totals Greater than Specified Amounts, St. Croix 22 5. Average Rainfall for Each 10-Year Period, 1852-1911, of St. Croix 23 6. Night and Day Rainfall, Charlotte Amalie, 1888 25 7. Rainfall Intensities Measured at Jolly Hill, St. Croix, 1940-1941 28 8. Rainfall Intensities Measured at Anna's Hope, St. Croix, 1940-1941 28 9. Percentages of Days with Specified Amounts of Rainfall, Christiansted, St. Croix, 1852-1907 29 10. Average and Extreme Numbers of Days with Rain, Christiansted, St. Croix, 1852-1907 29 11. Chemistry of Rainfall at Anna's Hope, St. Croix, 1911-1915 32 12. Temperature and Relative Humidity, January 15-21, 1914, at Anna's Hope, St. Croix 35 13. Ste.-Claire Deville's Temperature Observations at St. Thomas during 1840 37 14. Mean Temperatures at Different Hours at Christiansted, St. Croix, 1913- 1915 38 15. Temperature Difference between Charlotte Amalie and Louisenhoj, St. Thomas 38 16. Monthly Mean Humidity 39 17. Vapor Pressure, Relative Humidity, and Cloudiness at Christiansted for Dif- ferent Hours of the Day, 1913-1915 39 18. Average Monthly Wind Velocity at the Evaporation Pan at Experiment Sta- tion, Anna's Hope, St. Croix, 1920-1938 44 19. Percentage Frequency of Wind Directions at Christiansted, St. Croix, 1875- 1907 46 20. Sugar-Cane Yield, and Rainfall of St. Croix, 1862-1938 51 21. Departures of Rainfall and Sugar Yield in Extreme Years of St. Croix 52 22. Frequency of Annual Rainfall Totals and Correlated Sugar Yields of St. Croix 52 23. Rainfall of Five Wettest and Five Driest Years of St. Croix, 1852-1914 .... 53 Appendix Tables 1. Register of Stations and Available Records 82 2. Synoptic Table of Annual Rainfall Totals, Virgin Islands, 1875-1939 84 STONE: METEOROLOGY OF THE VIRGIN ISLANDS 3 PAGE 3. Average Monthly Rainfall, St. Croix Stations, Danish Period, 1838-1917, and St. Bernard's, Tortola, 1831-1833 85 4. Average Monthly Rainfall Totals and Days with Rain, St. Thomas Stations 86 5. Average Monthly Rainfall Totals and Days with Rain, St. John Stations 98 6. Average Alonthly and Extreme Rainfall Totals and Number of Days with Rain, American Period 1917-1941 ; and Tortola, 1900-1935 110 7. Average Monthly Rainfall for Estates in the Sugar Area of St. Croix, 1921- 1930 114 8. Water Loss by Evaporation from Standard Weather Bureau Pan, Experi- ment Station, St. Croix 115 9. Average Monthly Temperatures, Virgin Islands (Danish Records) and Tor- tola 116 10. Average and Extreme Maximum and Minimum Temperatures 117 11. Average Number of Clear and Cloudy Days, and Prevailing Wind Direc- tions, at Virgin Islands Stations, 1920-1930 120 12. Monthly Temperatures and Rainfall at Bourne Field, St. Thomas 121 13. Relative Humidity, Bourne Field, St. Thomas 123 14. Visibility, Bourne Field, St. Thomas 124 15. Cloudiness, Bourne Field, St. Thomas 125 16. Sea Temperature, Cloud Motions, and Ceilings, Bourne Field, St. Thomas 126 17. Mean Upper-Air Temperatures at Bourne Field, St. Thomas, in °C 128 18. Mean Upper-Air Relative Himiidities, St. Thomas 129 19. Equivalent-Potential Temperature (0e) and Specific Humidity (q) from St. Thomas. Airplane Soundings, 1937, and Mean Noon Vapor Pressure (e) at San Juan, P. R 130 20. Mean Upper-Air Pressures in Millibars at St. Thomas 131 Figures 1. Effects of Weather on Vegetation 16 2. Rainfall Map of St. Croix, 1921-1930 21 3. Average Lapse Rates over St. Thomas, 1937-1938 55 4. Upper-Air Sounding at St. Thomas, March 23, 1938 56 5. Upper-Air Sounding at St. Thomas, November 5, 1940 56 6. Surface Pressure and Height of the Trade (Ts) Inversion, July-November 1938, at St. Thomas _. ._ 58 7. Surface Pressure and Upper-Air Temperature and Specific Humidity at 3000 Meters, July-August 1938, at St. Thomas 59 8. Scheme of Upper-Air Structure over the Virgin Islands 64 9. Mean Relative Humidity in the Upper Air over the Virgin Islands, 1937- 1938 _ 64 10. Monthly Mean Rainfalls, Mean Rainfall Intensities, in Inches per Rain Day and per Rain Hour, and Mean Maximum and Minimum Temperatures, at Bourne Field, St. Thomas, 1937-1938 64 \^ ACKNOWLEDGMENTS This part of the volume on meteorology was not envisaged by the late Dr. Oliver L. Fassig, who restricted his projected volume to Porto Rican meteorology alone. While the present author was in Porto Rico in 1939 on invitation of the School of Tropical Medicine, the opportunity arose to extend the scope of the study to include the Virgin Islands. The appro- priateness of this extension is obvious ; but it would not have been possible to offer such a comprehensive discussion had the writer not been able to visit the U. S. Weather Bureau at San Juan and the U. S. Marine Corps Aerological Station on St. Thomas. We are happy to acknowledge the favors of many people in the islands, who, in one way or another, aided the work on this part of the monograph. Special thanks are due to Mr. Morris de Castro, Secretary to the Virgin Islands Government, to Col. Francis P. Mulcahey, in command of the U. S. Marine Corps base at St. Thomas in 1939, and to Mr. N. N. Nichols, Director of the Experiment Station, St. Croix, for facilitating the acquisi- tion of data and general information in the field ; to Sgt. Michael Davido- vic, U.S.M.C. Aerographer, formerly stationed at the Marine Corps base at St. Thomas, who under authority from his superior officers was most helpful in putting at my disposal the facilities and observations of the sta- tion at Bourne Field ; to Mr. R. W. Gray, official in charge, and to his as- sistants, Messrs. Kronberg, Cintron, and Maldonado, of the U. S. Weather Bureau, San Juan, for their help in copying and tabulating data from their files ; to my own assistants at the School of Tropical Medicine, Miss Maria Ruisanchez and Mr. George del Toro, Jr., for their faithful and intelligent handling of many tedious computations and tabulations ; to Hon. W. C. Roy, Agricultural Officer of the British Virgin Is., for a transcript of weather observations at the Experiment Station, Tortola. Dr. George W. Bachman, Director of the School of Tropical Medicine, San Juan, and Prof. Gleason W. Kenrick of the University of Porto Rico, have made available for this part of the work all the resources which were also pro- vided for the Porto Rican section, acknowledgment for which is made in detail in Part 3. The Blue Hill Observatory of Harvard University, the U. S. Weather Bureau, and the Department of Meteorology of New York University likewise supported, amply and in many ways, the preparation of this part of the monograph along with the other parts. I am particularly indebted to Prof. Charles F. Brooks, Director of the Blue Hill Observatory, for hav- ing first initiated me to the interesting task of completing Dr. Fassig's 6 SCIENTIFIC SURVEY OF PORTO RICO project and for his continued support through many delays and difficulties. He kindly read the manuscript of this part in its preliminary stages, mak- ing many valuable suggestions. Through discussions with him some of the difficult meteorological problems of the region were considerably clarified. The author had the good fortune to collaborate on a study of Caribbean Meteorology at New York University, in the course of which many ideas of value for the present work were gleaned from discussions with his col- leagues, Prof. G. E. Emmons, Mr. J. E. Miller, and Mr. R. N. Culnan. Prof. Earl B. Shaw of Worcester (Mass.) State Teachers College has generously allowed us to reproduce passages and illustrations from his Doctoral Dissertation on the Virgin Islands (Clark University, 1931). Mr. Harold Larson, in charge of the Territorial Section of the National Archives in Washington, kindly searched the Virgin Islands records in the Archives for climatological material ; he discovered the large series of hitherto unpublished Danish West Indies weather observations which we have summarized and printed herewith, a most remarkable and valu- able find. We are also indebted to him for translating several articles from the Danish. Mr. J. B. Kincer, Chief of the U. S. Weather Bureau's Divi- sion of Climate and Crop Weather, provided facilities of his staff for com- puting the averages from these data. We are grateful to Mrs. H. W. Dooley, Mrs. G. W. Kenrick, Misses Maria Velasquez, Rose Vickers, Edna Scofield, Leoni Bleston, Mr. Ralph Burhoe, and Mr. B. Acevedo, for kindnesses and help in certain particu- lars. INTRODUCTION The chapters in this section are in the form of topical notes designed to assist in interpreting the weather ohservations summarized in the accom- panying tables and in the literature (see annotated bibliography at end). No attempt is made to give a complete or well-rounded analysis of all the elements nor a synthesis of the general character of the climate, as it does not differ sufficiently from the climate of Porto Rico which is described in Part 3 of this volume. The meteorological background of Porto Rican cli- mate also applies generally to that of the Virgin Islands, because the lati- tude, wind system, and surrounding ocean surfaces are essentially the same. The differences are due mainly to the smaller area and lesser eleva- tions of the Virgin Islands. From the standpoint of climate, geology, soils, and vegetation, the islands of Vieques and Culebra, political dependents of Porto Rico, should be considered as part of the Virgin Islands ; but the climatic data for them are given in the section on the meteorology of Porto Rico, in order to con- form to the political alignment, for the convenience of those who will use the observations. HISTORY OF THE OBSERVATIONS The history of weather observations in the Virgin Islands is rather in- teresting. Excellent data were compiled during the Danish regime, from about 1825 on (appendix table 1). Educated Europeans working in the islands early took an interest in the climate and as a result there are extant several series of regular observations by instruments from the period be- fore the Danish Government officially organized the taking of observa- tions in 1876-77. Some official Danish stations were still reporting in 1917, but thereafter the United States Government gave no special attention or encouragement to these or other weather observers, except at Charlotte Amalie and Christiansted, until the U. S. Weather Bureau began to estab- lish more "cooperating" stations in 1920. The Bureau increased the num- ber of cooperating stations in 1938-39. Since about 1830 many of the sugar estates on St. Croix and some on St. John and St. Thomas measured rain- fall for their own interest. Some of these observations found their way into print, but most of them were inaccessible until 1911 when the Agri- cultural Experiment Station began to collect the current data and publish them. Many of the early records are lost, though some are on file at the Danish Meteorological Institute in Copenhagen. Finally, the observations of travelers and visiting ships, though covering very brief periods, may 8 SCIENTIFIC SURVEY OF PORTO RICO illustrate some important details of the weather. Numbers of these appear in the literature, but as there is no bibliographic guide to them, they remain mostly unknown to those who might use them, HOMOGENEITY OF THE RECORDS It must be emphasized that as most of the various weather records for the Virgin Islands are not strictly comparable with one another because they cover different periods and were made in different ways, great caution must be used in drawing conclusions from them as to geographical distri- bution of temperature and rainfall. The interpolation of missing observa- tions for certain days or months in a broken series, by comparison with the records of nearby stations, is seldom justified in this region, except pos- sibly with temperatures, because the weather is marked by decided local contrasts. Localities only half a mile apart at the same elevation and in the same valley can have very different rainfall values ; likewise showers are usually so limited in area that within a day or month it is largely chance that two neighboring places receive the same rainfall. These anomalies of locality and of short periods tend to equalize over longer periods, so that the total annual rainfalls of neighboring stations generally show a parallel or definitely correlated course, however different their normal or average precipitation may be.* In the tables, the records of the Danish period are in most cases sepa- rated from those of the American period, because the kinds of instruments used, instructions followed (see appendix A), and other circumstances affecting the official records were uniformly different. Unofficial observers both in the Danish and American periods probably used a variety of in- struments and conventions. In most cases these circumstances are un- known, and therefore it is well to accept such records with reservations. On the other hand some of these private records are undoubtedly more accurate than some of the official ones. TABULATIONS In APPENDIX TABLE 1 is a Hst of the stations and their periods of records, with elevations, elements recorded, and place of publication or file. In- quiries in the Virgin Islands and in Denmark would probably turn up much more of such essential information. The records of the Danish co- lonial government of the islands and files of some of the island newspapers. * The parallelism of the mean rainfall of two neighboring stations is tested by the constancy of the ratios, month by month or year by year, between the rainfalls of the two. The parallelism of the mean temperatures at neighboring stations is tested by the constancy of the differences between the means, month by month or year by year. STONE: METEOROLOGY OF THE VIRGIN ISLANDS 9 now deposited in the National Archives in Washington, were examined for weather records by Mr. Harold Larson in charge of the territories sec- tion in the Archives. He discovered the manuscript reports of the official Danish observations for St. Thomas and St. John from 1877 to 1917 ; com- plete except for a few missing months. As these had never been published, except from 1877 to 1888 in the "Set. Thomae Tidende", an abstract was made of them and means computed (appendix tables 2 and 3). The offi- cial observations from St. Croix apparently were mostly sent to Denmark where some were published and the rest probably filed in the Danish Ar- chives or the Danish Meteorological Institute. Their inaccessibility will not be so keenly felt, because we have fairly extensive published records for many St. Croix stations since 1911, as well as the Christiansted record from 1875, and some other miscellaneous data. For the other islands, how- ever, the Danish period supplies the chief material, though very recently in the American regime considerable data are again being obtained. Relatively much less material is available for the British Virgin Islands. In fact, Schomburgk's paper of 1837 is still the best discussion of the cli- mate. The modern records cover only a few decades at Tortola and Virgin Gorda. The absence of plantation agriculture in the last hundred years no doubt accounts for the lack of interest in keeping weather records there. Tabular summaries of pressure, temperature, rainfall, humidity, evapo- ration, sunshine, cloudiness, winds, visibility, and some upper-air condi- tions are given in text tables 1 to 23 and in appendix tables 2 to 10. GENERAL CLIMATIC FEATURES The general climatic features do not dififer from those in certain parts of Porto Rico, e.g., Vieques, Guayama, Arecibo, and Isabela, but on the aver- age the rainfall and relative humidity are less, the sunshine greater. In- deed the traveler from Porto Rico or from other rainier parts of the Carib- bean will be impressed by the prevailing moderate dryness or semi-aridity of the Virgin Islands, which the vegetation reveals at every hand. The islands are so small that the climate approaches more nearly that of the surrounding ocean than in the case of Porto Rico. Some of the ex- tremes of temperature and rain recorded in the latter island are probably never attained in the Virgin Islands. Owing to their more easterly location, the influence of continental cold-air outbreaks from higher latitudes is weaker than in Porto Rico. In the winter half-year storms passing across the Atlantic in higher latitudes cause the same constant heavy ground swells from the north as are observed in the islands farther west (a fact noted and correctly explained by Schomburgk in the 1830's) . 10 SCIENTIFIC SURVEY OF PORTO RICO Before introducing details about the variation of each of the weather elements over the islands, it may assist the general reader as well as the specialist to have a concise view of all the elements for one typical station on St. Croix and one on St. Thomas. The official Danish observations at Christiansted from 1875 to 1916 are the most complete for any Virgin Islands station and serve this purpose well. The averages of each element are tabulated together in text table 1 for ready comparison. Further de- tails for Christiansted are given in other tables under special headings and in the appendix tables. The summary for Charlotte Amalie (text table 2) is a copy of the table prepared by the U. S. Weather Bureau for use in the publications of the U. S. Hydrographic Office. The figures in these ta- bles do not agree in detail with some of those given for the same place else- where in this monograph, owing to differences in period of years covered, or in the source of the data. Also, the wind- velocity and cloud-amount data for Charlotte Amalie cover such a few years that they are probably mis- leading as to details of the month to month variations. VEGETATION AND CLIMATE It has long been recognized that types of virgin vegetation as well as various species of plants are usually limited in their geographical distribu- tion by some climatic factors. In earlier times this obvious fact was often used to judge the suitability of new lands for crops and settlers. Early travelers to the Virgin Islands described its vegetation. Later observers compared the contemporary scene with past accounts. In this way some curious notions about the climate, particularly as to alleged changes with time, have become popular in the islands. The results of modern botanical and geographical studies do not reveal any simple correlation of climate and vegetation. But when proper allow- ance is made for non-climatic factors, such as soil, plant succession, effects of deforestation, grazing, and cultivation, introduction of new species, etc., a general association of the boundaries of the "climax vegetation" types with certain mean temperature and rainfall limits is evident.* "Climax vegetation" is the ecologist's term for the virgin growth which represents the most luxuriant that the given climate will support, the final equilib- rium stage of the plant succession. In the absence of extensive systematic instrumental observations of the climatic elements, the climax vegetation may serve as a rough indication of the climatic type. It is, however, a problem for the well-trained and ex- * Well described in James, P. E. "An outline of geography", N.Y., 1937; and in "Climate and Man", 99-127. U. S. Dept. Agric, Yearbook for 1941. !*• ^h fva H . r* t-" On ^O - dj o 3 C U ft ? *J 1 ^ ° ego < D.-S o < ID CO 00 ro vO \0 O C^J \0 VO ^ m 00 t^ t^- t^ CO vO VO Ov . .rvi ooo Tf OO rO (NJ ro O vO "^ !^ t-" !i>+ 6ta Sri 00 -w '' ^ r/) (^ "5 "^ ^ c t. v_,.t: c : g >• „ & 3 k! -o c g g «J S JS^ ^, --^ P "! - ^ c I S = o o -H ^ 3 "* c „; ,; .„ S^. 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O 3^ o ts |°° «r t^2^ m 12 STONE: METEOROLOGY OF THE VIRGIN ISLANDS 13 perienced ecologist or geographer ; the amateur is apt to overlook impor- tant factors and thus come to unscientific conclusions. Even the existing "secondary" vegetation, greatly altered as it often is by the artificial influences of human settlement, may suggest climatic con- ditions which are otherwise unnoticed or outside the scope of ordinary in- strumental observations of the weather elements. This is particularly use- ful in studying local influences of the vegetation on the microclimates, the small-scale variations in the air layers near the ground which have much practical importance for man and agriculture. A description of the vegetation (not the flora) is therefore not out of place here. Since practically no detailed quantitative ecological field studies have been made in the West Indies, the interpretation of the vegetation- climate relationships is necessarily conservative and general. B^rgeson, writing in 1898, sketched the forest situation succinctly: "In consequence of their location, the islands have a tropical vegetation, but the comparatively scanty rainfall does not, as a rule, permit the development of as rich and luxuriant a plant life as is found in other places in the Tropics where the precipi- tation is greater. In spite of this, however, the vegetation may be almost as rank as that of the jungle in valleys possessing a sufficient supply of moisture. As in all small islands, with a relatively large population, man's intervention has left its stamp on the vegetation. The story goes that at the time of their discovery they were covered with thick forests of which, however, hardly anything is left. During the period of colonization the forests were burned off and the little that probably sur- vived, along with what later grew up, has been partly destroyed by reckless cutting, a practice which is indulged in up to the present time. There is nevertheless some forest vegetation in the northwestern part of St. Croix, on some mountain slopes of St. Thomas, and especially in St. John, but mostly of secondary growth. The origi- nal vegetation in all probability is best preserved in the mangrove swamps along the coasts and in the forests growing on the sandy beaches, where the quality of the soil gives no encouragement to cultivation". Although the vegetation was undoubtedly originally "forest" (i.e., merely arborescent) in most parts, it probably included some of the very dry thorn-bush landscape which appears more widely today. St. Croix, St. Thomas, and St. John, at least, were all deforested during the 18th and early 19th centuries by cutting to provide timber and by burning to clear for cultivation. On most of St. John the bush has been allowed to return, and it is now returning to much of St. Thomas, although St. Croix, except for the dry eastern end and the hilliest northern sections, remains largely under cultivation for sugar cane or in pasturage for cattle. Grazing is also an important industry on St. Thomas. Grass, thorn forest, and bush are generally conspicuous on St. Croix today, which, owing to its greater range of climates and soils, is said to have a greater variety of vegetation types than the other islands. 14 SCIENTIFIC SURVEY OF PORTO RICO There are numbers of endemic species, but they do not seem to have specialized climatic adaptations. Britton and Wilson (1923: 4) observed that "... they are scattered in distribution, not being restricted to wet, dry, high or low districts or to specific types of soil, though many of them appear from our present knowledge to be very local in distribution. All the endemic species are more or less closely related to other species inhabiting other West Indian islands, indicating community of origin, and differentiation through isolation". The vegetation has been so altered by man tliat much caution must guard any inference about climate from the present character and distribution of the vegetation. The heaviest woods on St. Thomas are now growing in certain ravines on the north side of the island, but one cannot conclude that the rainfall is therefore more abundant in that vicinity ; evaporation, to- pography, desirability of the soil for other purposes, charcoal burning, grazing, lumbering, and accidents of plantation abandonment also have to be considered. In connection with this question, the following statement in a recent official publication* is of interest, "It has been generally supposed that the Virgin Islands were once covered with mahogany and other valuable cabinet woods and that dense forests of these trees were cut off in the early days and then again later to make place for cane when the slave labor cultivated even the steep hillsides. Qualified foresters, however, consider this extremely doubtful, and do not believe that these cabinet woods were ever plen- tiful on these islands. They do grow well here, but the specimens scattered sparsely throughout the islands or standing in clumps about old ruins and along the sides of many roads, are believed to have been imported. Little natural young growth of these trees is to be found, but thousands are now being set out, and seem to do well. Most of St. Thomas and St. John and much of St. Croix is thickly covered with many different types of trees, but these are not of accepted commercial value, nor of size or character to be considered forests. Where the land is not cleared for cul- tivation or grazing, these scattered trees are interspersed with dense growths of underbrush and vines that rapidly reclaim any land that is left uncared for even a year or two. Many of these trees afford food in the form of wild fruits, such as the mango, the soursop, etc. ; but most of them are valuable only as wood for the burn- ing of charcoal — the universal fuel of the Virgin Islands". The dry vegetation of eastern St. Croix, where the rainfall probably averages but 25 inches or less a year, considerably resembles some of that of the semi-arid southwestern United States and northern Mexico.t The trees and bushes on exposed slopes have been forced by the prevail- * "The Virgin Islands of the United States", p. 5. U. S. Dept. of the Interior, Washington, D. C. 1935. t Cf. Shreve, F., Lowland vegetation of Sinaloa. Bull. Torrey Bot. Club 64: 605-613. Dec. 1937. Britton, N. L., Cactus studies in the West Indies. Jour. N. Y. Bot. Garden 14: 99-109. 1913; The vegetation of Anegada. Mem. N. Y. Bot. Garden 6: 565-580. 1916. Gleason, H. A. & Cook, M. T., Plant ecology of Porto Rico. Sci. Surv. Porto Rico and V. I. 7 : 1-173. 