222 MONTHLY WEATHER REVIEW AUGUST 1933 WET AND DRY PERIODS IN PUERTO RICO, 1899-1932 By C. L. RAY [Weather Bureau Office, San Juan, P.R., August 19331 This paper gives the chronological classification of wet Gulf States; and no. 4 the Northeaste,rn States including and dry years that occurred in Puerto Rico from 1899 to New Jersey, Pennsylvania, and Michigan. The de- 1932, mclusive. Following the plan of A. J. Henry’s pendability of the annual rainfall is best indicated by its “The Calendar Year as a Time Unit in Drought Sta- tendency to adhere closely to the normal from year to tistics” (MONTHLYWEATHER REVIEW, April 1931, vol. year, and it will be noted that Puerto Rico, in terms 59, pp. 150-154) the island is divided into sections- bf this test, compares favorably with the United States north, east, south, and west-in order to indicate the groups, as shown in table 4. The no. 4 group of New variations in rainfall in different parts of the area. In the Engla.nd States with an est,rerne range of 49 percent is south portion droughts are somewhat more frequent than thus the most dependable of the United States groups, as in the east, owing to the east-west course of the mountains, Professor Henry has stated, ouing to this small vanation which therefore cause the heavier rains to occur in the from the normal in extreme years; Puerto Rico closely north and east parts of the island. Comparing the values, approaches this figure with a range of 52 percent. Several expressed in terms of percent of the normal, with similar factors may be said to favor a coinpa,ratively small range data for the continental United States as given by Pro- between