1927. STONE: METEOROLOGY OF THE VIRGIN ISLANDS 15 ing trade winds to grow with a bend to the leeward ; this asymmetry is presumably due to the effect of the salt carried by the wind and deposited on the exposed branches. The wind-deformed vegetation points rather uni- formly towards west by south, suggesting that the prevailing wind direc- tion is from slightly north of east. Actually such a wind direction predomi- nates only for the winter season, so that probably the stronger winds and more salt spray of the winter months have a predominating effect on the vegetation (see figure 1A). ATMOSPHERIC PRESSURE Barometric observations in the Danish period are available from Horn- beck's record (1833-51) at Charlotte Amalie, from Ste. -Claire Deville's (1848) and Stenzel's visits (1886) to St. Thomas, and from the official Danish station at Christiansted (1875-1916). During the hurricane sea- son (July to October), special daily barometer reports have been made at St. Thomas since 1878, first by A. Walloe and later by the harbor masters. These were cabled or radioed to San Juan or to the United States for the U. S. Government weather service. It appears that these observers read the barometer in other months also, but only the means computed from these reports for the years 1919-28 are available (text tables 2 and 3). Such reports were also made from Christiansted, but no tabulations were published. For some years after 1911, the Experiment Station at Anna's Hope, St. Croix, maintained an aneroid barograph, and photographs of the daily charts for several years are reproduced in the first two reports from the station. These give as adequate a picture of the pressure varia- tions as necessary for most purposes, and their value is enhanced by the concurrent temperature curves which are reproduced in the same reports. The extremely low pressures experienced in hurricanes are illustrated in Appendix B. Barograph records are now being kept at the aerological sta- tion at Bourne Field, U. S. Marine Corps, on Lindbergh Bay, St. Thomas, and probably at the new Army air bases on St. Croix. No pressure read- ings for the British Virgin Islands are available except those of Schom- burgk (1833-37), which are not reduced to sea level. All the early 19th century observations are from instruments of doubtful accuracy. As in Porto Rico, many planters, merchants, and professional men own a barometer of some sort which is watched during the hurricane season for signs of local disturbance, in spite of the efficient broadcast warning serv- ice now available from the U. S. Weather Bureau at San Juan. These in- struments are often improperly calibrated, uncompensated, and exposed 16 SCIENTIFIC SURVEY OF PORTO RICO Figure I . Effects of weather on vegetation. A. Tree deformed by prevailing wind. Picture taken a few miles east of Christiansted, near the north coast of St. Croix. (Photo by E. B. Shaw, 1930.) B. Dry weather is sometimes fatal to the palm trees as well as to the sugar crop. These palms "lost their heads" during a severe drought period on St. Croix. (Photo by E. B. Shaw, 1930.) STONE: METEOROLOGY OP THE VIRGIN ISLANDS 17 at various elevations, so that the readings from them are usually reliable only for showing relative changes in the pressure rather than the absolute values. Pressure Variations The barometric pressure mean values and variations observed in the Virgin Islands are entirely similar to those experienced at San Juan where an extensive and accurate series of readings has been kept by the Weather Bureau since 1899. The average is lowest in May and November and high- est from January to March and July to August. The semi-diurnal daily cycle of the barometer (i.e., double maximum-double minimum at about 10 A.M. and 11 P.M., 4 a.m., and 4:30 p.m., respectively) is the most pro- nounced feature, as everywhere in the tropics, and at San Juan has an average amplitude of .063-.085 inch (=2.14-2.98 millibars), largest in February and July, least in May and November. There are occasional spells of some days' or weeks' duration that may occur in any season, when the regular daily cycle is largely suppressed or greatly disturbed by various factors, including cloudy or rainy spells, hur- ricanes, and longer-period pressure "waves". In addition there are gentle swells in the general level of the barometer curve, rising and falling in long steady swings lasting several days to several weeks. These are often asso- ciated with noticeable changes in temperature and rainfall and are due to disturbances of the trade winds set up by passing cyclones and anticyclones in higher latitudes in winter, and in summer to the changes in the intensity and position of the Azores-Bermuda anticyclone and passing allobaric systems.* In the winter half of the year nearly all the coolest nights re- corded at the Experiment Station at Anna's Hope, St. Croix, occurred during the middle of a few-days period of relatively low pressure for that season, and vice versa the warmest days (highest temperatures) come at times of higher than normal pressure for the season. This relation is prob- ably general, at least for all sheltered locations in the interior of an island. Knox (1852) correctly observed, "... with NE winds the barometer almost invariably rises, and generally falls when the wind is SE or S. We have observed it to rise to 30.15 inches with a fresh NE breeze, but this great tide seldom takes place. Passing showers seldom or never cause the mercury (i.e., the barometer) to fall ..." The little fluctuations on the barograms when showers or thunderstorms pass are usually less than one millibar in ampHtude. ^OAl * Frolov, S., Bull. Am. Met. Soc. 22: 198-210. 1941; id., "On the synchronous variations^ pressure in tropical regions", in the same journal, (5) 1942. Dunn, G., Bull. Am. Met. Soc. 215-229. 1940. A _ 2 LIBRAR %>^- K a! H (« fo be w H .s J c/:! '3 < H H <; P^ t/5 s ;3 en o P4 J .2 o >■ ; o :z; rO o *-» O; 00 o O^ <> a o; Ov o» o^ C/3 O bi) O; O; 3 < d CN o _>;. o O; "3 d c> 1—) ro c 3 >-> >. rt 2 C a < »J , rt S • ^' Pi( c ta >— 1 e c 4J g « s H E s ui l-l "H ca rj ca >. >. •a c 6 6 •-* *-* So 00 Q rvi CM 1 1 c _ca o> ON O^ o\ < ^^ N-' i-T i S 3 O •i R^ K CO mD ^ « o , oo2 + c 1— I 3 S ^.6 .Q On P ^H &1^ a,E •>^ SE — ^ C oo B^ o >. 3 o C b ■S.o E c c JS r a 3 c o 1 t/i rt o aj .a T3 w 1-1 c m Ix s < CJ 4-1 rC -^ H J3 a; rt fc >< ■" & I; ^ 3 ta ^ o z V ^ -c x; p. S ^ c Pi ca S M < ctf s -^ c "^-^ ** s "■ c/l ■^ ^ c ^ rt J3 >, CM rt rt M ^ H T3 C £ ►J n 3 a < ra ? ta-o H II a E a d (1^ 60 o o 3 ■B O V ^5 OJ "« S3 ^ c ca OJ c ^ in c .2 ca £l o c c 3 c rt ^ E > E > 3 f' > J3 ^ g " 'Eb ^ V OJ _C ca 3 J3 L. ., , B3 *^ ra c J3 T3 3 7 c la ta !2 ca ^ O O 3! OJ ^ "o C/2^ 2" XI ti = fa "ta T3 E S OJ c. m c j: ^^ B s Oj <2 3 a j: o- c "?> ■n n C ca ■2 Ov E rt •- 5 o ^ c e i-> 3 OJ t^ e? 3 H J5 ca 18 STONE: METEOROLOGY OF THE VIRGIN ISLANDS 19 The nature of the shorter-period pressure variations in relation to the weather and the general circulation are discussed below under The Upper Air and General Circulation, etc. PRECIPITATION Rain is the climatic element of most practical concern in the islands be- cause it is often insufficient to mature sugar cane in one or two seasons ; a drought of six or nine consecutive months occurs every decade or so, caus- ing much hardship to the townspeople and small native farmers as well as to sugar and cotton estates and cattle ranches. Since early in the nineteenth century rainfall in the Virgin Islands has been measured in a unique unit of depth, called the "line". The reason for the adoption of this measure is not known. It is an old English measure, in which 1 inch = 8 lines (rr 25.40 millimeters). In Denmark they once used the Paris measure of 12 Linien = 1 Tomme (Paris inch) = 27.07 milli- meters ■= 1.0658 inches. 1 Paris line = 2.256 mm = .0888 inch ^ %44 foot, whereas the Danish West Indian (or English) line = 3.175 mm = Yg inch. It is conceivable that as many of the residents were British this "line" was adopted locally from using English rain-measuring glasses or sticks graduated in eighths of an inch. Since the American occupation inches have been used. Accuracy of the Measurements The accuracy of rainfall measurements is a difficult problem in gen- eral, and is especially serious in tropical countries.* We have already re- ferred to the lack of standards in the instruments and observation pro- cedures at Virgin Islands stations, and here we must add that where the rainfalls are frequently light and the monthly and annual totals are small the errors of measurement are greatest on a percentual basis. The common practice of measuring the catch only once each 24 hours allows some water to evaporate from the gage before it is read, particularly in a warm windy climate. The use of a funnel is common and tends to cut down the evapo- ration. Where most of the rain falls at night, it is better to read the gage in the morning, and where it falls more in the day an evening observation hour is preferable ; two readings a day would be still better, and best of all the use of recording gages or the habit of reading the gage after each shower. It has been shown that a considerable difference in a given * For a comprehensive discussion see Brooks, C. F., Need for universal standards for measuring precipitation, snowfall, and snowcover. Trans. Meet. Int. Comm. Snow and Glaciers, Int. Assoc. Hydrol. Bull. 23: pp. 1-52. Riga. 1938. 20 SCIENTIFIC SURVEY OF PORTO RICO month's total may result at the same spot between a gage read each morn- ing and a gage read each evening. But it is difficult to estimate the magni- tude of this effect in the Virgin Islands except to say that the results from gages read only in the morning are probably somewhat lower than they would be if read only in the evening. The hours of observation at the vari- ous stations are not stated or known in many cases and at some stations they were changed from time to time. Rain gages of different diameter and dift'erent height of orifice above the ground do not give comparable catches, but it is believed nearly all the gages used in the Virgin Islands since 1870 have been of the standard 8-inch diameter with rim about 3 feet high (cf. appendix A). The wind eddying around the gage may keep away some of the rain that should go in the gage. In windy places the catch may average 20 per cent too low from this cause, but we judge from tests. made elsewhere with shielded gages that this error in the Virgin Islands probably does not average over 10 per cent (i.e., readings are 10 per cent too low on average from the wind effect alone). If we may assume that this error applies roughly equally to all the gages in the Caribbean region, it may be overlooked in practical comparisons. However, the error due to wind effect increases as the wind velocity increases and therefore the catch during severe storms, hurri- canes, is apt to be more than 10 per cent too low. High wind sometimes blows the gage over resulting in loss of a large catch of rain. Occasionally during heavy rains the gage may overflow before it is read. Considering all these sources of error, it is evident that on the average the recorded rain- falls are systematically lower than the true rainfalls. In addition there may be mistakes and falsifications on the part of ob- servers, which are unsystematic in their effect on the results and largely hidden in the averages. An inspection of the daily entries and the reputa- tion of the observer are the only bases for accepting observations as genu- ine, where the stations are not under regular inspection of an efficient na- tional weather service. We have not found any record of inspections by the Danish government, and the U. S. Weather Bureau inspections have been too infrequent to be effective. General Distribution From APPENDIX tables 2 and 3 we note that the mean annual rainfall differs considerably at the various stations, ranging between about 35 and 70 inches. The absolute range between driest and rainiest years at these stations is not much larger, however, the extreme annual totals ranging from about 25 inches to nearly 95 inches (appendix table 1). If we had STONE: METEOROLOGY OF THE VIRGIN ISLANDS 21 45 - 50 50 - 55 55 - 60 SUGAR ESTATES REPORTING SCALE OF MILES Figure 2. Rainfall map of St. Croix, 1921-30. (From Shaw, 1932.) records from eastern St. Croix and from the mountain tops, these ex- tremes would be greater, probably reaching from 15 to more than 100 inches. A rainfall map of most of St. Croix is shown in figure 2. The seasonal distribution generally shows two maxima, a smaller one in May or June and a larger one in October. The winter minimum is much more pronounced than the summer one. The lowest monthly amounts on record indicate that severe drought conditions can occur in almost any month ; even October has sometimes had less than 2 or 3 inches at most sta- tions (see TEXT TABLE 4). Rosc points out that the middle and western sections of St. Croix have somewhat opposite tendencies in departures of rainfall from normal — from 1903 to 1908 the middle was drier than the west, but from 1909 to 1915 the middle was wetter, and after 1915 the middle was again the drier. This may possibly be due to a quasi-cyclic shift in the relative frequency of winds from slightly north and slightly south of east, which would be accompanied by changes in the average tem- peratures and humidities of the trade winds as well as contrasted oro- graphic effects. Forests and Rainfall E. Taylor in his "Leaflets from the Danish West Indies" (London, 1888: 42) suggests that St. Croix formerly had a greater rainfall be- 22 SCIENTIFIC SURVEY OF PORTO RICO cause an early book on the islands by Oldenclorp (1777) reported a greater amount of forest growth than is now found. Although a change of climate is possible, the present condition is better explained by the known destruc- tion of the forest by the inhabitants. Text table 5 shows no permanent change in the rainfall of St. Croix since 1852. St. John and Tortola have the most forests at present because they are too mountainous for economi- Text Table 4 Frequency of Monthly Rainfall Totals Greater than Specified Amounts, St. Croix Average of 3 stations for 63 years, 1852-1914 (From Ravn) Month Number of years with rainfall Over 20 lines Over 40 lines Over 60 lines (2.S0 in.) (5.00 in.) (7.50 in.) January 25 2 _ February 13 1 _ March 13 1 _ April 33 S 1 May 37 24 11 June 38 19 9 July 44 12 3 August SO 22 8 September 57 32 10 October 60 38 18 November 54 30 13 December 39 11 4 cal sugar-cane culture, though at one time both were under considerable cultivation. There is no reason to believe that either St. John or Tortola receive much more rain than St. Thomas or St. Croix merely because they are now more forested. Indeed, the rainfall observations (cf. appendix TABLES 2-6) lend no support to that notion. Orographic Effects The rainfall increases with elevation on all the islands, as residents and travelers can readily observe and as one would expect. But rain-gage sta- tions are lacking at high elevations, except Pearl, Maf olie, Liliendal, Wint- berg, and Dorothea. Shaw's rainfall map (figure 2) based on rainfall records (see appendix table 7) of sugar estates on St. Croix leaves no doubt that even moderate elevations are better watered. Yet the rate of in- crease of rainfall with elevation does not here seem to be as large as in the parts of Porto Rico where the mountains rise steeply to 3000 feet or more directly in the path of the prevailing winds. Rose suggests that the rain- fall of the islands is not as great as one would expect from the topography because the winds blow mostly parallel to the mountain trends. The reason O O CO »^ oo ^o oo fo ^ i^ o^ r^ ■^ ^ ^ -4- ^ ^ OO O f^ t^ 00 »0 O ':}• ro O O O C^J ro c^j ro ^ ^'l ^ fO tJ- O ^ C^, ^ O^ ««f Tf *o lo tn -^ 1-1 lo ta , CO ^ iyl^\0 W -r \0 \0 OO c^ 1— " * ^ + tT 'I-" ■* •^ • (1^ >H O CO J Sfc ly-) —1 CO O '^_ -^ fO t^ CO ■^ "^ CM ■vo SO 00 r^ O O t^ CO -^ O; so ■*_ CO c^ ^ fO ^ rO >iO >0 (M rO*0 >fO»-^ u OS i-H OO t^ ^ » rt ^ « _H 0\. so ^ I- OS O ^ > tr, K lis s s 23 24 SCIENTIFIC SURVEY OF PORTO RICO for this may also be contained in some observations of the writer : on sev- eral occasions during his stay at St. Thomas in June, 1939 when the sum- mit of the island (1800 feet) was visited, he noticed that any large cumulo- nimbus cloud that had been initiated by forced ascent of the wind over the island would lean to the leeward so that most of the rain falling from it would fall on the ocean surface somewhat to the lee of the island. In other words the orographic influence on the rainfall was not fully enjoyed by the island itself owing to its small size and narrow form. This observation is confirmed (oral communication) by Sergeant Davidovic, the Aerographer stationed at the U. S. Marine Corps Fleet Air Base on St. Thomas in 1939. In general the annual rainfall does not seem to increase more than about 10 inches between sea level and 1000 feet elevation, but some of the lower stations have as much rain as places high up on the leeward slopes or in high protected valleys (compare Adrian and Cinnamon Bay, or Barracks and Liliendal, in the same years) (appendix table 2) . In generally rainy years or months the rainfall differences between stations of different ele- vation are much greater than in generally dry seasons. At the U. S. Marine Corps station on Lindbergh Bay three rain gages have been set a few hundred yards apart in a line from the water to the foot of the mountain. These gages show a decided increase in rainfall (ap- pendix TABLE 12) as the mountain is approached, although they are all about at the same elevation. This demonstrates how sensitive the rain- producing process is to the topography. For this reason, within the hilly town of Charlotte Amalie, or of Christiansted, the average annual rainfall probably varies considerably (up to 5 inches?) from block to block ; hence records taken at different spots in such a town cannot justifiably be com- bined as if from one station. Likewise different parcels of an estate often have very different rainfall (e.g., Eden, Emmaus, Caroline ; Adrian, Su- sannaberg). We have not attempted to construct rainfall charts of St. Thomas and St. John owing to the non-homogeneity of the records. Shaw's map of St. Croix (figure 2) is based on a homogeneous though short (10 years) series of 26 records from the flatter parts of the island, which should give a reliable and consistent pattern. Year to Year Variation The variability of the mean annual rainfall is of prime economic conse- quence because in over half the years the actual rainfall is well below the normal rainfall,* which is just about sufficient for an annual yield of sugar * It is characteristic of the frequency distribution of either daily, monthly or annual rainfalls, that the most frequent value {mode) is generally much less than the average, and in some cases the zero value is most frequent. STONE: METEOROLOGY OF THE VIRGIN ISLANDS 25 cane, long the chief crop, and the cane yield suffers accordingly (Dr. Shaw discusses this prohlem with respect to St. Croix, in paragraphs quoted be- low). Du Tertre and Oldendorp mention great droughts in 1661 and 1753 ; the poor crops of 1841, 1864, 1869, 1872 to 1877, 1891, 1892, 1899, 1904, 1923 to 1924 were due to low rainfall (see text tables 20 to 23). Con- trary to the impression of many residents and travelers, there is no real evidence that the rainfall is slowly and steadily decreasing from century to century (see Forests and Rainfall). The question of cyclic variations has not been scientifically studied here, but results elsewhere generally show long quasi-periodic fluctuations of considerable amplitude in the rainfall. These undoubtedly exist here too but the records are not long enough to reveal any but the shortest "cycles". The average rainfall is so near the critical limit for sugar cane that even the small short-period fluc- tuations are important. It does not contribute much either to fundamental understanding of the variations nor to practical precautionary measures for the farmers merely to describe the rainfall curve as quasi-periodic. All attempts to forecast the fluctuations by means of extrapolating "cycles" derived from analysis of past records have been failures. Scientific bases for long-range forecasting are being sought in many directions but the solutions offered do not yet give results of practical value and general ap- plicability, however promising the method or enthusiastic the advocates. The most successful results so far are for certain special conditions and places, none of which have been in the West Indies. Diurnal Variation The diurnal distribution of the rainfall, as at San Juan, shows a much greater amount of rain by day than by night, judging from Mr. A. Wal- loe's observations at Charlotte Amalie, published in the "Set. Thomae Tidende", 1888. He gives the following figures. Text Table 6 Night and Day Rainfall, Charlotte Amalie, 1888* Month (1888) Total By day By night July 38.4 26.8 11.6 August 77.2 SS.9 21.3 September 69.0 44.6 24.4 * In lines; 8 lines := 1 inch. The frequency of rain is no doubt also greater by day but the contrast is probably not so pronounced because the intensity of the day showers is heavier. 26 SCIENTIFIC SURVEY OF PORTO RICO At sea the rainfall frequency is a maximum at 6 a.m. with a secondary maximum at about 10 p.m. The amplitude of this daily variation is pre- sumably smaller than the one observed over the islands, where the maxi- mum comes in the afternoon. It is very likely that the sea maximum at 6 A.M. affects the islands, or at least their shoreward margins, causing a secondary maximum at that hour. No hourly observations are available from the islands but the sunrise shower seems to be recognized by the residents as a more or less regular phenomenon. The daily double period in the rainfall is of course reflected in the cloudiness (text table 17) and in the frequency of thunderstorms. Intensity and Frequency The rainfall in this low latitude and oceanic situation is entirely of the shower type, and therefore it is of great practical importance to know how frequently showers occur, how long they last, how much rain falls per shower, and what are the average and maximum rates of fall over short periods of time. Unfortunately systematic observations using recording rain gages were begun in the islands only very recently, so we are forced to infer much from the usual rainfall observations which give only monthly totals and numbers of rainy days. The average rainfall per rain day (appendix tables 10 and 12; figure 10) indicates some important characteristics. The "showers" of the winter and spring seasons are characteristically brief and light, often mere sprinkles, from cumulus clouds of small or moderate size and spaced by large intervals of blue sky (cf. text table 9) . Sometimes "norther" efifects cause a low overcast cloud deck with driz- zling rain punctuated by occasional heavier showers, which condition may persist a day or two. However, very heavy rains up to 2 or 3 inches in a day have fallen even in the driest months. In the "rainy season", from May to November, heavier and more enduring showers, with squalls or thunder and lightning at times, are to be expected much more often ; at least one shower of some sort then falls almost every day. Heavy rains lasting as much as 6 or 8 hours, even with brief intermissions, are normally very rare, but passage of a hurricane within 50 or 100 miles can cause enor- mous rainfall totals (over 10 inches) in a day or two from virtually con- tinuous downpours. The high wind during hurricane weather adds greatly to the destructive efifect of the rain. Some significant deductions can be made from the results of the re- cording rain gages, in spite of the short period they have been in use. At the Marine Barracks of Bourne Field on St. Thomas a recording rain STONE: METEOROLOGY OF THE VIRGIN ISLANDS 27 gage has been operated since 1935. An analysis of the results (appendix TABLE 12 and figure 10) indicates that the average rainfall per rain day and also per rain hour for each month is proportional to the greatest rain- fall during any 24 hours of the corresponding months. This is a very in- teresting relation because in the absence of recording rain gages at other places we can assume that the "greatest rainfall in 24 hours", which is tabulated by the U. S. Weather Bureau for all its stations, gives a rough basis for estimating the average intensity of rainfall per day and per hour. Since February 1940, the Soil Conservation Service has been tabulating rainfall rates monthly from recording gages at Anna's Hope and Jolly Hill estates on St. Croix. An abstract of the results appears in text tables 7 and 8. Although the period of observation is too short to give any definite averages or extremes likely to occur, the figures are already significant. A study of the tables reveals a closer correlation by months between the total rainfall and the maximum intensities than between the total rainfall and the average intensities. This is not surprising because one or two intense showers probably have more effect on the monthly totals than the more numerous lighter showers. There is nevertheless some tendency for the average intensity to be greater in the rainier months than in the drier months. It will be noted, however, that the average intensity in the spring months appears to be as high as or higher than in the autumn months, whereas the total rainfall is usually much greater in the autumn than in the spring. This is a curious fact which we have already suspected from the greater frequency of hail in the late spring and early summer than in the late summer and autumn. Over a period of many years the average in- tensity of rainfall will actually be greatest in the autumn or late summer because of hurricanes. The important conclusion is that, if hurricanes are excluded, winter and spring showers probably have as great average in- tensity as the autumn rains, but the ina.rimum rates of rainfall in short pe- riods, as shown in text tables 7 and 8, are generally two or three times greater in the "rainy season" than in the winter and spring. It is impossible to infer to what extent this conclusion is justified for all parts of the islands, as the topography may greatly aflfect the rainfall intensities as well as the totals, but the Bourne Field results (appendix table 12) seem to show similar features to those of Anna's Hope and Jolly Hill estates. Any practical interpretation of the average rainfalls reported in the Virgin Islands, especially on St. Croix, should take into account the fact that a large proportion of the rain falls in light showers and brief sprinkles (see text tables 9 and 10) . Many of these light rains are measured in the rain gages and they augment the total rainfall out of proportion to their significance for crop growth and for vegetation because they barely wet 28 SCIENTIFIC SURVEY OF PORTO RICO Text Table 7 Rainfalx Intensities Measured at Station SCS No. 18 F. S. A., Jolly Hill Estate, St. Croix, V. I. (From U. S. Soil Conservation Service) Total Rainfall. Total Duration, Average Intensity, Maximum Intensity for Different Intervals Month inches hours* in./hr. S-min. 10-min. 20-min. 60-min. 1 20-min. 1940 February 0.90 15.02 0.06 1.00 0.75 0.35 0.13 March 0.52 1.92 0.27 April 1.97 6.42 0.31 2.00 1.50 0.80 0.50 0.28 May 7.10 30.00 0.24 3.50 2.00 1.40 1.10 0.70 June 3.05 8.10 0.38 2.00 1.50 0.90 0.55 0.30 July 2.14 5.07 0.42 S.OO 3.50 2.80 1.30 0.65 August 3.05 12.37 0.25 3,00 1.75 0.95 0.30 0.15 September 4.19 12.65 0.33 7.00 5.00 3.40 1.40 0.75 October 7.47 22.45 0.33 4.50 2.75 2.80 1.70 1.05 November 7.15 25.37 0.28 5.00 3.50 2.40 1.45 0.90 December 3.47 20.25 0.17 3.50 2.25 1.60 0.85 0.45 1941 January 1.97 5.02 0.39 2.00 1.75 0.90 0.40 0.20 February 0.21 0.80 0.26 _ _ March 1.33 1.13 1.17 1.50 1.00 0.50 0.18 _ April 2.28 9.45 0.24 3.75 2.50 1.40 0.56 0.30 Intensities of less than 0.10 in./hr. are not included. Text Table 8 Rainfall Intensities Measured at Station SCS No. 15 F, S. A., Anna's Hope Estate, St. Croix, V. I. (From U. S. Soil Conservation Service) Total Total Average Maximum Intensity for Different Intervals Rainfall, Duration, Intensity, Month inches hours* in./hr. 5-niin. 10-min. 20-min. 60-min. 120-min. 1940 January 0.35 2.77 0.13 _ _ _ _ _ February 2.09 17.18 0.12 1.80 1.25 0.75 0.30 0.18 March 0.99 4.92 0.20 1.00 0.75 0.35 0.13 _ April 1.55 4.20 0.37 3.50 1.75 1.20 0.80 0.43 May 2.88 15.05 0.19 2.00 1.50 0.80 0.40 0.25 June 1.56 4.60 0.34 3.00 2.25 1.20 0.40 0.20 July 1.17 4.23 0.28 1.00 0.75 0.35 0.13 _ August 1.75 7.48 0.23 3.00 2.00 0.80 0.30 0.15 September 5.24 6.67 0.82 7.00 5.00 4.20 2.30 1.18 October 8.43 22.05 0.38 4.00 3.00 2.00 0.80 0.50 November 6.05 14.67 0.41 7.00 5.50 3.30 1.10 0.60 December 2.36 14.10 0.17 1.50 1.00 0.45 0.18 - 1941 January 3.67 12.25 0.30 4.00 2.50 1.60 0.60 0.38 February 0.19 2.50 0.08 - _ _ - _ March 1.05 2.83 0.37 1.50 1.00 0.50 0.20 O.IS April 2.48 • 7.25 0.34 4.00 3.00 2.60 0.92 0.48 Intensities of less than 0.10 in./hr. are not included. STONE: METEOROLOGY OF THE VIRGIN ISLANDS 29 the vegetation and the top of the soil and do not sink into it, and so are quickly evaporated by the sun and wind. Text Table 9 Percentages of Days with Specified Amounts of Rainfall. Christiansted, St. Croix, 1852-1907 (From Willaume-Jantzen and Ravn) , > 20 mm > 50 mm 0-S mm (0.79" (1.97" Month (0-0.20") or more) or more) January 67 5 February 64 3 March 66 4 April 55 16 2 May 56 10 4 June 46 14 3 July 55 10 2 August 54 11 4 September 45 15 4 October 44 18 7 November 48 12 3 December 52 8 2 Year 54 11 3 Text Table 10 Average and Extreme Numbers of Days with Rain, Christiansted, St. Croix, 1852-1907 (From Willaume-Jantzen) Highest Lowest Month Mean in any one year in any one year January 11 20 2 February 9 23 1 March 6 14 April 7 13 2 May 11 26 3 June 10 20 4 July 11 17 4 August 11 17 4 September 13 19 6 October 12 19 6 November 14 20 4 December 13 19 6 Year 128 177* 84* * These figures are not sums of the columns above, but are the extreme totals on record for any one year in the period covered by the table. 30 SCIENTIFIC SURVEY OF PORTO RICO Evaporation The actual water loss from the ground by evaporation and by transpira- tion of plants is probably high, judging from the general weather condi- tions and from the measures of evaporating pozver of the air made at the Experiment Station (see appendix table 8). Consequently, the roughly 45 inches of measured average annual rainfall in the Virgin Islands is by no means the equivalent for plant growth of 45 inches of measured pre- cipitation in rainier parts of the West Indies or in the southern United States. Thunderstorms, Squalls, and Hail Thunderstorms occur, as in Porto Rico, chiefly from July to October, according to the records at Christiansted and Bourne Field (text table 1 and appendix table 12). Schomburgk in 1837 reported that 5 to 10 per cent of the days in a year had thunderstorms, mostly in September and October, which roughly agrees with the Christiansted data, although at Bourne Field more of the storms occur in July and August. Most storms probably occur in the afternoon, as at San Juan. They are apt to be squally and inflict wind damage at times, but lightning damage is usually slight. Squalls are sometimes associated with heavy showers and probably with most thunderstorms. The familiar downrush of cold air under a thunder- storm or tall cumulonimbus cloud can be so violent as to capsize small boats and damage dwellings, trees, and crops. When the observer is located on the sunny side of the cloud, it may appear white until after the squalls reach him, giving rise to the term "white squall" of the West Indian na- tives ; but when the observer is under or on the shaded side of the cloud, it appears very dark and ominous, so the accompanying gusts are called "black squalls". White squalls are also reported without heavy clouds nearby, but these are merely gusts when the trades are blowing strongly. The West Indian sailor well knows that the squalls are apt to be especially violent and dangerous to boats along a coast which rises to high mountains immediately back of the shore. Hail is rarely reported and most residents spend a lifetime in the islands without seeing any. There are enough authenticated reports to leave no doubt that it falls at least every few years, even several times in some years in which conditions are favorable for it. Much hail, with cold and rainy weather, occurred in Virgin Gorda in January 183v3, according to Schom- burgk, who also wrote of hail on the north side of Tortola in November 1829. Knox mentions that hail as big as hen eggs fell in St. Croix on April 13, 1844; and that a Mr. Nissen told him of a hailstorm at St. STONE: METEOROLOGY OE THE VIRGIN ISLANDS 31 Thomas on May 13, 1829. A reliable eyewitness reported that hail fell in St. Thomas in 1938. Although in Porto Rico most hail comes in spring and early summer, the cases cited for the Virgin Islands are chiefly in the win- ter and spring ; perhaps the winter hailstorms are more phenomenal and thus more likely to be remembered or noticed. Chemistry of Rainwater Chemical analyses of rainfalls were regularly made at the Experiment Station from 1911 to 1915 (see text table 11). Rainwater (fresh) con- tained an average of 9.6 parts per million of chlorides, 0.262 parts of nitrogen in the form of ammonia, and 0.324 parts of nitrogen as nitrates. These figures varied greatly from month to month and storm to storm. The amounts do not seem to have a definite seasonal variation, nor do they appear to depend on the total monthly rainfall, though it is quite possible that they are related to the intensity and amount of the individual shower. These chemical constituents of the rain have an important efifect on the soil and the nourishment of crops, a subject much studied by agronomists. Water Supply and Irrigation Owing to the small and erratic rainfall, the dry porous soils, high evapo- ration, and the few permanent streams, it has been a serious problem to obtain domestic water supply. Rain water is diligently caught from roofs and stored in cisterns, and several acres of steep slopes are paved with con- crete to catch rain for use by the town people. Drought periods enforce strict economy in use of water and sometimes necessitate importing water. Shallow dug wells are used for cattle and gardens, and salt water is pumped for flushing the sewers. The cisterns for drinking water are stocked with "mosquito fish" and screened, for in the past malaria was spread chiefly by mosquitoes that bred in uncovered cisterns. Irrigation of La Grange plantation from a dammed mountain brook in northwestern St. Croix was started in 1910 and perhaps similar projects are feasible, but not on a scale sufficient for all fields during a drought. Deep wells have not yet been tried. Stored surface water might deposit alkali or salt in soils on which it was used for irrigation. TEMPERATURE Geographical Controls Temperatures in the Virgin Islands are more uniform than at most Porto Rican stations owing to the more insular exposure and relatively small land area available for local radiational and insolational influences. 32 SCIENTIFIC SURVEY OF PORTO RICO Text Table 11 Chemistry of Rainfall at Anna's Hope, St. Croix (Experiment Station) (From Reports V. I. Exp. Sta., 1911-15) Rainfal Parts per million of I in lines Nitrogen as Nitrogen as Nitrogen Date (1 line = Ys in.) Chlorine ammonia nitrates total 1911 Sept. Oct. 29.8 1 S0.3 5.5 0.290 0.234 0.524 Nov. 17.0 4.0 0.260 0.074 0.334 Dec. 43.6 7.S 0.290 0.124 0.414 1912 Jan. Feb. 10.1 ) 19.5 ] 11.0 0.370 - - March April 14.9 ) 12.2 f 14.5 0.390 0.310 0.700 May June 10.9 ) 33.5 J 9.0 0.440 0.254 0.694 July Aug. 22.58) 10.24( 10.0 0.070 0.500 0.570 Sept. Oct. 6.72) 7S.28| 6.0 0.420 0.220 0.640 Nov. 70.32) 8.0 0.100 0.380 0.480 Dec. 10.96) 1913 Jan. Feb. 28.24) 12.64 j 14.0 0.180 0.274 0.454 March April 21.361 32.00) 9.0 0.370 0.423 0.793 May June July 66.72) 3 68) • 8.0 0.100 0.266 0.366 19.20 14.0 0.180 0.710 0.890 Aug. Sept 14.88 contaminated 29.60 10.0 0.400 0.810 1.210 Oct. 42.40 18.16 contar ninated 0.320 Nov. 9.0 0.440 0.760 Dec. 19.60 8.0 0.160 0.230 0.390 1914 Jan. 21.76 10.0 0.100 0.280 0.380 Feb. March 14.401 9.60' [ 17.0 0.400 0.260 0.660 April May June 7.76; 86.96 i 9.0 0.320 0.110 0.430 28.00 contaminated July Aug. 11.04' 19.04 [ 11.0 0.240 0.450 0.690 Sept. Oct. 10.64' 20.40 \ 8.0 0.500 0.340 0.840 Nov. Dec. 38.32 23.28, I 6.0 0.200 0.120 0.320 1915 Jan. 11.28 11.0 0.080 0.130 0.210 Feb. March 6.88 20.00 1 13.0 0.300 0.160 0.460 April 102.00 10.0 0.080 0.160 0.240 May 80.40 8.0 0.060 0.180 0.240 June 95.20 10.0 0.060 1.120 1.180 Average 9.6 0.262 0.324 0.S86 STONE: METEOROLOGY OF THE VIRGIN ISLANDS 2>Z The climate is fairly sunny, and the insolation on some days, especially in spring, is very intense. Owing to the small number of tempefature station records and their lack of homogeneity, it is not feasible to construct reasonably accurate iso- thermal charts of the islands. However, a true isothermal map would show very much the same pattern as the topographic contour map, because the mean temperature varies chiefly with elevation, though the vertical gra- dient varies somewhat with exposure and distance from the coast (cf. Vertical Temperature Gradient, infra). The geographical distribution of the temperature shows much less range and complexity than the rainfall distribution and is of correspondingly less practical concern. The nearness of the sea and the prevailing trade winds prevent excessive maximum temperatures from local insolational heating in the interior. The mean temperatures (computed from observations recorded in the standard louvred thermometer shelter) do not differ much from place to place, and are controlled chiefly by the temperature of the ocean surface to the wind- ward, and by elevation (appendix tables 9-10 and text table 15). In sheltered interior valleys such as Anna's Hope, St. Croix, the night mini- mum temperatures are somewhat lower and the wind dies down more at night than at coastal points or on exposed slopes. Dew is frequently re- ported on clear nights especially at such interior places, and is not unknown at any point. It is the impression of many residents and students of the Virgin Islands that St. Croix is somewhat warmer than the other islands. There is no doubt that on any of these islands the windward sides must be, and do "feel", somewhat cooler than the leeward sides, if only because of a con- trast in cloudiness and wind velocity. Consequently, when comparing Christiansted or Fredericksted with Charlotte Amalie, the latter is usually rated somewhat cooler, although the mean temperatures are similar. It has even been thought that St. Croix as a whole is noticeably warmer because of its more southern location. There is as yet no observational evidence from sea-water nor air-temperature data to justify such a conclusion, and if it be quaUtatively true, the magnitude of the difference must be small. As St. Croix is only 40 miles south of St. Thomas, equal or greater differences are to be expected from one place to another on any of the islands, except possibly the smallest "rocks". It is quite misleading to attempt to differ- entiate climates within the islands from the available weather records.* * Owing to the fluctuations of the mean temperature above and below the normal from year to year, the averages of the shorter-period weather records are probably not representative and should not be compared with the longer records without some allowance for the trends. About 10 years are required to give an acceptable approximation to the temperature normal under tropical conditions, but 30 years are needed to smooth out the small-amplitude so-called sunspot-cycles, while a large- amplitude cycle of about 100 years must be reckoned with in most climates. (Rainfall fluctuates much more than temperature, so that 30 to 100 years are needed to give a reliable normal.) 34 SCIENTIFIC SURVEY OF PORTO RICO The limited area of forest cover on St. Croix, compared to well-wooded St. John and Tortola and partly-wooded St. Thomas, is certainly a factor that may tend to elevate the temperature. In the mean temperatures for the whole 24 hours of the day, the effect of the greater insolational heating of a barren land is offset to a large extent by a greater radiational cooling at night. Thus we may expect, other things being equal, St. Croix to have a greater daily range of temperature than the other islands. The relative flatness and somewhat greater extent of St. Croix also favor a greater daily range. Comparing the temperature records (appendix tables 9 and 10), note that Anna's Hope in central St. Croix has an annual average daily range of 14° F., whereas the coastal stations have about 9 or 10° daily range. The mean minimum temperature at Anna's Hope is slightly lower than at any other station and the mean maximum sligh-tly higher. Extremes : Cool Spells and Warm Spells The coldest winter nights at Anna's Hope generally follow immediately after a series of abnormally warm nights (minima above 75° F.). A slight shift of wind takes place between these episodes from south of east to north of east. The warm nights are due to the deep current of moist air imported with the southeast component of the wind, which prevents much cooling of the ground by radiation at night ; but the cold nights are due to the drier upper air and lack of clouds that follow in the wake of the northerly component in the trades. The first night with more northerly air may remain quite warm due to low clouds resulting from cooler air pass- ing over the warm sea, but the second and third nights will usually clear off and become cool unless the normal air circulation has completely re- established itself by this time. It is not unusual for three or four nights in succession to have minima of 60° F. or less in the winter. Schomburgk mentions a minimum of 36° F. occurring once at Tortola in the early part of the last century, which caused the natives much suffering, but such a low temperature seems impossible and must be a misprint for 56° F. It will be recalled, however, that temperatures were abnormally low in the United States in the early decades of the 19th century, when there must have been many severe "northers" in the Caribbean.* The weather of January 16-20, 1914, is an excellent, if somewhat ex- , * The "Set. Thoitiae Tidende" for March 15, 1837, reported, "The weather has been cool for some time past, and of late we have had frequent stiff breezes from the north [jic/], both probably arising from the prevalence of cold weather in America". On March 22 of the same year the "Tidende" related, "... the Packet Express . . . arrived here late on Monday, in IS days from Barbados, being, it is said, the longest passage ever known, of either mail boat or packet from windward . . . it is accounted for by the strong northerly wind that has prevailed for some time past". STONE: METEOROLOGY OF THE VIRGIN ISLANDS 35 treme, example of a cold winter night between rather warm nights, as is shown by the data of text table 12. Text Table 12 Temperature and Relative Humidity, January 15-21, 1914 AT Anna's Hope, St. Croix (Data from V. I. Agric. Exp. Sta. Report, 1914) Date Min., A.M. 8:30 A.M. 12 noon Max., P.M. 4 P.M. 9 p.m. Rel. Rel. Rel. 1914 Temp. Temp. Hum. Temp. Hum. Temp. Temp. Hum. Temp. January ° F. ° F. % " F. % " F. °F. % "F. IS 70 79 80 81 81 82 79.S 84 74 16 66 74 88 78 76 80 78 68 71 17 68.5 75 72 78.5 72 79.5 78 — 66 18 52 64 _ 79.8 _ 80 79.5 — 59.5 19 [55?] 77 78 81 88 86 75 89 75.S 20 74 76 81 82 86 85 82 — 76 21 68 78 79 82 - 83.5 80 72 70 This is a striking example. Note how the temperature failed to fall be- low 68.5° on the early morning of the 17th in spite of the rather low hu- midity which had already set in on the afternoon of the 16th. The warm night was due to clouds which also kept the temperatures from getting ab- normally high during the day. The night of the 17th was clear and cold (52° min.) and likewise the next night (18th), with continuing low hu- midity. The 19th and 20th were clear and the sun heated the air up to 86° and 85°. The humidity rose again on the 19th and it was cloudy that night so that the minimum on the morning of the 20th was high, 74°. No rain fell from the 16th to the 20th. Long cloudy, rainy spells sometimes reduce the daily range of tempera- ture to less than 5°, a disagreeable type of weather in the tropics, but it does not happen as a rule more than five to ten times a year in the Virgin Islands. What might be called hot spells occur practically every year, particularly between May and November. These are times when the daily maximum temperature exceeds 88 or 90° F. for several days in succession, especially when the humidity is unusually high and the wind light at the same time. The daily maximum at low-level stations holds rather constantly from 83 to 88° F. in the warmer half of the year, wherefore an occasional day with 90 or 95° F. is very noticeable to the residents. Spells with daily maxima above 90° F. are disagreeable not so much because of the maxima as be- cause of the minima, which are often about 78 to 80° F., so that the nights do not allow one to recuperate from the heat of the days. 36 SCIENTIFIC SURVEY OF PORTO RICO Character and Effects of Changes Changes from season to season, from day to day* and night to day, or because of storms, wind-shifts, or showers are small as compared to those in high latitudes and in more continental parts of the subtropics, but they are important to the health and comfort of more or less permanent resi- dents in the islands. This was noticed long ago, and is mentioned by many writers. Knox (1852) noted a sudden drop in temperature of 5.4° F. dur- ing the passage of a shower, but this is unusual. Nevertheless, he says it had as serious an effect on the health of the inhabitants as a drop of 26° F. would in New York, because "influenza, ague, or bowel complaints, etc., succeed such falls of temperature". In July and August 1851, 4000 of the population were affected by influenza. Knox wondered if the epidemic were not caused by the few cool hours of rain which followed a period of more than usual heat and drought — a theory which may be discounted in this case as epidemic influenza, once begun, rapidly spreads to all cor- ners of the earth as fast as the germs can be transported. Nor can we agree with Knox's conclusion that, "It is owing to the very minute daily varia- tions that this climate is healthy, and so happily adapted to the individuals suffering under pulmonary attacks." The chief drawback to this climate is the lack of stimulating variations, which lowers the resistance to unusual changes when they do come and permits a marked slackening of tone in those who fail to return to the higher latitudes at intervals.t Indeed, many residents and physicians (and Knox himself) have admitted that a change to a temperate climate every few years is almost as necessary in the present relatively hygienic era as in the more disease-ridden Danish times. Schomburgk observed at Tort.ola a very unusual drop of temperature on April 29, 1832, from 79.5° F. at 9:15 a.m. to 73° F. at 9:25 a.m. to 71° F. at 12:15 p.m. ; there was a quick drop of 6.5° F. in 10 minutes, and of 8.5° F. in 2 hours. Daily Cycle Of course the regular diurnal temperature changes are a source of physiological stimulation. Everywhere they average less than 15° F. and at many localities are less than 10° F. (see appendix tables 9, 10, and 12), but for the individual days the range varies considerably depending on the * The mean interdiurnal (day to day) temperature variability at San Juan is a little more than 1° F. in all months. The figures for the Virgin Islands stations would be about the same, and cer- tainly less than 2° F. even at interior locations. t Stone, R. G. "Some results of modern physiological research in relation to acclimatization in the tropics". Appendix 1 in Price, A. G. "White Settlers in the Tropics". New York, 1939. STONE: METEOROLOGY OF THE VIRGIN ISLANDS 37 amount of clouds, wind velocity, and rains. Normally the hottest hour comes between noon and 2 p.m., perhaps averaging a little later in winter than summer; sunrise is the coolest (see text tables 13 and 14). In clear weather the diurnal cycle is surprisingly regular, but irregularities in the thermograph traces are common, due to the passage of clouds and rain, and to changes in wind velocity or direction. There seem to be various characteristic types of these daily curves. Knox illustrates three from the month of September 1845, and Stenzel's 1886 figures are interesting. The many curves for Anna's Hope in the Experiment Station Reports for 1911 through 1915 show all the likely variations. Especially noteworthy is the characteristic alternation of spells of con- trasted types of daily temperature cycles, for example : (A) small daily range (B) large daily range 1. high maximum temperature and 1. high maximum temperature and high minimum temperature (due low minimum temperature to clouds at night?) (clear day and night?) 2. low maximum and low minimum 2. normal maximum and very low (due to heavy rain or clouds by minimum (dry air, some clouds day?) by day, none at night) 3. low maximum and high mini- 3. very high maximum and normal mum (due to steady rain, and minimum (dry air, light wind clouds day and night) and clear by day, partly cloudy or humid and windy by night) Other combinations are also observed. A curious secondary rise in tem- perature often appears around 2 a.m. and abates by sunrise, perhaps due to a night-time increase in the wind velocity. The uniformity of the daily range from day to day is closely related to the regularity of cloudiness. Text Table 13 Ste.-Claire Deville's Temperature Observations at St. Thomas DURING 1840 (broken SERIES)* Temperature Hours of Hours at which the Mean difference extremes mean temperature Months - temperature 6 a.m.-I p.m. (approx.) is reached March-April n'.g" F 39 6* F 6 a.m., 1 p.m. July-August ll'^jl p- 39-4! p* 6 A.M., 1 P.M. October-November RnT v' xtr' v ^ A-^-> ^ ^ 80.2° F. 37.8° F. * Hombeck's averages of S years' observations at Charlotte Amalie, 1829, 1830, 1834, 1835, 1836, are: 77.9° F. at 7 a.m., 81.7° F. at 4 p.m. and 78.8° F. at 8 p.m. These seem to give too small a range, prob- ably owing to defective exposure of the thermometer. 38 SCIENTIFIC SURVEY OF PORTO RICO Text Table 14 Mean Temperatures at Different Hours at Christiansted, St. Croix, 1913-15 (From "Meteorologisk Aarbog") Year 8 A.M. 2 P.M. 9 p.m. 1913 26.0° C. 78.8° F. 27.6° C. 81.7° F. 25.3° C. 77.5° F. 1914 26.2° C. 79.2° F. 28.4° C. 83.1° F. 25.7° C. 78.3° F. 191S 26.7° C. 80.1° F. 28.3° C. 82.9° F. 26.1° C. 79.0° F. Vertical Temperature Gradient The expected decrease of temperature with elevation is definitely ob- servable in the range of elevation from sea level to over 1000 feet, but this amounts to only a few degrees on the average (compare Canaan, Wint- berg and Charlotte Anialie, St. Thomas ; and St. Bernard's and Roadtown, Tortola). Ste. -Claire Deville in 1840 found that the average gradient was 1° C. per 100 meters up to 500 meters. Knox made ( 1845 ?) the following comparisons between Charlotte Amalie and Louisenhoj (778 ft.) : Text Table 15 Temperature Difference between Charlotte Amalie and Louisenhoj, St. Thomas Charlotte Amalie Louisenhoj Difference Hour (20-100 feet?) (778 feet) {ca 700 feet) 6 a.m. 76.1° F. 72.1° F. 4.0° F. 2 P.M. 83.8 79.1 4.7 8 p.m. - 78.8 73.7 S.l These differences are rather large and lead us to suspect they may rep- resent unusual days or else the thermometers or their exposures were not comparable or proper. On the other hand, since Louisenhoj is on a sum- mit well ventilated by the trade winds, and Charlotte Amalie is sheltered therefrom and overheated by virtue of its stone surfaces, it is conceivable that these "superadiabatic" gradients can exist much of the time. Airplane soundings made from Bourne Field (appendix table 17) also reveal slightly superadiabatic lapse rates for the lowest 500 meters in the sum- mer months (8 a.m.), but those data are open to various interpretations and criticisms.* * See McDonald, Bull. Am. Met. See. 23: 75-76. Feb. 1942; Conrad, V. Meteorological results of the "Meteor" expedition 1925-27. Bull. Am. Met. Soc. 23 (4): 143-147. 1942. STONE: METEOROLOGY OP THE VIRGIN ISLANDS 39 HUMIDITY, SUNSHINE, AND CLOUDS Absolute Humidity The absolute humidity may be judged here from the vapor-pressure ob- servations because the barometric pressure varies so little. Text table 17 gives the values for Christiansted. The specific humidity (grams of water Text Table 16 Monthly Mean Humidity Period Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year Relative Humidity, per cent of saturation Frederikssted, 7 a.m. mean 79 81 80 85 84 78 83 83 85 Christiansted, mean (8 a.m. + 2 p.m. -\- 9 p.m.)/3 73 72 72 74 76 74 75 77 78 Bourne Field, mean (8 a.m. -1- 12 m. -f- 4 p.m.)/3 70 68 72 73 72 72 74 71 74 1879 17 years (1900-16) 4 years (1935-39) 77 73 83 (77) 81 78 74 77 74 75 72 Specific Humidity, grams per kilogram Bourne Field, 8 a.m. 1937 14.9 15.4 14.8 15.7 17.5 17.6 17.6 18.7 19.2 16.6 18.4 15.4 16.8 Text Table 17 Vapor Pressure, Relative Humidity, and Cloudiness at Christiansted FOR Different Hours of the Day, 1913-15 (From "Meteorologisk Aarbog") Year Vapor Pressure (mm.) Relative Humidity (per cent) Cloudiness (in tenths of sky covered) 8 A.M, 2 p.m. 9 P.M. 8 a.m. 2 P.M.* 9 p.m. 8 A.M. 2 P.M. 9 P.M. 1913 1914 1915 18.4 19.2 20.4 18.7 19.2 20.5 18.7 19.5 20.4 74 75 78 68 67 71 78 80 81 4.1 4.5 3.0 4.2 3.8 3.7 4.9 4.7 4.0 * Mean 2 p.m. relative humidity for 4 years was 70 per cent, ranging from 65 per cent in March to 75 per cent in November; see text table 1 for details. per kilogram of moist air) is also given for Bourne Field in text table 16. After 1916 no vapor pressures are available from the Virgin Islands, but San Juan, Porto Rico, values may be taken as a rough indication of mean conditions at sea level in the islands. 40 SCIENTIFIC SURVEY OF PORTO RICO Relative Humidity At Christiansted, the mean annual relative humidity (text tables 16 and 17) for the hours 8 a.m., 2 p.m., and 9 p.m is 3 per cent lower than that for San Juan, Porto Rico; the 2 p.m. mean (text table 1) is 6 per cent lower than the noon mean for San Juan, It seems likely that these figures indicate a real, though small, difference in the respective climates, which might be expected from the lower rainfall and more sheltered situa- tion of Christiansted. The four-year record at Bourne Field, St. Thomas (sea level, southwest coast), shows a mean annual (8 a.m. + 12 m. -j- 4 p.m.)/3 relative humidity (text table 16) of 72 per cent; the (9 a.m. + 9 p.m.)/2 mean for these years at San Juan was 80 per cent, and the 8 a.m. mean at St. Thomas was about 5 per cent below the 9 a.m. mean of San Juan for the same period ; thus St. Thomas is appreciably drier than San Juan. Even where the rainfall supports only cacti, the humidity is probably still rather high (but 5 to 15 per cent below that at sea) owing to the proximity of the sea and the constant draft of moist ocean air circulated over the islands by the trades. This high humidity and the salt content of the sea air naturally cause corrosion and deterioration of metal, furniture, and buildings, though by no means so severe as in the rainier tropical and equatorial climates. The average diurnal range of the relative humidity is certainly less than 1 5 per cent, since at both Bourne Field and Christian- sted the difference between 8 a.m. and 12 noon or 2 p.m. readings is well under 10 per cent. The highest humidity may be expected at sunrise, when the temperature is lowest; and the lowest between noon and 2 p.m., when the temperature is highest. October and November in the rainiest season, and March in the driest season are the periods of highest and lowest aver- age humidity respectively, but the difference is only 6 per cent or less. (Note : The "hygrodeik" readings published in the St. Croix Experiment Station Reports, 1911-15, are from an unventilated instrument, and not comparable to, nor as accurate as, the standard ventilated psychrometer observations.) Typical day to day variations are illustrated in text ta- ble 12, Sunshine and Radiation No measures of solar radiation are available but the intensities probably do not differ greatly from those at San Juan where several years' records are available. Slightly more insolation than at San Juan may be expected due to lesser cloudiness and humidity. Whether there is an abnormally early diurnal maximum of total radiation and sunshine (between 11 a.m. and noon) in the rainy season, as at San Juan, cannot be surmised. STONE: METEOROLOGY OF THE VIRGIN ISLANDS 41 Cloudiness Records of cloudiness from Christiansted and Bourne Field (see text TABLE 1 and APPENDIX TABLE 15) are probably reliable. The Charlotte Amalie record is short and hence erratic (text table 2). The tables (ap- pendix TABLE 1 1 ) of clear and cloudy days for the other stations should not be relied upon too closely as the different observers were allowed to judge for themselves what constitutes a "cloudy day", etc. (In Porto Rico, the results of such observations were found to be very misleading.) In general, the cloudiness (and, complementarywise, sunshine) varies con- siderably with location where there are mountains sufficient to interfere with the trades. The windward side of the upper slopes frequently is shaded by, or shrouded in, clouds, especially in the rainy season ; however, one can observe in the Virgin Islands how the large cumulus clouds which originate where the air is pushed over the mountains lean far out to the leeward and shade the lee side as much as or more than the windward. It is recorded in a book cited by Rose that Crown Mount, 1750 feet high, is often in the clouds during the rainy season. Judging from the Christian- sted (text table 17) and Bourne Field records (appendix table 15), the amount of clouds, or rather the percentage of the sky covered by them, averages only slightly greater during midday and afternoon than in morn- ing and early evening. However, these are coastal stations whose horizons encompass a large part of the sky over the water where the afternoon clouds from the land may not reach. The real diurnal range of cloudiness over the islands proper is certainly greater than the figures available sug- gest. The diurnal range of variation in amount of cloud cover is as great as, if not greater than, the annual range; the diurnal variation is much more marked over land and around the mountains than over the open sea, since it results chiefly from an insolational increase in vertical convection. Over the open sea a maximum of clouds (and rainfall) occurs between 4 and 6 a.m., and this undoubtedly also affects such small islands as the Virgins so that they have in addition to a chief maximum in the afternoon a secondary one near sunrise. Except possibly in the rainiest months the high clouds (chiefly strati- form) are generally much more extensive than (though not so dense as) lower ones (mostly cumuliform). The cloudiness does not vary greatly from month to month ; the dry months of February and March usually average between a third and a half of the sky covered ; the rainy period of July to November averages between 0.4 and 0.6 of the sky covered, de- pending on elevation and exposure of the station. Perfectly clear or en- tirely overcast days are rare. 42 SCIENTIFIC SURVEY OF PORTO RICO The prevailing character of the clouds in these two seasons is apt to be rather different, however. The rain-season clouds are more often cumulo- nimbus or cumulus of considerable vertical development with much cirrus, whereas the clouds of the dry season are smaller fair-weather cumulus and broken stratiform types (stratocumulus and altocumulus) for the most part. One's impression is that the sky exhibits a greater variety in form, density, and arrangement of clouds from season to season than the observations of "amount of cloudiness" would suggest. The cloudscapes also reflect certain subtle changes, not yet fully understood, in the general weather situation ; these changes tend to offset some of the monotony of other aspects of the weather. STORMS The characteristics and forecasting of "hurricanes" in this region are discussed in our report on Porto Rican meteorology ; the characteristics and effects of hurricanes in the Virgin Islands do not differ significantly from those in Porto Rico and the other Antilles. The list of hurricanes in appendix B, compiled from many sources, is probably complete for only the 19th and 20th centuries. Only nine storms were recorded between 1700 and 1800, whereas twenty-six were noted in the next century and seventeen so far in the present century. Some small and weak storms were reported in recent years, but before 1900 many such storms must have "grazed" the islands without exciting special mention in the annals. Some of the storms listed were recorded as passing only close enough to cause heavy rains and moderate gales with but minor, if any, damage. Somewhat less than thirty hurricanes actually struck one or more of the islands in full force and with very serious damage since about 1770 (when fairly complete annals begin). The summary in appendix B indi- cates about 43 per cent of the storms have struck in August, 35 per cent in September, 18 per cent in July, and the rest in October. It is not minimizing the danger from these storms to say that they rarely strike with great severity. The hurricane of August 1772 was described in a now widely-quoted letter, written by Alexander Hamilton (appendix B), then a youth of St. Croix, who left the island soon after for North America. Perhaps the most destructive storm at St. Thomas was that of October 29, 1867, of which graphic accounts were published in the "Set. Thomae Tidende" in early November of that year ; a severe earthquake and tidal wave on November 18 added to the misery. That of September 1876 was also devastating. Of recent years the storms of October 9, 1916, August 28, 1924, and September 13, 1928 (St. Croix), were especially SrONE: METEOROLOGY OF THE VIRGIN ISLANDS 43 damaging. It should be noted that only about a third of the storms that pass over St. Thomas pass over St. Croix or cause damage there, and vice versa. Only twenty of the fifty-one storms since 1738 listed in appendix B affected both islands, though they had about the same total number of storms (St. Thomas 39, St. Croix Z2>). This is easily explained by the small diameter of the damaging part of most hurricanes and by the various directions from which they approach the islands (from E through SSE, approximately ).§ Hurricanes passing to the south of St. Thomas, even a hundred miles away, can severely damage shipping and docks in the harbor merely from the heavy southerly swell, though no gale or rain may be observed; the harbor is protected from swells if the storm passes well to the north. For details of some of the past storms in the islands see ap- pendix B and the bibliography. WIND Surface Wind Velocity and Direction We have little representative data concerning wind velocities and di- rections. The anemometer on the Custom House roof at Charlotte Amalie apparently has only been used for spot readings, which in the hurricane season have been cabled or radioed to the U. S. Weather Bureau ; means for several years appear in text table 2.* At Bourne Field a pressure- tube anemometer has been in operation for several years and the records are on file there and at the U. S. Navy Department, Bureau of Aero- nautics, in Washington. Mean velocities cannot be obtained easily from this kind of instrument but the maximum gust velocities are given in ap- pendix table 16. At Anna's Hope, St. Croix, an anemometer has been exposed beside the evaporation pan (1^ feet above the ground in a spot somewhat sheltered by nearby buildings). The record is summarized in text table 18. These values must be multiplied by at least 4, or 5, to ob- tain an estimate of the velocity that would be recorded on top of a tower 30 feet above the ground but are representative of a typical microclimate. It is interesting to notef how the wind dies down at night near the ground at Anna's Hope ; the nocturnal radiation forms a shallow layer of cool, stable air which remains nearly calm in the valley because it is denser and exerts more f rictional resistance than in daytime. The result is a more or less pronounced diurnal cycle in the wind like that in Porto Rico with high- est speeds in the afternoon, lowest in the early morning. § See the maps in Tannehill, I. R. "Hurricanes, their nature and history". Princeton, 1938. * No severe hurricane struck the island during this period, though some passed near enough to cause high winds. The anemometers are usually carried away by a full hurricane hence the highest velocities are not known; they often exceed 150 miles per hour. t The anemograms were inspected by the writer at San Juan and Anna's Hope. 44 SCIENTIFIC SURVEY OF PORTO RICO Text Table 18 Average Monthly Wind Velocity at the Evaporation Pan* AT Experiment Station, Anna's Hope, St. Croix (From U. S. Weather^ Bureau, San Juan) Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year 1920 1.8 1.4 1.9 1.4 1.8 2.2 2.8 2.4 1.8 1.1 1.2 1.7 1.8 1921 — - - _ — — 2.1 1.7 1.4 1.0 1.3 0.7 1.9 1922 1.2 2.0 2.1 2.0 2.3 2.3 2.5 2.3 2.0 1.1 1.1 1.6 1.9 1923 2.0 2.3 2.4 2.0 2.0 2.6 2.8 2.6 2.0 2.4 0.7 1.9 2.1 1924 2.3 1.8 1.5 1.6 2.1 2.1 2.5 2.2 2.0 0.8 0.9 1.4 1.8 1925 1.5 0.8 1.7 1.3 1.2 1.1 1.0 1.1 0.7 1.2 1.2 1.0 1.2 1926 1.9 1.7 1.3 2.0 1.6 1.7 1.6 1.2 0.9 0.8 0.9 1.5 1.4 1927 1.3 1.3 1.2 1.5 1.4 1.5 2.0 1.0 0.6 0.7 1.2 1.2 1.2 1928 1.5 1.4 1.5 1.6 1.2 1.5 1.6 1.2 2.9 0.8 0.5 1.2 1.2 1929 2.1 1.6 1.8 1.4 1.6 1.5 1.9 1.6 1.0 0.9 1.6 2.0 1.6 1930 1.9 1.0 1.3 1.5 1.4 2.2 2.2 1.8 1.7 1.2 1.5 1.6 1.6 1931 1.3 1.4 1.6 2.0 1.6 l.S 1.7 1.8 1.0 0.4 0.5 1.9 1.4 1932 2.3 1.8 1.8 2.1 1.8 2.0 1.8 1.7 1.3 1.4 1.7 1.7 1.8 1933 1.7 3.0 2.7 1.9 2.2 1.8 2.4 1.5 1.6 1.2 1.5 1.9 1.9 1934 1.5 1.4 2.3 1.9 2.1 1.7 2.1 1.5 1.0 1.1 l.S 2.0 1.7 1935 0.9 1.1 1.8 1.2 1.3 1.3 1.7 1.2 1.4 0.7 0.7 1.2 1.2 1936 1.8 1.4 0.8 1.4 1.0 0.9 1.2 1.0 1.4 0.8 _ 0.5 — 1937 1.2 0.6 0.9 0.9 0.9 1.1 1.1 1.1 0.8 0.6 0.7 1.0 0.9 1938 1.2 1.2 1.2 1.0 0.9 1.0 0.7 0.8 0.5 0.2 0.6 0.4 0.8 Totals 29.3 26.2 29.8 28.7 28.4 30.0 35.7 29.7 26.0 18.4 19.3 26.4 Averages (Mi./ hr.) 1.63 1.41 1.65 1.59 1.57 1.66 1.88 1.56 1.37 0.97 1.07 1.39 1.48 * The anemometer is mounted about 1 }4 feet above the ground and hence gives velocities which are much lower than those read from anemometers as usually exposed on high poles or tops of buildings. The more exposed, especially windw^ard, parts of the Virgin Islands do not experience such a locally-produced night calm, though the w^inds may die dov^^n slightly at night, because at sea the trade w^ind has a diurnal ve- locity range of less than 1 mile per hour. Palgrave {infra) thought that the vv^ind force at St. Thomas often showed a secondary increase between 2 and 6 a.m., thus tending to produce a double maximum for the day (after- noon and early morning). We have no data from the Virgin Islands to verify this. Since it is not in accordance with observations from ships over the tropical seas,* if real it can only be the effect of the diurnal tempera- ture changes over the island on the stability and f rictional stress in lowest air layers, which normally occurs in more striking form only over larger islands ( see second paragraph infra ) . Leeward slopes and shores have some protection from the full force of the trades, but it is surprising how these winds pass over and around the islands without much diminution in velocity so that few spots lack a rela- tively fresh "sea breeze" most of the day whenever the trades attain con- siderable velocity over the surrounding sea. * Conrad, V. Meteorological results of the "Meteor" expedition 1925-27. Bull. Am. Met. Soc. 23 (4): 143-147. 1942. STONE: METEOROLOGY OF THE VIRGIN ISLANDS 45 From day to day the general velocity of the trades often varies decid- edly ; one day it may average 5 miles per hour and the next 1 5 miles per hour. Trades in excess of 15 miles per hour of course occur more fre- quently in the months of highest average velocity, w^hich are in winter and in mid-summer. Calm spells may be expected during May, June, Septem- ber, and October. The winds at sea blow as hard from one quarter as an- other, except in October and November when SE and S winds are usually lighter than others. The prevailing fresh breezes, coupled with the open vegetation, dry soil, ample sunshine, and moderate rainfall explain the reputation of the islands for a relatively healthy and comfortable sub- tropical climate. The changes in wind velocity from day to day are probably more important as physiological stimuli than the temperature changes, which average only 1° or 2° from day to day. On St. Croix the agriculturalists have found a need for windbreaks, especially in the flat southwestern section, to protect garden crops and or- chard fruit trees from the effects of the constant trade winds. The fruit trees have an unthrifty appearance in unprotected sites on St. Croix and it is found that such trees suffer from heavy scale infestations. In the lee of windbreaks of suitably chosen type the scale insects are held in check by the fungi that thrive better in the more humid conditions provided by the protection of the windbreak; less spraying of the trees is necessary in the lee therefore. The Experiment Station has been trying out various plants for windbreaks. In any case the protective effect does not extend more than several hundred feet to the leeward. In some parts of the island there is ample natural protection from woods and hills. The islands are too small to develop a noticeable system of land and sea breezes as is found in Porto Rico. Since the wind has some diurnal varia- tion in velocity and normally speeds up during the morning and falls off at night, residents often speak of the "sea breeze" arriving in the morning. There may be a relative calm at night (as at Anna's Hope), especially in sheltered valleys and on leeward sides, which increases the discomfort on summer nights, but no true off-shore "land breeze" at night seems pos- sible. Sometimes shallow layers of radiationally cooled air may be detected slowly flowing down the hillsides into valley bottoms in the early morning hours, but this is a very weak and restricted phenomenon here compared with what it is in the Greater Antilles and in higher latitudes, for it pro- duces neither a marked local cooling nor a noticeable "breeze" (less than 1 meter per sec). Usually the trades blow strong enough to mask almost completely any tendency for land and sea breezes to change the zvind di- rections. The wind-direction data from Christiansted (text table 19), St. 46 SCIENTIFIC SURVEY OF PORTO RICO Thomas (text table 2), and other stations (appendix table 11) are not exactly representative of the general winds because of the interference of nearby hills and mountains.* Text Table 19 Percentage Frequency of Wind Directions (1875-1907) Christiansted, St. Croix (mean of 3 observations per day) (From Willaume-Jantzen) Direction N ■ NE ^e SE S SW W NW Calm January 3 28 51 9 1 8 February 3 25 47 13 2 1 9 March S 29 43 10 2 1 2 8 April 3 21 43 18 5 1 9 May 1 10 46 27 7 9 June 10 57 25 3 5 July 20 62 15 1 2 August 1 22 51 16 3 7 September 1 IS 42 19 5 1 1 16 October 1 13 36 19 7 1 1 1 21 November 2 25 42 12 2 1 16 December S 29 45 8 2 1 10 Year 2 21 47 16 3 <1 <1 1 10 Upper Winds The directions of the lower clouds, as given in appendix table 16 for Bourne Field and in Palgrave's notes below, are roughly indicative of the undisturbed trade winds. The pilot-balloon ascents made by the Navy at Bourne Field have not been summarized. A technical discussion of the Caribbean upper-air circulation appears elsewhere. f Winds and Weather Types (after Palgrave) W. G. Palgrave, a one time authority on the West Indies, observed the following features during residence at St. Thomas (written in 1874). "During an average period of nine months in the year the regularity of the air- currents over the Virgin group resembles clockwork. The surface, or lowest cur- rent, is formed by the trade wind, which blows briskly from the east-north-east, with a slight variation northward during the night and early morning, and a cor- responding deflection southward from noon till near sunset. Its greatest strength is usually at or near Z-A a.m. ; and about the same hours p.m. It generally bears with it light masses of cumulus, from which there fall occasionally showers, heavy, but very short in duration. This air current is known as the trade wind of these regions. * Wind roses for the surrounding waters will be found on the "Pilot Charts of the North Atlan- tic" (monthly), published by the U. S. Hydrographic Office. t Stone, R. G. On the mean circulation of the atmosphere over the Caribbean. Bull. Am. Met. See. 23 (1): 4-15. 1942. STONE: METEOROLOGY OE THE VIRGIN ISLANDS 47 Next above this current comes tlie south-west wind, rarely absent ; it brings vvith it light cirrus clouds, but seldom cumulus or other indications of rain. . . . Highest of all the west wind reigns, manifested by very light cirrus clouds, rapidly formed and as rapidly disappearing. These three winds blow with scarcely any interruption from November to June inclusive ; almost the only variation being then afforded by the north or north-north-east wind which sometimes prevails, but near the surface only ['norther' influence], for a few days together during three winter months. When — a rare but much desired event [for the crops and water supply] — a south- erly current occurs about this time, it brings heavy clouds and abundant rain. While the wind is from the north and north-east [relatively], great dryness is indicated by the hygrometer. But in the months of August, September, and October, and often in the latter half of July, the polar or north-east current loses its strength and is often neutralized or even conquered by the southerly winds. These during the summer are usually light, and accompanied by a clear and serene sky, only clouded when the north-east, regaining for a time its supremacy, drives the south back, and precipi- tates heavy showers, amid thunder and lightning, sometimes lasting for three or even four hours ; after which the wind veers round again to the south-east and south. The same phenomena, when intensified, concentrate themselves into a hurri- cane or cyclone — a rare occurrence in this island, not more than four of any great severity having taken place at St. Thomas in the course of the present century [1800-74]. "Another phenomenon peculiar to the winter and spring months are the white squalls, which take place on calm days, generally at noon, and most often at no great distance from shore ; their area is very limited, and their duration does not exceed a few minutes ; in some respects they resemble a miniature hurricane, and appear to be due to similar causes ; but neither have I witnessed in them nor heard re- corded any instance of circular motion. They are much dreaded by the small craft of these seas ; a slight fall of the barometer is their only premonitory indication". ["Black squalls" with dark shower clouds are also feared by the sailors along the West Indian coasts. — R. G. S.] The above note shows a good elementary comprehension of tlie wind system for one writing at such an early date. In general the upper winds over the Virgin Islands are no different from those over San Juan, Porto Rico, which have been given special study by Fassig, Ray, and Stone.* Quin (1907) has pubhshed observations of the wind behavior over the Virgin Islands preceding hurricanes passing the vicinity. VISIBILITY The visual range ("visibility") seaward from the U. S. IVIarine Corps Base at Bourne Field on St. Thomas is given in appendix table 14. The visual range is more constantly good than at San Juan, as there is no smoke or land haze. On specially clear days the mountains of eastern Porto Rico, some sixty miles away, can be seen. The factors affecting the visibility are discussed in Part 3. The range is never less than 2.5 miles except during heavy rains. * Stone, R. G. Bull. Am. Met. Soc. 23 (1) : 4-15. 1942. 48 SCIENTIFIC SURVEY OF PORTO RICO CLIMATE, DISEASES, AND HEALTH* We have already mentioned some effects of temperature changes and winds on comfort and health. Of course, in earlier times the knowledge of medicine and public hygiene was insufficient to cope with the predisposing conditions for certain diseases and debility which a warm, moist, monoto- nous climate offers. It was formerly believed that the climate itself directly produced tropical diseases, and much was unfairly blamed on the climate which we now recognize as only indirectly, if at all, related to the climate. The monotony and mildness tend to lower the resistance of the body, but better individual and public hygiene can probably offset this danger. A certain percentage particularly of white people cannot tolerate the climate well in spite of good hygiene, yet this is true in some degree of any climate. Constitutions differ due to inherited or acquired idiosyncrasies, and some are better adapted to one place than another. In another decade we expect research in tropical physiology will make it possible to select by previous examination the persons who will adapt well to a tropical climate, and many of the tragedies that follow assignments to tropical stations will be largely avoided. On the other hand, it must be recognized that the benefits of hygiene and medicine are achieved in the tropics at a greater cost of money and inconvenience than in higher latitudes because the continually warm moist conditions cause a greater growth of pathogenic organisms which have to be combatted, than in a climate with a cold winter. It can be said in favor of the subtropical climate that the tendency to degenera- tive functional diseases of the heart and to metabolic diseases is distinctly less than in higher latitudes with their many sharp weather variations that can overtax the body. Knox (1852) described conditions which are not much different from those of today as follows : "... in winter and spring, fever and ague are apt to prevail in the low grounds and towns ; bilious fevers make their attacks more generally in the fall, induced by ex- posure to rains and the hot sun or intemperance [author's italics]. Consumption car- ries off many of the inhabitants [as today]. Rheumatism and neuralgia are common. Dysentery and influenza are epidemic. Whooping cough, scarlet fever, and measles are almost unknown. The continuous heat of summer and winter ultimately debili- tates the system and induces disease, especially bowel complaints. Foreigners are less sensitive to cold in the West Indies than the Creoles, but they feel the heat more." Owing to defects in diagnosis, the Danish official statistics of disease and death are probably in part misleading as to the prevailing ailments. After 1917 the United States administration has paid more attention to public health, and conditions are now vastly improved. Some prevalent dis- * Stone, R. G. "Health in tropical climates", in "Climate and Man". 246-261. U. S. Dept. Agric, Yearbook for 1941. STONE: METEOROLOGY OE THE VIRGIN ISLANDS 49 eases were formerly not understood or barely mentioned, such as hook- worm, venereal diseases, and elephantiasis. The relative dryness of the islands considerably limits malaria, schistosomiasis, hookworm, and prob- ably other parasitic diseases. St. John, being heavily wooded and thus hav- ing moist ground, has more malaria. The swamps, crabholes, and ponds near the coast, and moist crypts of trees and bushes, as well as cisterns and ditches, form the foci for local endemism of malaria, elephantiasis, and other mosquito-borne diseases. An unusually rainy season is followed by an epidemic increase of malaria (Shaw). McKinley ("A Geography of Disease" 1935) lists the following as important diseases in Porto Rico and the Virgin Islands : Malaria, lep- rosy (a few cases), syphilis, typhoid and paratyphoid, sprue, ascoriasis, hookworm, schistosomiasis, filariasis, elephantiasis, ulcus tropicum, mal- nutrition, measles, German measles, varicella, influenza, colds, pneumonia, tuberculosis, meningitis, diphtheria, nephritis, pellagra, and cardio-vascu- lar ailments. It is of interest to note the terrible consequences of a hurricane (see Red Cross report on 1928 storm) — the death rate increases not only because of injuries at the time, but also from overcrowding and malnutrition dur- ing the next few months. The intensity of the sunshine and its richness in ultraviolet have an im- portant indirect influence on health by drying the soil and killing bacteria and parasites. The ultraviolet (UV) will give the untanned skin an in- tense sunburn (erythema) in a fraction of an hour when the sky is clear and the sun is high. Although severe erythema should be guarded against, the general exposure to UV produces vitamin D in the skin and helps un- derprivileged folk to withstand malnutrition and especially rickets. Heat stroke and sunstroke are rare in the islands because the combina- tions of wind, temperature, and humidity are seldom so extremely uncom- fortable as are the hot spells of more continental climates. Milam and Smillie (1931) studied "colds" and oral bacterial flora of natives on St. John finding the flora very constant through the year with but few of the transient flora and seasonal changes found in northern United States. This is due both to the even cHmate and the isolation. CLIMATE AND SUGAR CANE IN ST. CROIX* Lack of sufficient moisture has always been the major limiting factor to the sugar industry of St. Croix. For an average cropf at least 45 inches of * Extract with minor changes and some additions by R. G. Stone, from "Geographic Studies on the Virgin Islands of the United States", by Earl B. Shaw, Ph.D. Thesis, Clark University, pp. 36-58. 1932. t The average production for plant cane is 17 tons per acre, for cane ratoon, 8 tons; these aver- ages and rainfall requirements for "good" and "average" crops were furnished by Glen Briggs (formerly) Director U. S. Agricultural Experiment Station, St. Croix. 50 SCIENTIFIC SURVEY OF PORTO RICO well-distributed rainfall are necessary, and from 50 to 60 inches are re- quired for what may be termed a good yield. Rainfall records for 80 years at Christiansted show that during only 43 years was the annual total 45 inches or more, while during the remaining, the totals range down to 29.48 inches (1873) (see text tables 20, 22, and 23). Not only is the yearly total rainfall unfavorable to sugar production but the distribution through- out the year is not dependable. There may be a yearly total of 50 inches or more, enough for a good crop if rightly distributed. But if approximately one-fourth to one-third of this rainfall comes during one month of the year, as it often does (see text tables 3, 6, and 7), due in most cases to hurricane influence, and the balance not always well apportioned during the remaining months, the yearly total has little significance. Although monthly rainfall is often unequally distributed, the data on the average number of days with rain (appendix tables 3-6, and 12) in- dicate that the daily distribution may be favorable. In general, this does not convey the whole truth. Rain may fall frequently throughout the growing season, but after the cane covers the ground it takes a heavy shower to reach the roots and to benefit the plant. Such showers come too infre- quently for ideal crop development, as St. Croix is too small and too low to bring about marked local heat-convectional or orographic rainfall. Wind is another climatic hazard to sugar cane in St. Croix. Tropical hurricanes may come anytime from June through November and some- times damage a growing cane crop or ruin it entirely (see appendix B). These storms not only cause loss of cane, but also inflict serious injury to buildings and the laborers. They are regarded with such dread by the people that special prayers for deliverance are made at the beginning of the hurricane season, and a thanksgiving service is held when the period of danger is past. The constant trade winds also cause a high rate of evapora- tion which is unfavorable to soil-moisture conservation. The mean annual evaporation from a water-filled pan over a period of years (1920-38) shows that the potential evaporation exceeds the mean annual rainfall by about 27 inches (see appendix table 8) ; but the actual evaporation from the land and vegetation is not as great as from a water surface represented by the pan. ■ On the other hand, temperatures and sunshine are favorable for sugar production, low latitude and altitude eliminate frost danger, and the large percentage of possible sunshine favors a high sucrose content (see text tables 1, 17, and appendix tables 9-12, 15). The rainfall map of St. Croix (figure 2) was made from data fur- nished by the Agricultural Experiment Station at Anna's Hope. Rainfall records (appendix table 7) from 26 stations (mostly sugar estates), in- STONE: METEOROLOGY OF THE VIRGIN ISLANDS 51 eluding Christiansted and Fredericksted, for a period of ten years were used in drawing the isohyets. Only one station (Cotton Grove) with but one year's precipitation records was available for the far eastern section, but it is known that eastern St. Croix has a considerably lower rainfall than other parts of the island (see discussion of Vegetation and Climate). Northwestern St. Croix is the rainiest section, and the south has less precipitation. The trades (prevailing direction ENE-ESE) blowing over the northwest upland account for the greater precipitation there. In eastern Text Table 20 Sugar-Cane Yield and Rainfall of St. Croix, 1862-1938 Years 1862-1902 Years 1900-1938 (From St. Croix Gov't: "Statistics concerning sugar production . . .") (From U. S. Dept. Interior: "The Virgin Islands of the U. S.", 1935) Yield Yield Rain- Year Acres *hogshead/ Rainfallt Year Acres short fallt fiscal taxed acre "lines" fiscal taxed tons in. 1862 18,074 0.72 330 1900 16,298 8,614 45 1863 17,535 0.56 272 1901 16,441 15,111 67 1864 17,449 0.38 316 1902 16,428 15,937 61 186S 17,602 0.47 396 1903 15,820 9,419 45 1866 17,475 0.73 371 1904 15,704 11,231 37 1867 17,505 0.60 373 1905 15,194 4,978 53 1868 17,326 0.64 302 1906 15,068 8,541 56 1869 17,276 0.37 382 1907 13,986 7,940 38 1870 17,276 0.54 410 1908 13,550 4,859 45 1871 17,277 0.99 287 1909 14,007 6,444 52 1872 17,221 0.40 255 1910 13,901 7,516 43 1873 17,137 0.40 237 1911 13,710 7,209 45.S 1874 17,089 0.18 385 1912 13,397 4,831 37 1875 16,835 0.61 245 1913 12,744 4,203 39 1876 16,554 0.32 286 1914 11,898 3,653 37.5 1877 16,608 0.30 391 1915 12,474 3,037 65.5 1878 16,576 0.62 454 1916 12,220 15,334 59 1879 16,574 0.53 541 1917 12,627 7,725 39 1880 15,664 0.64 331 1918 12,718 5,841 46 1881 15,980 0.62 464 1919 12,498 9,723 Sl.S 1882 15,963 1.02 354 1920 12,847 13,329 34.5 1883 16,486 0.68 386 1921 11,854 — 30 1884 16,700 0.94 356 1922 9,662 6,345 26.5 1885 16,508 0.91 365 1923 9,014 1,948 33 1886 16,548 0.80 433 1924 9,208 2,385 39 1887 16,511 0.90 383 1925 9,585 10,653 52 1888 16,440 0.81 438 1926 9,196 6,343 39 1889 16,479 1.16 437 1927 9,250 6,860 48.4 1890 16,489 0.98 243 1928 8,240 11,275 43 1891 16,333 0.21 403 1929 8,135 2,825 60 1892 16,404 0.94 282 1930 5,892 - 39 1893 16,587 0.62 448 1931 5,009 1,787 49 1894 16,633 1.22 352 1932 4,686 4,287 70 1895 16,011 0.85 446 1933 4,505 4,125 68 1896 15,974 1.15 448 1934 5,386 4,088 S3 1897 16,149 1.21 439 1935 5,800 1,670 40 1898 16,181 1.04 404 1936 6,277 3,725 68 1899 16,187 1.28 273 1937 5,266 5,749 44 1900 16,298 0.70 364 1938 6,112 4,664 43 1901 16,441 1.23 535 1902 16,428 1.29 491 * Hogshead — 1,500 lbs. t Averages of 2 or 3 stations, as in appendix table 3 and text table 5. 52 SCIENTIFIC SURVEY OF PORTO RICO St. Croix, the elevations are insufficient to produce much orographic rain- fall and the windward situation and narrow land cause fewer and less in- tense convection showers from local insolation than the wider leeward western portions. The TEXT TABLES 20, 21, and 22 show that the years of heavier sugar production generally follow those of increased rainfall. This is not always ■' Text Table 21 Departures of Rainfall and Sugar Yield in Extreme Years from Their Normals for 1881-1904 and 1882-1905, Respectively, in St. Croix (Mean rainfall was 1242 mm. ; mean sugar yield 1202 lbs. per acre) (From Ravn) A. Wet Years B. Dry Years Annual April— November Annual April-November No. days 9 No. days with with Sugar 20 mm. Sugar 2C 1 mm. Rain- yield Rain- or Rain- yield Rain- or fall de- departure fall de- more fall de- departure fall de- more Year parture (next yr.) parture ram Year parture (next yr.) parture ram % % % % % % 1881 + 19 + 8 +31 _ 1882 -IS — 26 -18 10 1886 + 11 — 4 + 6 16 1890 -38 — 76 -54 3 1888 + 12 + 26 + 15 11 1892 -28 -30 —24 7 1889 + 12 + 4 + 18 22 1899 -31 -24 — 26 — 1893 + 14 + 28 + 19 11 1904 — 24 -49 —31 5 189S + 14 + 14 + 25 +32 + 5 + 18 14 1896 15 —27 —41 —31 6 1897 + 12 + 9 + 14 17 1901 +37 + 40 +44 17 1902 + 2S — 6 -f22 - Mean + 17 + 16 + 19 IS Text Table 22 Frequency of Annual Rainfall Totals, 1852-1914 (average of 3 stations), AND Correlated Sugar Yields, 1878-1915, on St. Croix (From Ravn) Sugar yield Tons of cane Class intervals Number of per acre Hogsheads of sugar Inches of rainfall cases (1878-1915) per acre 26- -35 6 4.7 37 35- -45 20 7.0 58 45- -55 27 9.7 87 55- -65 8 10.6 98 65- -70 2 12.2 (97) SrONE: METEOROLOGY OE TEIE FIRCIN LSLANDS 53 the case, for sometimes a large percentage of ratoon cane and an epidemic of plant diseases or insect pests cause a low yield even after years of heavy precipitation. The distribution of rainfall through the year is equally as important as the mean annual rainfall. The data illustrate the erratic na- ture of the rainfall, a characteristic much emphasized in discussions of some of the temperate regions but not always stressed sufficiently for parts of the tropics. The area of production showed a gradual decline from ap- proximately 20,000 acres in 1850 to about 12,000 in 1900. Since then the acreage has decreased rapidly until in 1930 it dropped below 6,000. Text Table 23 Rainfall of the Five Wettest and Five Driest Years, St. Croix, 1852-1914, in Inches (average of 3 stations) (From Ravn) Wettest Driest 1861 58.8 1863 34.0 1879 67.6 1872 32.0 1881 58.0 1873 29.6 1901 66.9 1875 30.6 1902 61.4 1890 30.3 THE UPPER AIR AND GENERAL CIRCULATION IN RE- LATION TO THE WEATHER AND CLIMATE Aerological observations have been made at Bourne Field by the U. S. Marine Corps since 1935, airplane-meteorograph soundings being included since January 1937. The pilot-balloon ascents have reached only several kilometers on the average and have not been summarized. A lon^ series of observations of winds aloft at San Juan, discussed elsewhere,* is suffi- ciently representative of conditions over the Virgin Islands, except for the lowest 500 meters. Some of the most general features of the circulation are mentioned in the note by Palgrave above. More specifically, the troposphere in this re- gion stratifies itself into two primary layers which form two closely-related fundamental branches of the general circulation of the whole earth's at- mosphere, viz., the trades, an easterly current at the surface, and the anti- trades, a westerly current flowing above the trades. Both currents are present throughout the year but they vary seasonally in depth, velocity, di- rection, and steadiness. The average height of the center of the transition zone between the top of the trades and the bottom of the antitrades ranges * stone, R. G. Bull. Am. Met. Soc. 23 (1): 4-15. 1942. 54 SCIENTIFIC SURVEY OF PORTO RICO here from about 4.5 kilometers in winter to 10 or 12 kilometers in summer. The transition is rarely sharp but may be from tens or hundreds of meters to several kilometers thick ; also it varies rapidly in height from day to day about its mean height. Rarely the westerly winds may come down to the very surface, as in the front of approaching hurricanes or with "northers". The trades blow rather constantly from slightly north of, or due, east in winter and from somewhat south of east in summer (see text tables 2 and 3) ; but at the surface other directions appear more frequently than aloft due to the deflection by hills and the effects of showers and squalls, and possibly to land and sea breeze tendencies. The friction of the winds with the earth's surface causes a regular logarithmic diminution in the speed from about five hundred meters down to the ground, and also causes a slight turning of the direction more to the left (looking downwind) which means the surface wind comes from more south of east than aloft, on the average. All wind observations for the lower atmosphere, whether from cloud motions, balloons, or wind vanes and anemometers, must be interpreted with these facts in mind. Appendix table 15 gives a summary of mean resultant motions of lower, middle, and upper-level clouds at 3 observations a day at Bourne Field. The results show the general prevalence of trades and antitrades but there is much variation from year to year and month to month which is not to be taken as entirely real since the clouds are apt to be associated more often with certain directions than others. The monthly resultants from regular daily pilot-balloon ascents at San Juan do not show such large variations as do these cloud observations, and the former are for various reasons probably more representative of the true conditions. Appendix tables 17-20, containing the average temperatures, pres- sures, and humidities at fixed heights up to 5000 meters, are based on air- plane-meteorograph soundings made daily during 1937 and 1938 at Bourne Field. In 1939, 1940, and 1941 soundings were resumed during the hurricane season (July-November) and on special occasions (maneu- vers). The interpretation of individual soundings and their significance for various problems in tropical meteorology would be disproportionately long and out of place here ; but there are several features of the prevailing thermodynamic structure of the free air which should be described briefly because they determine to a large extent the rainfall regime of the islands. A perusal of the mean values of the upper-air humidities shows that the specific humidity is considerably higher in the summer (rainy) season than in winter both at the surface and upper levels ; but the relative humidity variation differs with height, for while aloft it is much higher in summer than winter, in the lowest 2.5 kilometers it is rather uniformly high through STONE: METEOROLOGY OF THE VIRGIN ISLANDS 55 iCj Jooo •o u. Zooo ... --^T_- ■ l\ 1 - ■^ \. - \ \\ -- - JAN \\ _ F.fS'^^^ ^ _ --- MAR \^*^ \ \- 1 1 ^ nX \ -10 O lo 20 3 DEO. C i\ hooo _ \Cv \ _ •> Im 3ooo ^ \ - lu 7.000 — JULY % _ 5 looo - AUG v,^ ■iCPT 1 1 ^ \ \: 30 ^ -10 PEG C Kn i\ 1 \\ _ \ i^ooo Xv \ _ Ul N 3ooo \j. >o ^\, '\' lu Zooo _- OZJ %s \ . \ _ WJ .,-.,. V - iooo _ D£C \^ 1 1 \ ^ ^x X DfG.C 10 DE&.C Figure 3. Average lapse rates over St. Thomas, 1937-38. Temperature in ° C. plotted against height in meters. One dry and one wet adiabat are added for reference, originating at 20° and 30° C. at the surface, respectively. the year. This distribution of relative humidity is graphed in figure 9. Now recalling the annual course of monthly average rainfall totals at sta- tions in the islands (appendix tables 3-7), it is evident that the "rainy season" from May through November coincides with the period during which the humidity in the levels above 3 kilometers is high. Such a relation is not surprising from what is generally understood or assumed about the nature of tropical rainfall processes, for the more active the convection and the further upward it reaches the greater rainfall that may result, other things being equal. The convection both carries moisture aloft with it and in turn is able to penetrate higher by virtue of the resulting increase in humidity (the condensation of which converts latent into kinetic energy). It remains to consider the reason for an increase of convective vigor in the so-called rainy season. A corresponding change in the vertical stability of the air column with season is indicated a priori, and readily verified by an examination* of both the mean monthly lapse rates (vertical tempera- ture gradients) and the lapse rates day by day through the year. Figures 3, 4, and 5 show the mean monthly lapse rates and two soundings of con- trasted types. * The adiabatic charts for each sounding were inspected at Bourne Field and at the U. S. Weather Bureau, Washington. 56 SCIENTIFIC SURVEY OF PORTO RICO Figure 4. Upper-air sounding at St. Thomas, March 23, 1938. Plotted on an emagram (co- ordinates: temperature and logarithm of pressure). The dashed lines are two wet adiabats and the sloping full straight line is a dry adiabat. The right hand curve of the sounding is tempera- ture with specific humidity values written beside the significant points. The left hand curve of the sounding is the wet-bulb temperature, with relative humidity values added for each signifi- cant point. The range of pressure from bottom to top of the chart is from lOSO to 500 millibars. This sounding reached to about 18,000 feet. St. Thomas, Nov. 5. 19*0 Figure S. Upper-air sounding at St. Thomas, Nov. 5, 1940. Plotted similarly to Figure 4. This sounding reached about 14,000 feet. SrONE: METEOROLOGY OF THE VIRGIN ISLANDS 57 The soundings made at St. Thomas were the first extensive series from this part of the suh-tropical Atlantic. From them the writer has noted that the structure of the trades in the Caribbean is similar to that foiuid years ago from soundings in the eastern Atlantic off the Canary Islands and Cape Verde. This structure explains why the whole region of the sub- tropical North Atlantic (or "Azores") anticyclone has relatively little cloudiness and rain, for the moist unstable surface air layers in which the trade-wind cumulus clouds form are constantly overlaid by a deep warm, dry, stable layer (or layers) which generally prevents any convection originating in the surface layers from reaching high enough to produce heavy showers. The formation and maintenance of this upper stable layer are not yet fully understood, except that it undoubtedly has its immediate origin in the subsidence of the antitrades into the core of the anticyclone. The surface moist current is separated from the upper dry current by an inversion of the temperature lapse-rate, or by at least a zone of isothermal lapse rate or of greater stability than below and above. This is called the trade in- version. Presumably in many cases the so-called Ts, or S, inversion often noted above tropical Atlantic air-mass invasions into the United States were originally trade inversions. The St. Thomas soundings show the trade inversion in some degree during most of the year. This is not surprising since the Virgin Islands are nearly always under the southwestern margin of the Azores anticyclone, but what it probably really means is that the inversion is being constantly carried along outwards from where it is formed in the central and eastern parts of the anticyclone. A comparison of soundings made in different parts of the anticyclone shows that the height of the base of the inversion tends to rise in all directions from the center of the anticyclone.* Thus near the Cape Verde Is. it is only about 500 meters above sea level on the average but increases to 1800 meters near the equa- tor and over the Caribbean. The stability, djryness, and thickness of the up- per current tend to decrease as the height of the inversion increases, and vice versa. When the daily soundings at a fixed station, such as St. Thomas, are compared successively it is immediately apparent that the inversion and the air above it vary remarkably in height, thickness, intensity, and other char- acteristics from day to day. Furthermore a closer analysis leaves an im- pression that the day to day changes in cloudiness, rain, and wind are somehow related to the changes in the stability of the air aloft. But to date it has not been possible to find a simple or clear-cut correlation, owing to the complications introduced by local convection and surface influences, * von Ficker, H. Die Passatinversion. Veroffentl. Meteorol. Inst. Univ. Berlin 1, Heft 4, 1936. 58 SCIENTIFIC SURVEY OF PORTO RICO Figure 6. Surface pressure and height of the trade (= "Ts") Inversion, July-November 1938, at St. Thomas. (After Dunn, Bull. Am. Met. Soc. p. 22S, June 1940.) and to inadequate data.* The attempts of Frolov and of Dunn to find clues to this problem through isallobaric analysis should be studied by those interested in further details.f It appears from daily North Atlantic weather maps that the Azores anticyclone fluctuates from day to day in size, intensity (maximum pressure), and in position, which in turn causes some of the day to day changes in the trade inversion at St. Thomas men- tioned above. Note in figure 6, from Dunn, how the surface pressure (Barometer Curve) has a rough tendency to be high when the height of the inversion (Ts Inversion Curve) is low. Dunn found that in the July period (shown on this figure) there were eight major "waves" of pressure change and that in each case the pressure-fall phase of the "wave" was as- sociated with a lifting of the inversion and usually also with an increase of specific humidity and temperature at the 3000-meter level. Hurricanes * See ref. to correlation coefficients computed by Bice, Bull. Am. Met. Soc. 21: 219, also fig. 2 on p. 218. 1940. t Bull. Am. Met. Soc. 21: 216-229. 1940, and 22: 198-210. 1941. STONE: METEOROLOGY OP THE VIRGIN ISLANDS 59 are observed to develop through continued "deepening" of a pressure-fall of this type. Of course, the great majority of these falls do not become hurricanes but remain of small amplitude (.01 to .11 inch) or die out. J. E. Miller and G. Emmons have investigated this phenomenon further.* All these pressure changes studied by Dunn were found by plotting maps of the 24-hour pressure differences which show 48-hour periodicities. The areas of rise and fall on these maps tend to move from east to west at 15 Figure 7. Surface pressure, upper-air temperature, and specific humidity at 3000 meters, July-August 1938, St. Thomas. (After Dunn, Bull. Am. Met. Soc. p. 218, June 1940.) to 25 miles per hour. However, Frolov has studied the 72-hour pressure differences which show six-day periodicities. These do not propagate from east to west but rise and fall in unison at all stations affected by the south- ern half of the Azores anticyclone from Africa to Central America. This fact can only mean that these longer fluctuations are produced by expan- sion and contraction of the entire Azores anticyclone. The shorter-period changes that propagate are superimposed on these longer periods. No cor- relation of Frolov's 6-day period in pressure with variations in the other elements has been made. An inspection of the daily rainfall tables in the "Climatological Data: West Indies and Caribbean Service" suggests that the chief variation of rainfall within a month has a rough period of about a week to 15 days, as a rule. This corresponds to the largest "see-saws" in pressure shown on one of Dunn's figures (figure 7). Apparently these * U. S. Dept. of Commerce, Weather Bureau, "Curso para el Estudio de los Huracanes, Tomo I: Caracteristicas del Tiempo en el Caribe y Metodo de Analisis". Mimeo., Wash., D. C, pp. 65-84. Feb. 1942. J. E. Miller, "The Significance of Allobaric Systems in Forecasting Cyclogenesis in the Carib- bean". MS Thesis. College of Eng., New York Univ. Oct. 1941. 60 SCIENTIFIC SURVEY OF PORTO RICO grosser fluctuations are due to shifts in the position of the Azores anti- cyclone in combination with changes in its size and maximum pressure. But they can be effectively studied only with the aid of weather maps of the whole northern hemisphere, because they are actually a direct manifesta- tion of changes in the intensity of the general circulation of the earth's atmosphere.* The east-to-west moving allobaric systems are apparently prevalent only in the "rainy season" (June-November). In the winter season they move from west to east. The winter systems are of different origin and character than the summer ones. The latter form closed circular areas whereas the former are long troughs that extend south or south west ward from cy- clones of middle latitudes well into the tropics if not often to the very edge of the doldrums. These troughs are originally associated with the cold fronts of polar-air outbreaks across the United States from Canada or the Pacific ; the warm Caribbean waters quickly efface any observable tem- perature discontinuity at the surface before the polar air reaches the Vir- gin Islands, except in the relatively rare cases of "northers." The anti- cyclones that follow the cold fronts often continue southeastward to merge with the west or north side of the Azores High ; such anticyclones are al- ready largely transformed from cold-air domes to warm-air dynamic anti- cyclones by the time they reach the subtropics. The airflow through the preceding trough and the warm ocean surface are then unfavorable to the continued existence of a temperature discontinuity, but the trough remains as a col between the newly developed dynamic anticyclone and the pre- existing one represented by the Azores High. The younger anticyclone does not simply amalgamate with the Azores High but often appears to "grow" and to move eastward at the expense of the older High. In this way the trough between them appears to move eastward across the Atlantic Ocean. A connection of this trough with the northern part of the cold front from which it originally derived may often be maintained during the eastward progress across the ocean, or else with a new secondary (or wave) cy- clone which may have formed on the front when it was off the eastern coast of America. In any case the passage of one of these troughs over the Virgin Islands and the other Antilles usually leads to a noticeable se- quence of changes in the weather : before the trough passes, a slight shift of winds toward the south with increasing warmth, moisture and clouds ; * Cf. Papers in Physical Oceanography and Meteorology. Mass. Inst. Techn. and Woods Hole Oceanogr. Inst. 8, no. 3. 1940; and 9, no. 1. 1941. Rossby indicates that the Azores High shifts north and south with changes of intensity in the zonal circulation of the westerlies (Jn. Mar. Res. 2 CI) : 38-55. 1939). Clayton correlates these shifts with sunspots (Trans. Am. Geophys. Un. 1941 (II): 420-423). STONE: METEOROLOGY 01' THE VIRGIN ISLANDS 61 then as it passes, a shift towards the north with more rain and cooler air ; after which, clearing and drier weather, gradually reverting to average trade conditions. The troughs tend to pass this region about once every 5 to 7 days, and thus the fluctuations which they cause may be considered as part of the "normal" picture for the season. The late Dr. O. L. Fassig in 1909 began to notice the "influence of passing highs and lows to the north" on weather in Porto Rico, but he never published his manuscript notes on the subject.* Only recently has the phenomenon again come to the atten- tion of American meteorologists. f The accompanying changes in the upper air and the mechanism by which the trough affects the clouds and rainfall have not yet been fully investigated. However, it appears that the belts of convergence and divergence produced by a sinusoidal field of flow, as demonstrated by J. Bjerknes.J: could account for all or part of the weather changes by raising and lowering the trade inversion. In addition, the southerly component in the forward side of the trough might import moister air aloft from lower latitudes. The intensity and frequency of troughs should be directly dependent on the character of the meridional interchange of polar and tropical air masses over eastern North America - — indeed, there is some evidence that during colder winters in eastern United States the weather is relatively cool, rainy, and cloudy in the An- tilles. The origin of the allobaric systems of summer is surmised to be in Af- rica ; from May through November systems of similar character parade from east to west across West Africa and out to sea past the Cape Verde Islands. Violent squalls ("tornadoes") are associated with the katallo- baric centers. Although the ship observations are too few to definitely trace the systems eastward to the Antilles, many meteorologists believe that at least they may occasionally develop into hurricanes upon approach- ing the Antilles. Hubert§ indicates a high probability that the great "New England Hurricane" of 1938 had such a history. But the "tornadoes" are always observed to weaken east of the Cape Verde Islands and therefore some process must favor their rejuvenation in the western Atlantic re- gion. || * These were edited by us and will appear in Part 3 of this volume. t See U. S. Dept. of Commerce, Weather Bureau, "Curso para el Estudio de los Huracanes", Tomo I: 57-62. The role of these troughs in the weather of the Canary Islands region was first recognized by Roschkott in "Festschrift der Zentralanst. f. Met. u. Geodynamik in Wien, zur Feier ihres 75 jahr. Best." Vienna, 1926, p. 121 fif. See also Piersig, W., Schwankung Luftdruck und Luftbewegungen. Archiv d. Deut. Seewarte. 54 (6): 1936. + Bjerknes, J., Theorie der aussentropischen Zyklonenbildung. Met. Zeit. 54 (12) : 462-466. 1937. Lt. Geo. P. Cressman, of New York University, has applied Bjerknes' principle to the winter troughs of the Azores High, in an unpublished manuscript, 1942. § Abstract by C. F. Brooks, in Trans. Am. Geophvs. Un. 1940 (II): 251-253; also critique by Portig, Ann. d. Hydrographie, 67 (7): 398-400. 1939. II Regula, H. Druckschwankungen und Tornadoes an der Westkuste von Afrika. Ann. d. Hydrogr., 64: 107-111. 1936; also Piersig, W., op. cit. 62 SCIENTIFIC SURVEY OF PORTO RICO The occurrence of east-to-west allobaric centers and of West African "tornadoes" is limited to the period when the boundary between the trades of the north and south hemispheres is well north of the equator ; it is there- fore a reasonable assumption that the Coriolis force is essential for the maintenance^ of these allobaric systems, though they apparently arise merely from the large diurnal period in temperature, pressure, and wind over subtropical Africa.§ However, we recognize the possibility that the allobaric systems observed in the Antilles may also have a more immediate origin over the ocean to the southeast due to surges of southern hemi- sphere trades pushing into the northern trades ; such surges are observed in the doldrums but it is generally believed that many of them are in turn consequences of the "tornadoes" from West Africa. j| The subdivision of the subtropical high-pressure belts into cells can per- haps be better understood in physical terms as a result of lateral mixing. Rossby and his collaborators* have shown that above the layer of surface influences the subtropical highs break up into a train of large anticyclonic eddies, in a manner called for by the assumption of a jet stream suffering lateral mixing with its environment on a rotating earth. In particular an analysis by Simmers* of a case of warm anticyclogenesis suggests that the lateral-mixing process may explain how the separate highs maintain their identity as they pass across the subtropical Atlantic in winter. In summer, too, anticyclonic eddies prevail aloft over the United States and Gulf of Mexico in the seasonal extension of the Atlantic high pressure belt north- ward and westward.* No studies have been made as yet to discover whether the weak summer allobaric systems are associated with observable eddy tendencies aloft.f We offer the tentative view that the winter cells and troughs of the Azores High are the result of jets of polar air driven into the westerlies, and the summer allobaric systems are the result of jets of equatorial or southwest-monsoon air driven into the trades. The much Ross id: Nouvelles etudes sur la meteorologie de I'Afrique Occidentale FranQaise. Paris, 1926; Fro- lov, S. Variations de la pression en A.O.F. Ann. de phys. du Globe de France d'Outre-mer, 3 (14): 46-50, 57-58. 1936; Weisse, L. and Barberon, J. Note au sujet du grain du 28 au 30 Juin 1933. fitudes met. sur I'Afrique Occidentale Fr., Publ. Comm. d'fitudes Hist. Sci. A.O.F., Ser. B No. 3: 47-61. Paris, 1937. Goualt, J. Vents en altitude a Fort Lamy (Tchad). Ann. Phys. du Globe Fr. d'Outre-mer, 5: 90-91. 1938; Farquharson, J. S. The diurnal variation of wind over tropical Africa. Quart. Jour. Roy. Met. Soc. 65: 165-184. 1939. II Ferraz, J. de S. Hurricanes and South Atlantic circulation. Bull. Am. Met. Soc, 20 (8) : 334-335, 1939; also Regula, H. Schwankungen der Passatgrenzen. Ann. d. Hydrogr. 65 (10): 458-460. 1937; Durst, C. S. The doldrums of the Atlantic. Geophys. Mem. (London, Met. Off.) 28: 1926. • Rossby, C. G. and collaborators. Fluid mechanics applied to the study of atmospheric circula- tions, Papers in Phys. Ocean, and Met., 7 no. 1, 1938. See also Namias, J. Isentropic analysis, in "An introduction to the study of air mass and isentropic analysis". 5th ed.. Am. Met. Soc, 1940, pp. 136-161. t Note, however, the isentropic charts for the hurricane of 1938 by Pierce in U. S. Mon. Wea. Rev. 66: 237-285. 1938, and the mean monthly isentropic charts published regularly in the same journal since 1939. STONE: METEOROLOGY OP THE I'IRCIN LSLANDS 63 larger, more intense development of the winter systems compared to the summer ones reflects the difference in latitude and hence in the respective Coriolis effects. The mean conditions of a month at a station such as St. Thomas are thus the result of a complex of fluctuations of various types and causes. The rainfall is increased by any process which weakens or lifts the inversion, whether it be simple heating from the surface, convergence from a passing pressure-change system, or a shift of the Azores anticyclone northward so that the lesser stability normal to regions nearer the equator spreads to somewhat higher latitudes. The annual march of rainfall in the West In- dies can be explained for the most part by seasonal migrations of the Azores anticyclone (dry) and of the equatorial Low (rainy). The ef- fect of this migration on the inversion over the Virgin Islands is thus a regular weakening from winter into the rainy season (May-November), with a brief partial return to winter-like conditions in July, and a rather rapid intensification of the inversion from November to January. Figure 4 is an example of the temperature-height curve when the trade inversion is very marked over the Virgin Islands region ; the opposite extreme, in which very little stability is present, is exemplified in figure 5. The more typical and frequent conditions are of course intermediate between these rather extreme types. In APPENDIX TABLES 1 7 to 20 are averaged together the 1937 and 1938 soundings month by month and level by level for temperature ( ° C. ) . pres- sure (millibars), and relative humidity (per cent), and for 1937 we have copied the monthly mean specific humidities and equivalent-potential tem- peratures as published. The levels for which the data are given are : sur- face, 500 m, 1000 m, 1500 m, 2000 m, 2500 m, 3000 m, 4000 m, and 5000 m. The mean temperature-height curves are plotted in figure 3 for each month. A wet adiabat is drawn to the right and a dry adiabat to the left on each diagram. The general prevalence of conditional equilibrium (lapse rate between dry and wet adiabatic, i.e., air is stable if unsaturated, but unstable if saturated, as when a cloud has formed) is very striking. Also the great stability, approaching inversion conditions, in the layer from 1000 to 3000 meters is prominent from December to April. The trade in- version is present on so many days that there is almost an inversion in the averages, in spite of the varying height of the inversion and the large in- tervals between the standard levels for which the averages are computed. In order to get a clearer picture of the stratification of thermal stability and instability, we have read oft' the lapse-rate types between each level for each month and tabulated them as follows : 64 SCIENTIFIC SURVEY OF PORTO RICO Types of Mean Lapse Rate above St. Thomas (1937-38) C stands for conditional equilibrium ; W for approximate wet adiabatic equilibrium; S for absolute stability (approaching isothermalcy or in- version) ; and U for absolute instability (dry adiabatic equilibrium) (limits given in meters above sea level) January February March C to 1500 m C to 1500 m C to 1500 m W 1500-2000 m W 1500-2000 m W 1500-2000 m S 2000-2 SOOm S 2000-3000 m S 2000-3000 m w 2500-3000 m C 3000-5000 m W 3000-5000 ra c 3000-5000 m April May June C ( to 1500 U to 500 U to 1000 w 1500-2000 C 500-1500 C 1000-1500 s 2000-2500 W 1500-2000 W 1500-2000 w 2500-4000 C 2000-5000 S 2000-2500 c 4000-5000 C 2500-5000 July August September C to 1500 U to 500 C to 5000 w 1500-2500 C 500-1500 c 2500-4000 W 1500-2500 December w 4000-5000 C 2500-5000 C to 1500 October November S 1500-2500 C 2500-4000 c to 5000 C to SOOO S 4000-5000 From this table a cross-section, figure 8, has been drawn in which the layers or zones of each lapse-rate type are separated by lines, smoothed somewhat to give a more simple schematic appearance and to interpret in a logical way some of the improbable angularities imposed on the data by the limitation of averaging at only a few fixed levels. Various interpretations could be given in this smoothing procedure, depending on the interpreter's preconception of the most probable scheme and on other information that may be available indirectly to corroborate or guide him, assuming of course that the observations were accurately made and reduced in the first place and that the number of observations is sufficient to give normal or typical values. The writer believes that the data for these two years indicate a nearly normal state of the upper air in the region in spite of the short record be- cause the conditions in this tropical maritime region are known to have a relatively small variability. More data would alter details but not the major features. An inspection of the individual soundings served to confirm the interpretation of the mean conditions given in figure 8. The vertical arrows in figure 8 indicate the relative intensities of up- ward convection and resistance thereto from stability aloft (downward arrows). When figure 10, showing the rainfall totals and intensities and temperatures at Bourne Field for the same years as the soundings cover, is compared with figure 8 and figure 9 the close relation of the mean rain- fall to the mean upper-air conditions becomes evident. s a s a g i/l ^ m c-l -:i 'a-s s ° s * s p xi ^ 3 Sot i.HO 0.^ 1=^ §< n ►^^ D.8 : SB - 0.S STONE: METEOROLOGY OP THE VIRGIN ISLANDS 65 The two chief rainfall maxima, in May-June and October-November, characteristic of this region probably have somewhat different causes, if these two years are typical. The June period had a few heavy showers last- ing some hours but many days with only light showers. The autumn period had relatively many more heavy showers but the average intensity was no greater than in the June period (cf. the discussion of rainfall intensi- ties, supra). Although the June maximum is nearly as rainy as the au- tumn one, the stability aloft is still marked in June whereas it is practi- cally absent in the autumn. However, the lowest 1000-meter layer is very unstable in June. It appears therefore that the spring rains are due to heating of the surface by the increasing solar altitude which forces the con- vection to penetrate the trade inversion occasionally with resulting intense showers. The mid-summer rains are apparently of similar character, for the solar altitude is a maximum again in July and instability in the surface layers is high through August. But the autumn rains require less energy at the surface since there is less stability aloft and any small convergence or heating will easily bring a good shower. We note that the second rainfall maximum is not in August, just after the second zenith-sun period, but much later when cooling into winter has already progressed for several months. This is not characteristic of most of the tropics nor of all of the Caribbean region, and therefore calls for some comment. The cooling of the North American continent in autumn is more rapid than the cooling of the Caribbean Sea, so that relatively cool air masses begin to encroach on the Caribbean from the north. There is as a result a tendency for convergence over the Caribbean. The slightest disturbance of this sort to the air column when it is very moist and in conditional equilibrium to high levels is apt to set off ample rains.* This happens more and more frequently towards winter and leads to greater and greater rainfall totals, until ultimately the return of the trade inversion in December chokes off the convection in the upper levels. The suddenness of the decrease in rainfall from Novem- ber to December is characteristic and is obviously the direct consequence of the sudden return of a strong trade inversion in December. At times in autumn a large flat low pressure area hovers over the Carib- bean for weeks, giving rise to almost steady rains over a wide region. The gentle incessant convergence up to perhaps 6 or 10 km under such condi- tions soon destroys any weak stability aloft that may have extended out from the now retracted Azores High. Some years, however, have rela- tively dry autumns when these conditions are presumably reversed, i.e., warmer than usual in North America and the High shifted more south- westward than usual. / * Examples analyzed in Externbrink, H., Kaltlufteinbruche in die Tropen. Archiv d. Deutschen Seewarte, 57, nr. 7. 1937; and Culnan, R. N., in, "Curso para el Estudio de los Huracanes". U. S. Dept. Commerce, Wea. Bur. Tomo 1 : 85-90. mimeo. Wash. 1942. 66 SCIENTIFIC SURVEY OF PORTO RICO Finally, we have to explain the large rainfall in February for the years 1937—38, which does not appear in the longer-period averages for this or other Virgin Islands stations. This abnormal rainfall was associated with an abnormal increase in the humidities aloft (figure 9). The lapse rate in February was very stable ; however, in the course of February 1937 some heavy rains occurred during a few brief periods when convection reached to abnormal heights, which is the explanation for the great difference be- tween the intensity per rainy day and that per rainy hour in that month. The diagram of monthly mean relative humidities plotted against height (figure 9) is very helpful in the interpretation of the lapse rates, in par- ticular for distinguishing the W layers that are stable (dry) from the un- stable cloudy ones (moist). The rapid extension of the high surface hu- midities into the upper levels between April and May is most striking ; as likewise the return to the winter dryness aloft between November and December. The spring transition period, moreover, coincides with the beginning of the seasons for some of the most severe instability phenom- ena, such as hurricanes, high rainfall intensities, hail, and thunderstorms. The question arises here whether the large accumulation of energy in- dicated by the high summer temperatures and humidities of the surface air parcels is sufficient to carry the low-level moisture to high levels by con- vection, penetrating the upper stability, or whether the horizontal advec- tion of air currents aloft from moister source regions (the Doldrums?) does not labilize the upper layers enough to permit the surface air particles to proceed to higher levels by free convection.* This problem cannot be discussed until much more upper-air data from the whole subtropical re- gion become available. It is interesting to note, however, the temporary drop in relative humidity during June and July between 2500 and 5000 meters, an indication of the closer approach of the anticyclonic core. During early winter the humidity reaches the remarkably high average of over 90 per cent in the lowest cloud layers around 800 to 1000 meters, owing to the steep lapse rates from the more rapid cooling into winter of the lands (and the air over it) than of the sea, and to the "lid effect" of the lower trade inversions ; in view of the error of the hair hygrograph there must have been clouds or near-saturation almost every day in these layers. It' is in March that the inversion is most intense, but by that time the fre- quent cold-air invasions from the continent bring cooler and drier trade- wind air (the wind roses on the Pilot Charts show fewer southerly com- ponents in March than at any other time). Hence the low levels are rela- tively driest in March. • See the interesting but speculative discussions by Externbrink, H., Met. Zeitsch. 54: 354-9, 413-17. 1937; and Scofield, E., Bull. Am. Met. Soc, 19: 225. 1938. APPENDIX A NOTES ON THE ORGANIZATION, INSTRUCTION, AND HISTORY OF THE OFFICIAL D. W. I. RAINFALL STATIONS, 1877-1917 According to records and correspondence now deposited in the National Archives, Washington, the Danish West Indies Government early in 1877 acquired twelve rain gages with measuring glasses from the Danish Me- teorological Institute, Copenhagen. Eleven observers in St. Thomas and St. John were given gages and glasses, and a copy of the printed instruc- tions of the Danish Meteorological Institute with amendments in ink indi- cating how to construct the gage supports and to substitute the local units of measurement, "lines," for the European metric ones. Blanks for enter- ing and returning the recorded rainfalls were also sent out. The returns were forwarded to St. Thomas and filed ; a summary or copy was also probably forwarded from St. Thomas to Copenhagen, but except for the years 1877-88 in the "Set. Thomae Tidende," the reports seem never to have been published. DELIVERY OF OFFICIAL RAIN GAGES TO ST. THOMAS AND ST. JOHN, 1877 Drafts of letter "No. 4", below, dated January 6, 1877, were sent in English translation to Th. Stevenson, C. Danielson, Wesselhdft (Smiths Bay), Rev. Warner, and Gottlieb, Esq., and in original Danish to Planter Harthmann, Dr. Miller, and Capt. Leigh (all residents of St. Thomas). According to your kind promise to take charge of a station for measuring the fall of rain on your estate, I hereby beg leave to forward to you a printed copy of 'Direc- tions for the measurement of the atmospheric precipitation (rainfall),' together with 12 monthly lists to be filled up in the course of the year, at the same time requesting that you please send to the Barracks for a rain gage and appurtenances thereto and to cause the same to be placed in a suitable locality on your premises. The lists, after being filled up, are to be sent to Government Office at the end of each month. Respectfully, [unsigned] The National Archives also contains a rough copy of the letter of Janu- ary 6, 1877, "No. 4", to the Country Sheriff at St. John, forwarding 3 rain gages, 3 copies of the printed instructions (see below), and a spare meas- 68 SCIENTIFIC SURVEY OF PORTO RICO uring glass (also monthly forms) ; one gage is for Cruz Bay, one for Adrian, and one for Caroline (with consent of estate owners) ; monthly completed rainfall reports to be sent to the Government, St. Thomas. There is also a rough draft of a letter, dated February 7, 1877, to the Country Sheriff at St. John, forwarding a reserve measuring glass and requests that rain be measured at Cruz Bay with both the old and new gage and reports sent in to St. Thomas. INSTRUCTIONS TO D. W. I. RAINFALL OBSERVERS, 1877 Translated excerpts from "Veiledning til Moaling af Nedbor, Udor- bejdet af Meteorologisk Institut (Copenhagen) 1873" (4 pp.), marked original, D. W. I. copy of which is in the National Archives (translation by Harold Larson). The equipment consists of a supporting stand, a can, and a measuring glass. The stand consists of four legs which are screwed fast at the top on each side of the four-covered frame and lower down are secured to each of the arms of the wooden crossbar [a drawing is entered on the margin of the copy in the National Archives]. The bottom ends of the legs of the stand are buried in the earth. It will be expedient to fasten to each leg a small crosspiece, so that after being covered with earth the framework cannot easily be pulled up or pressed down deeper into the ground. The can is placed in the framework from the top. It consists of a collecting rim at the top, therefrom a funnel and finally a container supplied with a spout (or lip) and a handle. The rim of the collector encircles a definite horizontal area (namely Vio square meter). The original circular form of the aperture must therefore be carefully pre- served. Its position must be horizontal and its height above the surface of the earth 6 feet, whereby the grounding of the framework must be directed. The funnel, in addition to a vent, is also supplied with two (2) other small holes, whereby air in the container can escape. These three (3) holes must always be kept clean. The tightness of the container should be examined into now and then by filling it with water. The spout should always be closed with a stopper in order to prevent the evapora- tion of the rain collected or the flowing in of rain in this manner. Measurement: by the depth of the rainfall is understood the depth to which the fallen rain would stand if it remained lying as an even layer over a level surface of the earth. This depth is given in lines, and the measuring glass into which the can is care- fully emptied, after being removed from the framework is precisely so divided that a quantity of rain caught by the collecting rim, and which makes a depth of rain of 1 line will fill the glass from one figure to the next. The space between the figures is also divided by small marks into 8 parts, so that one can read thereon both lines and eighths. Thus if the rainwater stands at 7 small marks above the figure 4 of the glass, it is read 4% lines. (While being read, the measuring glass must be held precisely vertical.) [NOTE: 1 mm=: 0.3115 of an English line; the Paris line used in Denmark was never used in the Danish West Indies. There are exactly eight English lines to an inch, but approximately 11.3 Paris lines to the English inch. Why the generally obso- lete English line was so widely adopted in the D.W.I, is not known, but possibly it STONE: METEOROLOGY OF THE VIRGIN ISLANDS 69 was introduced by the planters from British countries or by merchants seUing old English measuring devices. — R. G. S.] [The supporting stands for the rain gages, according to marginal sketches and notes on the Archives copy of these "Instructions", were constructed of pitch-pine sticks in a tripodal design, to hold the upper rim of the gage at 36 inches above the ground (the present standard height in many countries). Each stand cost $2.25 for the wood and 40 cents for painting, i.e., $2.65 per stand or $31.80 for the twelve gages. — R. G. 5".] LOVENLUND RAIN GAGE RECALLED, 1884 A rough copy of a request, dated October 12, 1884, to Planter C. Miller, Lovenlund, that the latter return to the Government Secretary the rain gage issued to him at the close of the year, is on file in the National Archives. TRANSFER OF RAIN GAGE FROM EMMAUS TO EDEN, 1892 St. John, Country Sheriff's OfiEice. 9 Mar., 1892. This is to report the removal of the rain gage, hitherto set up in the yard of the Pastor (Jacobs) at Emmaus; in view of Pastor's leaving the rain gauge was today moved to Eden (a parcel of Emmaus), the owner of which. Quarter Officer (Rev. E.) George, has promised to note rainfall and make monthly reports. APPENDIX B LIST OF HURRICANES PASSING OVER OR NEAR THE VIRGIN ISLANDS* Year St. Thomas St. Croix -Year St. Thomas St. Croix 1713 (?) (?) 1871 August 2 1 August 21 1738 August 30 ?) August 30 1876 September 13 September 13 1742 September 28 1881 August 23 1772 August 31-Sept . 1 August 31-Sept. 1 1889 August 16 1773 July 1889 September 3 1775 July 30 1891 August 18 1780 October 13-14 October 13-14 1893 August 16 178S July 25 (?)1894 October 13 1793 August 12 1899 August 7-8 1804 September 3 September 3 1906 September 2 September 2 (?) 1809 September 2 1908 September 10 (?) 1819 September 21 1910 September 6 1825 July 26 1916 July 14 July 14 1827 August 17 August 17 1916 August 22 1827 August 28 August 28 1916 October 9 October 9 1830 August 12 1917 September 21 September 2 1 1835 August 12 August 12 1919 September 3 September 3 1837 July 27 1921 September 9-10 September 9-10 1837 July 31 1924 August 17 August 1 7 1837 August 1-2 August 1-2 1924 August 28-29 August 28 1837 August 15 August 15 1928 September 12-13 1848 August 29-30 August 29-30 1931 September 10 1851 August 18 1932 September 26 1852 September 24 September 24 1933 July 26 1859 September 2 1938 August 8 August 8 1867 October 29 October 29 1940 August 5 August 5 * Compiled by R. G. Stone from the works of Garriott, Reid, Poey, Redfield, Alexander, Fassig, Mitchell, and histories of the Virgin Islands. Since 1900 the U. S. Mon. Wea. Rev. gives complete re- ports of storms. In later years (1900 — ) this list includes weaker disturbances which caused some rain, wind, or swell. Tally by Months, 1738-1941 June July August September October November Total St. Thomas St. Croix Both islands (no duplication) 18 15 22 39 33 51 ACCOUNTS OF SOME VIRGIN ISLANDS HURRICANES The following are quoted for convenience ; see parts on Porto Rican meteorology and references in bibliography for further information. HURRICANE OF AUGUST-SEPTEMBER 1772 During the last days of August and the first days of September a hurricane passed over the West Indies causing frightful havoc among the Leeward Islands. The disas- trous effects of this storm were felt nowhere more forcibly than at Santa Cruz where, it is said, the sea rose 72 feet above its usual height, carrying every ship at the island STONE: METEOROLOGY OF THE VIRGIN ISLANDS 71 on shore, some as far as 100 yards inland. Large stones were brought down from the mountains, and there was a terrific electrical display. Four hundred and sixty houses were thrown down at Christianstadts, and all but three at Fredericstadt. The maga- zines and stores were quite ruined. The total damage [in the W. I.] was estimated at $5,000,000. The damage at St. Thomas was placed at $200,000. (Garriott, Bull. H, U. S. Wea. Bur. 1900.) HURRICANE OF AUGUST 31, 1772 Excerpt from Alexander Hamilton's letter to his father describing the St. Croix hurricane of August 31, 1772. St. Croix September 6, 1772 Honored Sir : I take up my pen, just to give you an imperfect account of one of the most dreadful hurricanes that memory or any records whatever can trace, which happened here on the 31st ultimo at night. It began about dusk, at north, and raged very violently till ten o'clock — then en- sued a sudden and unexpected interval, which lasted about an hour. Meanwhile the wind was shifting round to the southwest point, from whence it returned with re- doubled fury and continued till nearly three in the morning. Good God ! What horror and destruction — it is impossible for me to describe — or you to form any idea of it. It seemed as if a total dissolution of nature was taking place. The roaring of the sea and wind — fiery meteors flying about in the air — the prodigious glare of almost perpetual lightning — the crash of falling houses — and the ear-piercing shrieks of the distressed were sufficient to strike astonishment into Angels. A great part of the buildings throughout the island are leveled to the grounds — almost all the rest very much shattered — several persons killed and numbers utterly ruined — whole families wandering about the streets, unknowing where to find a place of shelter — the sick exposed to the keenness of water and air — without a bed to lie upon — or a dry cov- ering to their bodies — and our harbors entirely bare. In a word, misery, in its most hideous shapes, spread over the whole face of the country. A strong smell of gunpow- der added somewhat to the terrors of the night ; and it was observed that the rain was exceedingly salt. Indeed the water is so brackish and full of sulphur that there is hardly any drinking it. . . . (Alexander Hamilton) HURRICANE OF 1830 This storm or hurricane was severe at the Island of St. Thomas, on the night be- tween the 12th and 13th of August, 1830. (Reid 1838.) HURRICANE OF JULY 26 TO 27, 1837 Copy of a manuscript report at Lloyd's, dated St. Croix (Reid 1838). About midnight on Wednesday, the 26th of July, it came on to blow smartly from the east-south-east, shifting by Thursday morning, the 27th of July, to south-east, blowing a gale of wind until towards noon, when it began to moderate. (Signed) Andrew Lang. 72 SCIENTIFIC SURVEY OF PORTO RICO STORM OF AUGUST 2 TO 3, 1837, AT ST. THOMAS A. Observations quoted by DeBooy : Hour Pressure (mm) Wind Direction and Remarks 1 P.M. 757 2 755 .... 3 754 3:45 753.3 n"' pressure began to drop rapidly 4 752 N strong gusts 5 747.5 NE storm increasing 6 744.5 NE hurricane force 6:30 740 NW « II 6:45 731 NW II II 7:05 731 NW " " 7:15 726 NW " " 7:25 718 NW " " 7:30 715 NW " " 7:35 713 calm 7:45 712 calm 8:00 712 calm 8:15 711.8 calm 8:20 712 S hurricane again 8:30 721 SSE " " 8:45 728.5 SE (since 8: 35) 9:10 731 SE hurricane 9:15 736 SE " 9:20 740 SE " 9:30 744.5 SE << 9:50 746.5 SE II 10:10 749.3 SE " 10:40 751 SE " 11:15 751.5 SE " 11:30 752 SE II 12:00 753 1:00 754 2:00 755 3:00 756 STONE: METEOROLOGY OF THE VIRGIN ISLANDS 73 B. Observations of Mr. Hoskiaer (quoted by Dove 1841, and Reid 1838) : Barometric Pressure (in "lines"; Hour 1 line = 2.16 mm) Wind Aug. 1; 18h Om 337 lines Aug. 2; 2h 10m 335 NW' 1 3 20 334 N 3 4 45 45 334 332 N N increasing tempest S 40 331.5 NE J 5 45 330 NE 1 6 30 328 NW 6 35 325.5 NW 6 45 324 324 NW NW f- hurricane 10 322 NW 22 318.5 NW 30 317 NW J 35 316.5 Dead calm ] 52 316 " ' , I calm center 8 10 316 * ' ( 8 20 316 " ' 1 8 23 320 SSE 8 33 321 SE 8 38 322 SE 8 45 323 SE ' 8 50 324 SE 9 326 SE 9 10 328 SE hurricane 9 25 329 SE 9 35 330 SE 9 50 331 SE 10 10 332 SE 10 35 333 SE 11 10 333.25 SE 11 30 333.5 SE 14 45 335 SE 20 336.5 SW 21 336.75 E C. Extract from the log-book of the brig, "Water- Witch", W. Newby, Commander, from Liverpool to St. Thomas (kept by the mate), made by Mr. Gilbert Ker, consignee of the vessel (in Nautical Time) : Hour Course Wind Remarks on board, Wednesday, Aug. 2, 1837 P.M. 2 4 6 8 10 12 A.M. 2 4 6 8 10 N.W.b.W. N.E. P.M. Fresh breezes and clear; people em- ployed variously; made the island of St. Kitts; in lower and all lee stud- N.W.J^W. ding-sails. At 2, made the island of Saba. At dusk, in all studding-sails, Saba bearing N.N.E.; and Eustatia E.N.E.; at 8, in flying jib and royals; midnight, fresh breezes and cloudy; in top-gal- lant-sails. A.M. Do. weather. At 7, made the island of St. John's, and shortly after that of St. Thomas. Noon, squally; double reefed the top- sails, and stowed the jib; the town in sight. 74 SCIENTIFIC SURVEY OF PORTO RICO D. Extract of a letter from Captain Newby, of the British brig, "Water- Witch", from Liverpool to St. Thomas, and which left Liverpool, July 19, 1837. Arrived ofif St. Thomas on the 2nd of August ; morning squally, and the Water- Witch was off St. John's, and standing for St. Thomas's, the wind north and north- north-west. Noon, shipping in the harbour visible ; at 1 p.m. squalls violent ; at 3 p.m. we had beat up within half a mile of the forts, when we could proceed no further for the violence of the squalls, and anchored in ten fathoms water ; sent down top-gallant- yards, &c ; did not suspect a hurricane. At 5 p.m. squalls ceased and began a heavy gale of wind, at that time off the land. At 7 p.m. a hurricane beyond all description dreadful ; the windlas capsized, and I could not slip my cables, ship driving until I was in twenty fathoms water ; a calm then succeeded for about ten minutes, and then, in the most tremendous unearthly screech I ever heard, it recommenced from the south and south- west ; I now considered it all over with us, for the wind was directly on shore, and the sea rose and ran mountains high. The foretop-gallant-mast (though struck) and the gig were carried up some feet in the air, and the vessel drove again into twelve fath- oms. We were obliged to steer her all night, and keep her head to wind, for when she got her bows to it she went down on her broadside. At 2 p.m. the gale abated some- what, and the barometer rose an inch ; at daylight, out of forty vessels, the Water- Witch and one other were the only two not sunk, ashore, or capsized. E. Extract from the log of H. M. S. "Spey". Sunday, August 6, 1837 a.m. Arrived at Tortola. Here the hurricane has destroyed the town and several plantations. One brig from St. John's, with a great number of small craft, total wrecks. P.M. at 2 : 30. Came to an anchor in St. Thomas's harbour and landed the mails. Here the hurricane of the 2nd appeared to have concentrated all its power, force, and fury; for the harbour and town were a scene that baffles all description. Thirty-six ships and vessels totally wrecked all around the harbour, among which about a dozen had sunk or capsized at their anchors ; some rode it out by cutting away their masts, and upwards of 100 seamen drowned ; but what was very extraordinary, there was not one English vessel in the port. The harbour is so chocked up with wreck and sunken ves- sels, that it is difficult to pick out a berth for a ship to anchor. The destructive powers of this hurricane will never be forgotten. Some houses were turned regularly bottom up. One large well-built house was carried by the force of the wind from off its foun- dation, and now stands upright in the middle of the street. The fort at the entrance of the harbour is levelled with the foundation, and the 24-pounders thrown down : it looks as if it had been battered to pieces by cannon-shot. In the midst of the hurricane shocks of earthquake were felt ; and to complete this awful visitation, a fire broke out in the back stores of Messrs. Stubbs and Co. Heavy tiles were flying about from the tops of the shaking and trembling houses, killing and wounding many persons. One fine American ship, 500 tons, was driven on shore under the citadel, and in an hour nothing could be seen of her but a few timbers. Several fine merchant ships and brigs are at anchor, dismantled, with cargoes; and not a spar or rope for standing rigging to be had in the island. No place hitherto has suffered so much from a hurricane in all the West Indies as St. Thomas's. Thank God we escaped so well out of it. (Signed) R. B. James F. Extract of a letter from Lloyd's Correspondent, dated at Santa Cruz. On Monday, 31st July, 1837, the weather was moderate; several ships sailed on Tuesday, the 1st of August; in the evening the wind was north-east and the weather SrONE: METEOROLOGY OF THE VIRGIN ISLANDS 75 moderate. On Wednesday the 2nd, the wind during the night had sliifted to the north ; the weather looked squally, cloudy, and suspicious, and continued so during the fore- noon ; the wind shifted gradually to the north-north-west. At 1 P.M. the falling of the barometer, the appearance of the weather, and the in- creasing wind, left us no doubt of the approaching storm, and it came on from the north-west, between 3 and 4 p.m. The mercury continued falling, and the gale in- creasing until half-past 6 p.m. when the wind became westerly. At 7 p.m. the mercury began slowly to ascend, but yet the storm increased in violence. At 8 p.m. it was blow- ing a hurricane from west-south-west to the south-west, coming in furious gusts until 10 p.m., when a certain decrease in their violence had taken place, which abatement continued until Thursday morning, the 3rd of August, when it blew a fresh gale from the south. (Signed) Andrew Lang HURRICANE OF AUGUST 1837, AT TORTOLA The brig, Jane, of St. John's, N.B., was driven on shore during the gale on the 2nd of August. (Tortola, August 6.) At Tortola the hurricane commenced at 3 p.m., and increased in violence until 9 p.m., when it began to abate. (Reid 1838.) HURRICANE OF SEPTEMBER 2, 1859 On September 2, a hurricane of rather mild form passed over St. Kitts, that is the center passed right over the island. It wrecked a number of boats besides other dam- age. It passed over St. Croix after leaving St. Kitts. (Garriott: 1900. op. cit.) HURRICANE OF OCTOBER 29, 1867 On the 29th October, 1867, at 7h a.m., the weather presented no unusual appearance. There had been several showers of rain during the previous night. At llh A.M., the weather became very threatening, the barometer falling slowly, and the wind blew occasionally in hard puffs from the N.W. At noon the wind had veered to the W.N.W., and began to blow furiously. At 7h a.m., the barometer indicated a pressure of 29.76 inches and it fell slowly and gradually until noon when the reading was 29.64 inches. After the reading at noon it fell very rapidly until Ih 30m p.m., when the reading was 28.50 inches. It remained stationary at this point until 2h p.m. when it began to rise rapidly, and at 6h p.m. had reached 29.65 inches. The wind blew very hard, capsizing or dismasting most of the small vessels in the outer harbor, until Ih 30m p.m. when a lull or calm occurred, lasting thirty minutes. At the expiration of the calm the wind commenced to blow from the S.S.E., the oppo- site direction from which it blew immediately before the calm, with greatly increased fury and power, and rain fell in torrents. Previous to the calm only the lighter vessels in the harbor had suffered, but as the wind increased the larger vessels were either driven from their anchors and swamped, or stove on the rocks and piers, or were sunk at their moorings. Very few rode out the storm, but of those that did the majority depended upon their anchors. The fury of the storm, after the lull, lasted only about one hour, but in that short time a fearful amount of destruction was accomplished. Between sixty and seventy vessels were driven ashore or sunk at St. Thomas and vicinity, and many others were entirely dismasted or otherwise damaged. Nearly all the small craft in the harbor were lost early in the storm, and there are no means of ascertaining the number of vessels of that class or the number of lives lost in them. On shore the destruction was nearly equal to that in the harbor. Many houses 76 SCIENTIFIC SURVEY OF PORTO RICO were unroofed, many toppled from their foundations, and others scattered in a con- fused mass of rubbish. Vegetation was Hterally scathed. A metalHc diving bell weigh- ing several tons was carried by the force of the wind a considerable distance. The Gas Works were completely destroyed. The large gas holder was rent outward near the top, in an aperture about eight inches in diameter. It has been estimated that at and about St. Thomas the value of the property de- stroyed was between four and five millions of dollars, and that at least five hundred lives were lost. More than one hundred and twenty bodies were recovered and buried. The thermometer was not observed during the progress of the storm, but it is affirmed by Mr. T. H. Jahnecke, who furnished the following data from his barometrical ob- servations on the 29th, that the temperature did not decrease more than 4° during the storm. Atmospheric Pressure at St. Thomas 29th October, 1867 Hour Pressure Hour Pressure 7 A.M. 29.76 8 " 29.75 9 " 29.73 10 " 29.72 11 " 29.69 12 M. 29.64 1 P.M. 28.86 m. 1.30 P.M. 28.50 2 28.50 3 28.90 4 29.20 S 29.48 6 29.65 The following list [of 79 ships, not reproduced] exhibits a statement of the casual- ties to vessels at and near St. Thomas during the storm. . . . T. H. Jahnecke estimates the velocity of the wind at St. Thomas at 74 miles per hour. ... At St. Croix the wind was N.W., W., and S.W. ... it is concluded that, at the time the storm passed St. Thomas the diameter of the destructive portion was about thirty-four miles . . . , the diameter of the central calm was about 7.5 miles. . . . (Eastman 1868: 8-9, 13-14.) HURRICANE AT SAINT THOMAS, W. I., AUGUST 21, 1871 The Island of St. Thomas, W. I., again claims public notice, and sympathy. On Monday the 21st instant, a terrific Hurricane burst over the Island, causing more suf- fering and misery among the poorer class of the Inhabitants than was experienced in the Tornado of 1867. Since Sunday evening the weather bore a threatening aspect, the wind blowing in strong puffs from N.N.E. to N.E., which continued all night until Monday morning, when the squalls increased in violence, and the Barometer indicated 29.90. By 9 a.m., the weather gradually became more threatening, Barometer steadily falling. At 1 p.m. the Quicksilver was down to 29 (in.) and the gusts of wind increased in fury, blowing from N.N.E. ; the Hurricane had now complete charge, heavy squalls continuing until 3.30 p.m., when the wind veered to the North, and came down over the hills in fearful whirlwinds, shifting from N.N.E. to N.N.W. From this time to 5 o'clock the worst part of the Hurricane was experienced, and nearly all the damage to the eastern part of the town was effected then. The first part of the Hurricane was now over, and a calm of about 30 minutes succeeded, as if the elements were prepar- ing for a greater outbreak. At 5 p.m., the Barometer had reached its lowest point about 28.10 to 28, the wind now changed to S.W. to S. and blew heavily, but the Mer- cury commenced to rise gradually and the Hurricane passed away to the Westward. The extent of damage which has been done in the Island has been very great ; the eastern part of the town has suffered most, and presents a desolate spectacle as the STONE: METEOROLOGY OF THE VIRGIN ISLANDS 77 Hurricane seems to have expended its utmost fury in that quarter, — in some streets only a couple of houses are standing, which are badly injured, whole squares of houses have been blown down, and the ruins lie jumbled together — the Military Hospital had the roof blown off ; the Roman Catholic Chapel in Cocoanut Square, is in a ruinous state. The French Masonic Lodge, "Les Coeurs Sinceres," No. 141, was com- pletely blown away, leaving only the walls of the foundation standing, but owing to the activity of a large number of the Members, the greater part of the valuables of this Lodge have since been saved from the debris, foremost amongst which, we may record the library and Archives. The residences of the Dutch Consul, and of the acting French Consul, have been unroofed, and otherwise badly injured. At Madam Bjerge, only two or three houses are standing, — this quarter, occupied by the poorer class, suffered much. In fact the effects of this visitation are dreadful, — in 1867, the damage was more general, and not as concentrated as it has been in this instance — the lower part of the town, the "Back-of-all," has suffered likewise, but not as much as the eastern part already alluded to. In the harbour the loss has been trifling, in conse- quence of the small number of vessels in port. The American steamer "Florida," cut away both masts, to ride out the gale. The French steamer "Sonora," was run ashore, after parting from her moorings. The steam tug "Vice-Governor Berg" stranded at the "Haul-over," and a couple of other small craft sustained some slight injury. The British barque "Duke of Wellington," with cargo sugars in hhds., lying off Prince Rupert's Rock, parted her cables, and drifted towards Water Island, where the crew was safely landed. The vessel was then blown about the Triangle rocks to the East of the entrance to the harbour, where she was wrecked. The British barque "Jane Lamb" and Danish brig "Axelstad" col- lided, sustaining some damage. But what is most distressing, is the loss of life and personal injuries sustained by many unfortunates, in the first blow ; many of the poorer people living in the vicinity of the parade ground and other streets were crushed by the falling houses, — in one house five persons were killed. By the Police Reports we learn that thirty bodies have been discovered and buried, and that about seventy are maimed. The Police Department acted with zeal and activity in rescuing the wounded, and rendering assistance to the destitute ; during Monday night and Tuesday morning the Constables were indefatigable in their efforts to do good to the sufferers, and the thanks of this Community is due to them, and the Military, for the promptness with which the streets have been cleared, and broken houses removed, thereby precluding theft, and permitting the necessary sanitary measures to be taken, as regards decom- posing matter. Our Colonial Council convened a meeting on the 22nd instant, and voted funds for the relief of the sufferers, and many of those who lost everything, and might under the circumstances have been without a morsel of food, have been at least placed above want by the promptitude of our Representatives. The Mercantile Community, with their accustomed liberality have nobly answered the appeal, and given freely, — the place is seldom invoked in vain, as unfortunately the visitations of past years has too well proved. The distress and suffering have been very great, but it is hoped that the worst has passed, and that we may be preserved from further calamities. — Printed by A. Wal- loe, St. Thomas, W. I. (Reprint from the "Set. Thomae Tidende", Aug. 23, 1871.) STORM OF SEPTEMBER 12, 1876 Early in the afternoon of the 12th a telegram was received (at San Juan, Porto Rico) from St. Thomas announcing that the barometer was falling rapidly there and that, according to information from the island of St. Kitts, a fierce storm was raging there, with signs of being a hurricane. 78 SCIENTIFIC SURVEY OF PORTO RICO Data St. Thomas Vieques Extent of barometric oscillation: 16 mm Hg 16 mm Changes in wind direction: , NE, E, NE NNE, NE, calm, SE, SSE Hours of greatest intensity: 4 a.m., 13th 6 a.m., 13th Thus the center of the storm passed S of St. Thomas, over St. Croix and Vieques. It traveled about 35 km per hr. Three ships at St. Croix and seven at St. Thomas were sunk or wrecked. (U. S. Mon. Wea. Rev. Suppl. 24, pp. 35-38.) HURRICANE OF AUGUST 22 TO 26, 1881 The center of the storm was reported at St. Thomas at 3 a.m., August 23, and at Turks Island, W. I., 3 p.m. on the 24th. (Garriott : 1900. op. cit.) ST. THOMAS-HATTERAS HURRICANE OF SEPTEMBER 3 TO 12, 1889 From the reports of vessels sailing to the eastward of the Windward Islands, this storm is thought to have been central September 1 in latitude north 14° and longitude west 57°. Passed over St. Christopher during the night of the 2d; barometer 29.50 wind northeast. The storm was central near St. Thomas on the 3d, and near to and north of Puerto Rico on the 4th while on the 5th it had moved about two degrees in a northwesterly curve away from Puerto Rico. (Garriott: 1900. op. cit.) HURRICANE OF OCTOBER 7 TO 12, 1916 After a few days' respite another disturbance appeared in the vicinity of the Island of Dominica, the barometer at 8 a.m. October 7 reading 29.84 inches with calm air and rain falling. Notification was sent at once to the Windward Islands and to the United States Naval Radio Service, and special observations called for. Nothing of value was received during the day, but on the morning of the 8th it was apparent that the disturbance was near and east of Puerto Rico and moving northwestward or northward. Advices to this efifect were sent to West Indian points and to Weather Bureau stations on the Atlantic and Gulf coast and broadcast by United States Naval Radio. No further reports were received until the morning of the 9th when the regu- lar reports showed the storm to be still east of Puerto Rico. Special evening reports afforded the first definite information as to the location of the storm center. These re- ports were to the effect that the storm center had passed over the Danish West Indies, Santa Cruz reporting pressure of 29.42 inches at 3 p.m. with a gale, and St. Thomas 29.26 inches at 6 p.m. This information was immediately given wide distribution over the wate'r and along the coasts and shipping warned to exercise great caution. By the morning of the 10th the storm had recurved slightly and had passed to the northeast- ward of Puerto Rico. Nothing further was heard from the storm until the captain of the barque Bellas reported by mail that he had encountered it, in the form of a severe hurricane with southeast to southwest gales, on October 12, in latitude 27° 40' N., longitude 62° 20' W. (U. S. Mon. Wea. Rev. 44: 583-584. October 1916.) The "Set. Thomae Tidende" for Oetober 14, 1916 remarked that the toppling of the coal crane at St. Thomas gave the best indication of the strength of the wind in the hurricane of October 9, since the crane was built to withstand 180 km/hr. STONE: METEOROLOGY OF THE VIRGIN ISLANDS 79 The Rainfalls Recorded at Some Stations in This Storm Were as Follows: Barracks: Oct. 9 61 lines Sekretariat: Oct. 9 30 lines " Oct. 10 25 lines Cruz Bay : Oct. 10 60 lines, estimated, as gage was blown down during storm. Susannaberg: Oct. 10 59 lines Pressures during the Virgin Islands Hurricane of October 9, 1916* Time Pressure Time Pressure Time Pressure 8:00 a.m. 29.80 4:40 28.99 6:10 28.70 12:00 M. 29.65 4:55 28.95 6:15 28.65 2 : 00 P.M. 29.50 5:00 28.90 6:40 28.73 2:50 29.45 5:15 28.75 6:50 28.70 3:00 29.35 5:20 28.70 7:10 28.80 3:15 29.30 5:25 28.75 7:15 28.85 3:30 29.15 5:45 28.70t 7:18 28.90 3:55 29.07 S:SS 28.68 7:25 28.95 4:30 29.05 5:57 28.65 7:30 29.00 * Observations by Miss Emily Quin, Christiansted, St. Croi.x. t A calm existed from 5: 25 p.m. to 6: 45 p.m. when the wind, which prior to 5: 25 had been blowing from ENE, returned in great force from the SW. STORM OF SEPTEMBER 3 TO 10, 1921 St. Thomas, September 11, 1921. Dear Sir : — From September 3rd to September 8th, 1921, light variable winds and calms, clear weather, high visibility, St. Croix 40 miles away seemed quite near and distinct. Max. temperatures 90° and 91° and very oppressive. Bright sunsets, red and copper colored, pressure during these days about .05 below normal. Sept. 8th. Small patches Cirrus and Cirrus-Stratus appeared in the sky from NE to SE. Pressure slightly below normal with a downward tendency. At 1 p.m. received first storm warning from Central Station at San Juan, Weather fine, light variable winds. Sept. 9th. At 8 a.m. the sea that had hitherto been very smooth, began to show signs of a swell rolling in from SE. At noon sky began to get overcast, about 6.10 Ci. and St. with patches of scud from E. At 6 p.m. swell increased considerably breaking heavily on south coast. Wind from ENE to E about 18 miles per hour, sometimes shifting to SE in gusts. Pressure 29.82, temp. 90°. Sept. 10th. At 3 A.M. tremendous swell from S breaking across mouth of harbor. Brisk E to SE wind, velocity 20 miles. Between 10 a.m. and 2 p.m. wind and sea at its height. Max. vel. 40 miles. At 6 p.m. signs of swell abating and coming from SW. Overcast. Sept. 11th. 6 A.M. Sea considerably smoother, wind SE. 20 miles. Cloudy. Presssure 29.84. At 4 P.M. marked improvement in weather generally. Note : At no time during the passing of this storm was the barometer below 29.80. Precipitation was practically nil. — Wm. O. Simmons (Acting Special Observer, U. S. Weather Bureau). (From letter to Dr. O. L. Fassig, then in charge, U. S. W. B., San Juan.) HURRICANES OF AUGUST 17 AND 28, 1924 During the latter half of the month two tropical disturbances reached the Lesser Antilles from the region to the eastward. The first of these was centered between 80 SCIENTIFIC SURVEY OF PORTO RICO Dominica and St. Lucia the morning of tlie 17th and the second a short distance northeast of Dominica the evening of the 27th. The tracks of these two disturbances were ahnost directly northwestward from the Lesser Antilles, the first at the rate of approximately 270 miles and the other at 200 miles a day. The former continued to move in a northwesterly direction until it reached a latitude 28° N and longitude 75° W. It then moved slowly in a westerly direction for 48 hours, after which it turned abruptly and moved north-northeastward with rapidly increasing speed and its course gradually changed toward the northeast. The first tropical disturbance was of only slight intensity in the region of the Lesser Antilles and of moderate intensity when its center passed between Puerto Rico and the Virgin Islands during the evening of the 18th. It increased gradually, however, both in intensity and size after passing to the north of Puerto Rico and within three days, when its center was in about latitude 27° 36' N and longitude 74° 30' W, the winds near the center had increased to hurricane force. The second tropical disturbance evidently developed much farther east than the first, inasmuch as it was already a storm of considerable intensity when it appeared near Dominica on the 27th. By the time it reached the Virgin Islands it had attained hurri- cane intensity. The barometer fell to 29 inches at St. Thomas at 3 a.m. of the 29th and great damage was done by the storm in these islands. A number of lives were lost, hundreds of houses were destroyed and thousands damaged, and much damage was done to crops. So great were the losses in the Virgin Islands that appeal was made to the American Red Cross for substantial aid. (U. S. Mon. Wea. Rev. 52: 410-411. August 1924.) A tropical disturbance at the beginning of September was central about latitude 25° N and longitude 70° W. The history and subsequent movement of this disturbance is discussed in the Monthly Weather Review for August [see above "Hurricane of August 17, 1924"]. Later information which has just come to hand indicates that the center of this storm passed between Antigua and Montserrat (Lesser Antilles) at 3 : 30 a.m. of the 28th. At 2 A.M. of the 29th the center with a reading of 28.56 inches passed over the eastern end of the island of St. John. The western end of the island of Tortola ex- perienced hurricane winds from 6 p.m. of the 28th to 6 a.m. of the 29th. The storm was accompanied by torrential rains and by winds estimated about 100 to 110 miles an hour. . . . serious damage resulted to crops along the path of the storm from Mont- serrat to St. Thomas. The observer at St. Thomas estimated the wind at 110 miles per hour from the north-northeast between midnight and 2 a.m. of the 29th. Esti- mates of 100 to 110 miles per hour were also made at Montserrat and Antigua between 3 a.m. and 4 a.m. of the 28th. (U. S. Mon. Wea. Rev. 52: 464. Sept. 1924.) Notes from the Daily Observation Book, San Jnan, U. S. Weather Bu- reau, August 27-29. 1924. August 27, 1924: Noon special observations were called for from all special meteorological stations, except- Santo Domingo and Puerto Plata. Special reports were also requested from Antigua, St. Croix and Martinique. Request for vessel reports was made through the Naval Radio Stations and several vessels reported giving their position, barometer reading, wind direction and force, etc. The following advisories were issued : Advisory 2 p.m. — Moderate disturbance over the Virgin Islands at noon to-day, apparently moving north west. Lowest barometer 29.82 inches at St. Thomas. The center will probably pass north of Puerto Rico tonight or Tuesday morning. No high winds expected over Puerto Rico. — Fassig. STONE: METEOROLOGY Of THE VIRGIN ISLANDS 81 Advisory 6 p.m. — Unsettled weather continued to prevail in the vicinity of the Virgin Islands. The disturbance is of moderate intensity and is moving very slowly northwestward and apparently not increasing. No high winds are expected over the Virgin Islands or Puerto Rico. — Fassig. The above advisories were duly disseminated. Cirrus clouds were observed moving from the southeast at 6 : 30 p.m. San Juan, August 27, 1924. Special observations were called for at 7 p.m. from St. Croix and Antigua. Based on p.m. reports the following advisory was issued: Advisory 8 p.m. — Indications of a disturbance of moderate intensity apparently centered near Dominica, Windward Islands. The disturbance will probably move slowly westward with moderate to strong northerly winds and rain over the Virgin Islands and Puerto Rico tonight and Thursday morning. — Fassig. The above advisory was duly disseminated. August 28, 1924: Noon specials were called for from all regular stations except Port of Spain, Santo Domingo and Puerto Plata. Special observations were called for at 8 a.m. and noon from St. Croix, Antigua, and Granada. Following hurricane warning issued for Vir- gin Islands : Observer St. Thomas. Hoist hurricane w^arning 10 a.m. Storm central over St. Kitts with barometer 29.56 inches. The storm will move slowly northwest- ward over the Virgin Islands accompanied by moderate to high northeast winds and rain today, changing to southerly winds to-night or Friday morning. • — ■ Fassig. Following advisories were issued : Advisory 10 a.m. The storm increased in intensity during the night and moved northwestward from Dominica to St. Kitts accompanied by moderate to high north- east winds and rain throughout the Virgin and Windward Islands. The lowest ba- rometer reported was 29.56 inches at St. Kitts. The storm will move slowly north- westward over the Virgin Islands and Puerto Rico with moderate to high northeast winds and rain today changing to southerly winds during tonight or Friday morning. — Fassig. Advisory at 3 p.m. At noon today the storm center was between St. Thomas and St. Kitts and moving slowly northwestward. The lowest barometer reading reported was 29.74 inches at St. Kitts. Moderate to high northeast winds and rain will occur this afternoon or tonight over the Virgin Islands and over Porto Rico, especially the eastern portion, changing to southerly Friday morning. Caution advised shipping in- terests. — Fassig. The above warning and advisories were duly disseminated. August 29, 1924: Advisory 9 a. jr. Storm center passed north side St. Thomas at 3:00 a.m. with northeast gales shifting through north to southwest by 7 a.m. Lowest barometer 29.00 inches reported by U. S. Tug Grebe in St. Thomas Harbor. Storm center is apparently moving slowly northwest. Moderate to strong southerly winds will prevail over the Virgin Islands today and moderate northerly shifting to southerly over Puerto Rico. — Fassig. The above advisory was duly disseminated. (From Notes in Daily record U. S. Wea. Bur. San Juan.) APPENDIX TABLES P^ H < 1-1 > n < < 1 a ^ ^ < w 2 ^ '4-t'V W^ pa M - . 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OjC/) Appendix Table 2 Synoptic Table of Annual Rainfall Totals, Virgin Islands, 1875-1940* Cowell's HiJI Barracks Nisky (Mission) Liliendal Ptarl Ldvcniund Ms Folic Klacr'.Hill Charlotte Amalie Estate (S6.61) S4.91 (60.91) 57.92 (63.51) S4.39 (59.91) — (55.17) 60,64 32.27 48.92 32.58 58.16 44.27 32.66 53.31 33.72 65.28 [50.521 32.58 48.51 35,80 60.40 41.92 40.73 56.38 34,66 — — 64.14 — 57.99 38.93 35.01 — 42.03 65.95 [44.241 [54.00] 40.86 — 41.49 43.99 Cinnamon Bay Est. [531 56.93 65.73 (58.89) [67.741 80.89 94.46 30.86 40.49 52,33 53,77 68.61 — 57.28 [52.97] (60,05) 29.56 42.57 51.68 59.36 [50.94] 54.58 67.59 49.35 [47.26] 40.14 60.91 S0.47 43.62 America Hill St. Cbodc Christianstcd Frcderiksled 37.18 54.9S SS.IO 61.01 59.43 32.70 28.62 49.25 1917 1918 19 1920 1921 1922 1923 1924 1925 1926 1927 1928 1930 1931 1932 1933 1934 1935 1936 1937 1958 1939 1940 Charlotte Amalie Estate Anna's Retreat Mosquito Bay St. John Cruz Day (Fort) Adrian America Hill St. Cigdc Chritliansted Fredenkstcdt Jolly Hill Annaly Bonne Espcmnce Angullla Bethlehem (Old Works) Bethlehem (New Works) Calquhoun (Mt. Pleasant) Castle Coakley 45.96 56,60 47.16 51.02 (36.32) 35.41 48,38 40.52 60.60 49.97 50.88 (38.50) 35.38 47.74 Gust. H. 42.79 49.55 47.41 44.48 36.90 41.05 49.27 41,91 42.09 60.48 57.34 35.47 48.41 61.35 46.81 52.63 (42.06) 34.78 25.41 41.24 34.09 53.80 45.00 41.57 41.25 54.53 51.41 (35.57) (37.69) 25.10 35,52 54.08 44.03 38.51 40.59 ? approximate. 33.90 73.94 50,87 37.13 mtli of observations 31,86 40,22 53,87 44,30 41,66 32,55 37.51 [32.38] 48.34 32,44 40.04 60,09 46.46 [30.65] 25.57 35.81 33.72 40.94 50.53 50.08 t55.72 46.14 55.82 34.42 44.37 37.97 37.23 42.98 68.23 31.53 52.79 46.48 65.66 35.74 54.28 35.29 57.89 46.05 50.80 51.15 44.30 63,63 S1.S2 made, these figures are too low by amounts up to possibly 10 inches. The official records for 1888, 1896, and January 1889 have not been found. St, Thomas and St. John stations were not opened until mid-January 1877. Dashes indicate less than a full month of observations. 39.18 65.43 57.03 68.67 36.18 32.67 34.94 29.81 37.35 61,79 53.39 64.77 35.50 44.38 78.61 60.94 74.87 41.49 31.47 66.08 60.95 75.00 80,21 67.22 82.53 59.55 67.59 43.83 41.39 39.14 t.37 37.32 36,74 37.32 37,07 — 49.73 t43.95 — 51.88 40.16 t Frederiksted = LaGrange Estate 1921-23; Prosperity Estate 1923-37: Fort 1938-, 43.00 31.83 (29.88) 38.97 — 26.88 — 37.42 26.49 30,99 16,04 (19.73) 48.29 31.91 35.67 33.75 23.70 31.72 44.70 31.59 (33.92) 35.15 25.76 40,80 30.53 31.06 42.19 25.71 31.75 56.28 46.56 (43.54) t; o CO 00 U-) oo r^ c r^ vo O coOiooo-^Ooo%t- »0 ^ ro t^ ) O ^O .-Hi ^ vO vO -^ ^ — * ^ I'H rO fO «0 MD .^H OO I (D t"^ O^ — H rO i "^ •-" \0 O O -^ •— 'fOt^O.— 'OOO^O 1 00 ^ O lo O O -H ^ 0^< ' ■§ "5 "^ ■£ "5 u ^S ^'^ ^'^ ^"^ ^ ^ S *j 3OOQOOOOOCOCOOOCO oOcCOO *-< ; 15 <■ fvi 00 ^ OOr- J '2 M ^^ — ( 30 00 :|s O 3 3 C Cc I 9 > a. 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The flights were made at 6 a.m. local time every day until May 10, 1937, thereafter at 8 a.m. ; 352 soundings were made in 1937, and 359 in 1938, 711 in all. Nearly all these flights reached 5,000 meters or more. The soundings were discontinued in mid-January 1939, but were resumed during the hurri- cane seasons of 1939, 1940, and 1941. The mean values for standard heights up to 5000 meters have been published regu- larly in the "U. S. Monthly Weather Review." Bourne Field is situated on the southwest coast of the island, practically at sea level; the mountains here rise steeply to 1700 feet within several miles back from the coast. In making the flights a competent aerologist always accompanied the pilot, mak- ing notes on clouds and visibility. During most of the flights the plane kept within 15 or 20 miles of the base. Generally the pilots flew up through any cumulus clouds pres- ent within range, but often entered or emerged from the sides of clouds. In the aver- ages, therefore, the conditions within and without the clouds are about equally rep- resented. The relative humidities in 1938-39 appear to be about 10 per cent lower than the true values due to the characteristics of the instrument, as it reads about 90 per cent even when in dense clouds. The results of each day's soundings were reduced and plotted on an adiabatic chart, later to be sent to the Bureau of Aeronautics, Aerological Division, U. S. Navy, Washington, D. C. ; copies of the adiabatic charts were also made for and deposited with the Aerological Division, U. S. Weather Bureau, Washington. The cloud notes were written on the adiabatic charts. The fact that the soundings from January 1 to May 10, 1937 were made at 6 a.m., whereas thereafter they were made at 8 a.m., introduces some heterogeneity in the re- sults for the lowest kilometer or so, because the surface temperature averages about 2° warmer at 8 a.m. than 6 a.m., and the relative humidity is correspondingly lower. u t^ fe m I I I t^in^o rr>Tt ao ^Oso -O^ovo cvfl'^oo ^-r^-rf oou^-H \0'«^o *^"^. 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NO -O o ^OO OOO On On On On On On . t^t>- NO NO vO ■) CO vO NO NO .— — ii— (^H nOvOnO OOo OOO ON On ON On On On 00 00 OO I ^ ^ OOO On 1 CO nO O -h O lOr^NO rOiOrJ- TfiOTj- OOO loioin OOO On On On 00 OO OO CO OO OO NO ^ lO CO 00 CO \0 CO t~>- OOO NO r^ NO -^ -^ -^ On On On On On On OO CO CO «^ ro <^ On On ^ On On v-h ^ ^ »(H OnOn^™, OnOn^_, OnONi, - s 131 ANNOTATED BIBLIOGRAPHY The bibliography inckides books, articles, pamphlets, and official publi- cations, which contain source material relating specifically to the weather and climate of the Virgin Islands, and to geological, botanical, medical, and agricultural works on the Virgin Islands which may be useful to the clima- tologist. Many secondary and trivial references have been excluded. Im- portant general and special works on the meteorology of the Caribbean and tropical North Atlantic regions are listed in Part 3 of this volume, which deals with Porto Rico. 1. American National Red Cross 1929. The West Indies hurricane disaster, Sept. 1928: Official report of relief work in Puerto Rico, Virgin Islands, and Florida. Wash- ington. 92 pp. 2. Anonymous 1843. Letters from the Virgin Islands: Illustrating life and manners in the West Indies. London. [Contains interesting notes on the devastation of the plantations of Tortola and St. Thomas by the hurricane of Sept. 1819, and ob- servations on health and comfort at that time. There was an ear- lier edition, 1839, under the author's name, S. Hovey.] 3. 1868. The hurricane in the island of St. Thomas, October 29, 1867. Translation from the "Diario de la Marina, Habana," January 5, 1868. Smiths. Inst. Ann. Rept. 1867: 464-465. 1868. [See also detailed accounts in the "Set. Thomae Tidende" for early November 1867; also see "St. Croix Avis" of that date.] 4. B0rgesen, F. 1923. On the vegetation of the Virgin Islands of the United States, for- merly the Danish West Indies. Pamph. 12 pp. Gov't Print. Off. St. Thomas. 1923. (Transl. by F. McFarlane of B^rgesen's paper in Danish, in "Atlanten," 1909: 601-632, figs.) [An "ecological" description of the flora according to the systems of Raunkiaer, Warming, and Schimpfer.] 5. & Paulsen, O. 1898. Om Vegetationen paa de dansk-vestindiske 0er. Bot. Tidskr. 22: 1-114, pis. figs. (Transl. in French, Revue Gen. Bot. 12: 99-107. 1900). 6. Britten, N. L. 1916. The vegetation of Anegada. Mem. N. Y. Bot. Gard. 6: 565-580. 7. & Wilson, P. 1923. Descriptive flora — Spermatophyta. Sci. Surv. P. R. and Virgin Is. 5(1): 1-158. 10 Au. 1923. [Preface refers to climate and vegetation.] 8. Butter, C. S., & Hakansson, E. G. 1917. Some first impressions of the Virgin Islands, medical, surgical, and epidemiological. U. S. Naval Med. Bull. 11(4): 465-475. figs. O 1917. STONE: METEOROLOGY OF THE VIRGIN ISLANDS 133 [Good description of health conditions, etc., in 1917, though they are much better now.] 9. Clayton, H. H. 1927. World weather records. Smiths. Misc. Coll. 79: 1016-1017. [Data from 1876 to 1925.] 1934. World weather records. Smiths. Misc. Coll. 90: 51, 376. [Data from 1921 to 1930. These contain the monthly and annual means of temperature and rainfall for Christiansted, 1875-1930.] 10. Dalhoff, N. 1907. Det tropische Klimas Inflydelse paa Menneskene. Atlanten . . . Aarg. 4: No. 45: 155-161. S 1907. [Mentions the D. W. I. but chiefly a review of W. P. Living- stone's "Black Jamaica."] 11. De Booy, T., & Paris, J, T. 1918. The Virgin Islands, our new possessions, and the British Islands. Phila. [Gives an account with good photos of the damage from October, 1916, hurricane.] 12. Dove, H. W. 1841. On the law of storms. Poggendorff's Ann. Phys. Chem. 1841. Translated into English in Taylor's "Scientific Memoirs" 3: 197; also in Am. Jour. Sci. Arts 44: 315-339. 3 figs. 1843. [Contains an account of the storm of Aug. 1837 at St. Thomas, with some discussion, and mentions the storms of Aug. 1827, Aug. 1830, and Aug. 1835, all apparently based on Col. Reid's work.] 13. Dunn, G. E. 1940. Cyclogenesis in the tropical Atlantic. Bull. Am. Met. Soc. 21: 215-229, figs. Ju. 1940. [Important discussion of pressure variations and upper-air condi- tions at St. Thomas.] 14. Du Tertre, J. B. 1667-1671. Histoire Generale des Antilles. Paris. 3 vols. 15. Eastman, J. R. 1868. Discussion of the West India cyclone of October 29 and 30, 1867. (For U. S. Navy Dept.) Gov. Print. Off. Washington. 17 pp. and map. 1868. [Gives data from St. Thomas with account of damage.] 16. Eggers, H. F. A. 1876. St. Croix's flora. Videnskab. Med. naturvid. Foren. i Kj0ben- havn. 1876: 33-158. 2 pis. 1876. Translated into English, 1879, A flora of St. Croix and the Virgin Islands. Bull. U. S. Nat. Mus. No. 13. [Contains notes and data on the climate, chiefly drawn from ear- lier writers.] 17. 1878. Naturen paa de dansk vestindiske 0er. Tidsskr. pop. Fremstil. Naturvid. V. 5: 1-34. 3 figs. 1878. [Contains data and information on the climate, taken from earlier sources.] 134 SCIENTIFIC SURVEY OF PORTO RICO 18. Gleason, H. A., & Cook, Mel T. 1927. Plant ecology of Porto Rico. Sci. Surv. Porto Rico and Virgin Is. 7: 1-173. figs. [Attempts to relate types of vegetation to soil and climate, but chiefly descriptive.] 19. Hayes, T. H. 1928. Hookworm as a new health problem in St. Croix. U. S. Naval Med. Bull. 26: 19-30. 20. Hornbeck, Dr. med. 1841. Nogle Bemaerkninger over St. Thomas' Geognosie. Forhand. Skand. Naturf. 2: 364-368. [Contains summary of barometric observations for 1833-39.] 21. Imperial Department of Agriculture for the West Indies 1921-1922 to date. Report of the Agricultural Department, Tortola, Vir- gin Islands. Annually, 1921-1922 to date. (Formerly "Reports of the Botanic Station, Economic Experiments and Agricultural Education," annually, 1900-1920.) [These contain rainfall and temperature data for Tortola and Virgin Gorda.] 22. L'Institut Meteorologique Danois (Det Danske Meteorologiske Institut) 1875-1916. Annuaire Meteorologique (Meteorologisk Aarbog), for 1875-1916, annually. Copenhagen, 1876-1917. Part II. Les Colo- nies (Kolonien). [Contains the monthly means for Christiansted, 1875-1916, for Frederiksted, 1875-79, and for Kings Hill, 1875.] 23. Jahncke, F. H. 1874. General remarks on the West Indian cyclones, particularly those from the 9th to the 21st of September, 1872. Quart. Jour. Roy. Met. Soc. 1874: 89-94. Ap. 1874. [Contains extract from the author's meteorological journal kept at St. Thomas from Sept. 7 to 14, 1872, when a hurricane passed to the south of the island, also mentions "white squalls."] 24. Kemp, J. F., & Meyerhoff, H. H. 1926. Geology of the Virgin Islands, Culebra, and Vieques. Part I. Sci. Surv. P. R. and Virgin Is. 4(1): 1-144. figs. 1-36. 2 pis. [Excellent treatise on physiographic features with many photos.] 25. Knox, J. P. 1852. Historical account of St. Thomas, W. I. New York: Chas. Scrib- ner. 1852. [Contains valuable data and notes on the climate, p. 179.] 26. Larsen, K. 1928. Dansk Vestindien 1666-1917. Copenhagen. 395 pp. illustr. 1928. [Popular work with innumerable illustrations that give a good idea of conditions as they were in the islands.] 27. Lassen, H. 1893-1894. Momenter af Vestindiens Geografi. Geografiske Tidsskrift (Copenhagen) 12: 63-79. 1895-1896. Momenter af Vestindiens Geografi. Geografiske Tidsskrift (Copenhagen) 13: 60-75. [The best discussion of geographical and climatic conditions in the D. W. I.; a valuable reference.] STONE: METEOROLOGY OF THE VIRGIN ISLANDS 135 28. Ledru, A. P. 1810. Voyage aux lies de Teiieriffe, La Trinite, Saint-Thomas, Sainte Croix et Porto-Ricco. 2 vols. Paris. [Observations on vegetation and settlement in 1796-98.] 29. Meyerhoff, H. H. 1927. Geology of the Virgin Islands, Culebra, and Vieques. Sci. Surv. P. I. and Virgin Is. 4(2) : 145-220. figs. 37-55. 30. Milam, D. F., & Smillie, W. G. 1931. A bacteriological studv of "colds" on an isolated tropical island (St. John, V. I.). Jour."Exper. Med. 53: 72>2,. figs. 31. Neuman, L. 1908. Klimaets Inflydelse paa Sundhedsforhold m.m. In, "De danske Atlanterhavs^er". Afsnitt 4: 583-586. 1908. [Very general remarks on nature of the climate and kinds of dis- eases found.] 32. Nielsen, V. 1910. Irrigationsanlaeget paa Plantagen La Grange. Atlanten: Med- lemsblad for foreninger de danske Atlanterhavs^er. Aarg. 7, No. 75: 493-498. Mar. 1910. [Describes new irrigation project at La Grange, one mile NE of Frederiksted at foot of mountains. Photos show catch basin and aqueduct.] 33. Oldendorp, G. G. A. 1777. Geschichte der Mission der evangelische Briider auf der ka- ribischer Inseln St. Thomas, St. Croix, und St. Jan. Barby. 1777, 2 vols. [Contains early account of climate, forests, crops, etc.] 34. Orsted, A. S., & Hornbeck, Dr. med. 1853. De dansk-vestindiske 0er. In, A. F. BergsjzSe's "Den danske Stats Statistik." (Copenhagen 1844-53.) 4: 557-711. [Contains extensive tabulation of the early climatic data with notes, pp. 571-80.] 35. Palgrave, W. G. 1874. Atmospheric currents as observed in the West Indies, and par- ticularly in St. Thomas. Nature (London) 10: 65. [Very good description of clouds and winds, weather types.] 36. Poey, A. 1855. Four hundred cyclonic storms of the East Indies and North In- dies and North Atlantic Ocean. Jour. Roy. Geogr. Soc. (London) 21: 291. (Reprinted as "Quatres cents ouragans cycloniques." Paris, Depot de la Marine. No. 348. 1862.) [A catalogue of recorded hurricanes.] 37. 1856. Bibliographic cyclonique. (ed. 2) Ann. Hydrographiques. 1856. (Reprinted, 1866. Paris. 96 pp.) [Complete bibliography of accounts of tropical hurricanes, down to 1855.] 38. Puleston, D. 1939. Blue water vagabond. Doubleday, Doran. New York. [Describes hurricane on West End of Tortola, pp. 88-95.] 136 SCIENTIFIC SURVEY OF PORTO RICO 39. Quin, J. T. 1907. Relation of the movements of the high clouds to cyclones in the West Indies. U. S. Mon. Wea. Rev. 35: 215-218, 510-511. [Also similar article in the "St. Croix Avis," Oct. 30, 1907.] 40. Ravn, F. K. 1916. Om Agerbruget paa St. Croix. Beretning om en Rejse foretagen i September, 1915. K0benhavn: J. Schultz. 23 pp. 1916. [Notes on climate and rainfall, pp. 4-6. Also contains a good dis- cussion of rainfall and sugar-cane yield.] 41. Redfield, W. C. 1854. On the first hurricane of September 1853, in the Atlantic, with a chart, and notices of other storms. Am. Jour. Sci. Arts II. 18: 1- 18, 176-190, map. figs. (Reprinted with different title, New Ha- ven, 32 pp. map. 1855.) [Map shows tracks of some earlv hurricanes passing near the V. I.] 42. Reid, Lt.-Col. W. 1838. An attempt to develop the law of storms. London, (ed. 1.) 1: 18, 55-59. maps. 1838. [Accounts of early hurricanes passing St. Thomas.] 43. Rose, F. 1930. Landeskiindliche Untersuchungen der Jungfern Inseln. Inau- gural Diss. Univ. Leipzig. Leipzig. 1930. [Contains summary of previous literature and an excellent bib- liography of the obscure Danish sources.] 44. Rutten, L. M. R. 1938. Bibliography of West Indian geology. Geogr. en Geol. Meded- deel., Physiogr. Geol. R. No. 16. Utrecht. 103 pp. 45. Sainte-Claire Deville, Ch. J, 1852. These de chimie et de physique presentee a la faculte des sci- ences de Paris, le 23 Nov. 1852. Paris. [Includes "fitudes de Meteorologie aux Antilles," which has ob- servations from St. Thomas.] 46. St. Croix Avis 1800 to date. Newspaper published at Christiansted since the early 1800's. [Contains some early estate rainfall returns and contemporary accounts of the effect of the weather on the cane crop, hurri- canes, etc.] 47. Sanct Thomae Tidende (and "St. Thomas Times") 1820-1917. Newspaper published twice weekly at Charlotte Amalie, 1820-1917. (Later years became the "St. Thomas Times.") [Contains scattered reports of observations and notes on the weather, including the St. Thomas and St. John rainfalls monthly from Feb. 1877 to Sept. 1886; some of A. Wallcie's observations, 1878-90; and descriptions of hurricanes.] 48. Schomburgk, R. H. 1837. Die Jungfrau-Inseln in geologischer und klimatischer Hinsicht. In, H. Berghaus' "Almanach den freunden der Erdkunde." 1: 367-455 (ref. on pp. 432 ff.) STONE: METEOROLOGY OF THE VIRGIN ISLANDS 137 [Most intelligent early account of natural history. Includes cli- matic data from Tortola.] 49. 1832. Remarks on Anegada. Jour. Roy. Geogr. Soc. (London) 2: 152- 170. 50. 1835. Remarks on the heavy swell along some of the West India islands, commonly called the "Ground" or "North Sea"; and on the set and velocity ... of the tides among the Virgin Islands. Jour. Roy. Geogr. Soc. (London) 5: 23-38. 51. Shaw, E. B. 1932. St. Croix's rainiest year causes an epidemic of malaria. Science 76: 566. D 16, 1932. 52. 1933. St. Croix, a marginal sugar-producing island. Geog. Rev. 23: 414-422, figs. map. [Contains rainfall map of St. Croix and discusses relation of rain- fall to sugar industry.] 53. 1935. The poorhouse of the United States. Scient. Month. 41: 131-140. Aug. 1935. [On page 135 is a photo showing trees deformed by the trade winds, and a windmill.] 54. [Smith, L.] 1913-15. Reports of the Agricultural Experiment Station in St. Croix for the years 1911-12, 1912-13, 1913-14. (3 vols.) Copenhagen. 1913, 1914, 1915. (For later reports see: Virgin Islands Gov't.) [Contain some of the most valuable climatic and weather data in print for the islands. Photos of thermograms and barograms for each day of the year, the weekh' rainfalls, daily dry- and wet-bulb temperatures and relative humidities, and the monthly rainfalls from each sugar estate; also chemical analysis of the rain water.] 1921. Sugar cane in St. Croix. Bull. No. 2, Agric. Exp. Sta., St. Croix. 55. [St. Croix Gov't] 1892. Statistics concerning sugar production in St. Croix from 1862- 89. Collected and published at the instance of the Government of the Danish West India Islands. St. Croix. 1892. 1905. Statistics concerning sugar production in St. Croix from 1890- 1902. Collected and published at the instance of the Government of the Danish West India Islands. St. Croix. 1905. [These contain rainfall totals from an average of two or three stations month by month.] 56. Stenzel. 1886. Die Inseln St. Thomas, Westindien. Ann. Hvdrogr. und marit. Met. 14:353-359. [Gives a table of complete hourly observations made in the har- bor of Charlotte Amalie from Feb. 5 to 11, 1886.] 57. Taylor, C. E. 1888. Leaflets from the Danish West Indies. London. [A descriptive handbook.] 58. Thorp, J. 1932. Soil survey (reconnaissance) of St. Croix Island, Virgin Islands. U. S. Dept. Agric. Techn. Bull. 315: 26 pp. soil map. 138 SCIENTIFIC SURVEY OF PORTO RICO 59. Tuckerman, J. 1837. A letter respecting Santa Cruz as a winter residence for in- valids; addressed to Dr. John C. Warren of Boston, Mass. Bos- ton: D. Clapp, Jr. 27 pp. 1837. 60. U. S. Coast and Geodetic Survey 1922 (repr. 1929). Maps, No. 3240, No. 3241, and No. 3242. Virgin Is- lands (St. Thomas, St. John, and St. Croix, resp.). [Topographic contour charts; scale 1:40,000; 50-foot interval, with culture; shows location and situation of nearly all places mentioned in this volume; indispensable for any careful analysis of the weather observations.] 1925. Geographic dictionary of the Virgin Islands of the United States, by J. W. McGuire. Spec. Publ. No. 103, 211 pp. [Study of existing maps, descriptions, place names, locations, co- ordinates, elevations, and bibliography of same.] 61. U. S. Department of Agriculture Soil Conservation Service, in cooperation with the Puerto Rico Reconstruction Administration 1939. Rainfall data for months of January 1939 to date, Stations SCS Nos. 15 and 18, F. S. A. Mimeo. San Juan, P. R., 1939—. 62. U. S. Department of Agriculture Weather Bureau (San Juan) 1920-41. Climatological data. West Indies and Caribbean Service. Monthly and annually Jan. 1920 through Sept. 1941. (21 vols.) [Gives daily and monthly total rainfalls, and mean monthly tem- peratures and extremes; with summaries of general weather con- ditions. Publication probably will be continued.] 63. 1934. Climatic summary of the United States to 1930: sec. 106: Puerto Rico and the United States Virgin Islands. Washington. 19 pp. 1934. [Contains tables of each month's rainfall, raindays, temperature, etc., 1920-30, at 7 V. I. stations.] 64. [Virgin Islands Gov't; after 1928, U. S. Dept. of Agric] 1920-33. Virgin Islands Agricultural Experiment Station, St. Croix. Reports, 1919-32. Wash., D. C, 1920-33 (no more issued). [Contain suminaries of meteorological observations at Anna's Hope, giving monthly means — ■ same data as published by U. S. Weather Bureau. Also notes on effect of weather on crops and crop pests, farming, etc.] 65. Willaume-Jantzen, V. 1908. Dansk Vestindiens Klima. In, "De danske Atlanterhavs(z(er". Afsnitt 4: 574-583. 1908. [Standard, and one of best sources of material on the climate in Danish times; however, no bibliography of sources of data is given.] PARTS ISSUED Volume I (complete in four parts, with index). Part 1. History of the Survey, by N. L. Britton. Geological Introduction, by C. P. Berkey. Geology of the San Juan District, with colored map, by D. R. Semmes. Part 2. Geology of the Coamo-Guayana District, with colored map, by E. T. Hodge. Part 3. Geology of the Ponce District, with colored map, by G. J. Mitchell. Part 4. The Physiography of Porto Rico, with colored map, by A. K. Lobeck. Volume H. Part 1. Geology of the Lares District, with colored map, by Bela Hubbard. Part 2. Geology of the Humacao District, with colored map, by Charles R. Fettke. Part 3. Geology of the Fajardo District, with maps and illustrations, by How- ard A. Meyerhoff. Volume HI (complete in four parts). Part 1. Tertiary Mollusca from Porto Rico, by C. J. Maury. Part 2. Tertiary Mollusca from the Lares District, by Bela Hubbard. Part 3. Fossil Corals of Porto Rico, by H. N. Coryell and Violet Ohlsen. Part 4. The Tertiary Foraminifera of Porto Rico, by J. J. Galloway and Caro- line E. Heminway. Volume IV. Geology of the Virgin Islands, Culebra, and Vieques. Part 1. Introduction and Review of the Literature, by J. F. Kemp. Ph3'-siography, with colored map, by H. A. Meyerhoff. Part 2. Physiography (concluded), with two colored maps, by H. A. Meyerhoflf. Volume V (complete in four parts, with generic index). Part L Descriptive Flora — Spermatophyta (Part), by N. L. Britton and Percy Wilson. Part 2. Descriptive Flora — Spermatophyta (Continued). Part 3. Descriptive Flora — Spermatophyta (Continued). Part 4. Descriptive Flora — Spermatophyta (Continued). Volume VI (complete in four parts). Parti. Descriptive Flora — Spermatophyta (Continued). Part 2. Descriptive Flora — Spermatophyta (Continued). Part 3. Descriptive Flora — Spermatophyta, with Appendix (Concluded). Part 4. Supplement to Descriptive Flora ■ — Bibliography. Index to Volumes V and VI. Volume VII. Part 1. Plant Ecology of Porto Rico, by H. A. Gleason and Mel T. Cook. Part 2. Plant Ecology of Porto Rico (Continued). Part 3. Palaeobotany of Porto Rico, by Arthur Hollick. Volume VIII. Part 1. Mycology, by Fred J. Seaver and Carlos E. Chardon, with contribu- tions by Rafael A. Toro ; F. D. Kern and H. H. Whetzel ; and L. O. Overholts. Part 2. Supplement to Mycology, by Fred J. Seaver, Carlos E. Chardon, Rafael A. Toro, and Frank D. Kern. Revision of the Myxomycetes, by Robert Hagelstein ; Myxophyceae, by Nathaniel L. Gardner. Part 3. The Diatomaceae of Porto Rico and the Virgin Islands, by Robert Hagelstein. Volume IX (complete in four parts, with index). Part 1. Mammals — Chiroptera and Insectivora, by H. E. Anthony. Part 2. Mammals — Rodentia and Edentata, by H. E. Anthony. Part 3. Birds — Colymbiformes to Columbiformes, by A. Wetmore. Part 4. Birds — Psittaciformes to Passeriformes, by A. Wetmore. Volume .X (complete in four parts, with index). Part 1. Amphibians and Land Reptiles of Porto Rico, by Karl Patterson Schmidt. Part 2. The Fishes of Porto Rico and the Virgin Islands — Branchiostomidae to Sciaenidae, by J. T. Nichols. Part 3. The Fishes of Porto Rico and the Virgin Islands — Pomacentridae to Ogcocephalidae, by J. T. Nichols. Part 4. The Ascidians of Porto Rico and the Virgin Islands, by Willard G. Van Name. Volume XI. Part 1. Insects of Porto Rico and Virgin Islands — Diptera or Two-winged Flies, by C. H. Curran. Volume XII. Part 1. Insects of Porto Rico and the Virgin Islands — Heterocera or Moths (excepting the Noctuidae, Geometridae, and Pyralididae), by W. T. M. Forbes. Supplementary Report on the Heterocera of Porto Rico, by W. T. M. Forbes. Part 2. Insects of Porto Rico and the Virgin Islands — Moths of the Family Noctuidae, by William Schaus. Part 3. Insects of Porto Rico and the Virgin Islands — Moths of the Families Geometridae and Pyralididae, by William Schaus. Volume XIV. Part 1. Insects of Porto Rico and the Virgin Islands — Odonata or Dragon Flies, by Elsie Broughton Klots. Part 2. Insects of Porto Rico and the Virgin Islands — Homoptera (excepting the Sternomynchi), by Herbert O shorn. Part 3. Insects of Porto Rico and the Virgin Islands — Hemiptera-Heteroptera (excepting the Miridse and Corixidae), by Harry Gardner Barber. Volume XV. Part 1. The Brachyuran Crabs of Porto Rico and the Virgin Islands, by Mary J. Rathbun. Part 2. Crustacea Macrura of Porto Rico and the Virgin Islands, by Waldo L. Schmitt. Amphipoda of Porto Rico and the Virgin Islands, by Clar- ence L. Shoemaker. Volume XVI. Part 1. A Handbook of the Littoral Echinoderms of Porto Rico and the Other West Indian Islands, by Hubert Lyman Clark. Part 2. Polychaetous Annelids of Porto Rico and Vicinity, by Aaron Louis Treadwell. Part 3. Bryozoa of Porto Rico with a Resume of the West Indian Bryozoan Fauna, by Raymond C. Osburn. Volume XVIII. Part 1. Porto Rican Archaeology, by Froelich G. Rainey. Part 2. A Large Archaeological Site at Capa, Utuado, with Notes on Other Porto Rico Sites Visited in 1914-1915, by J. Alden Mason. Appendix. — An Analysis of the Artifacts of the 1914-1915 Porto Rican Survey, by Irving Rouse. Volume XIX. Part 1. Meteorology of the Virgin Islands, by Robert G. Stone.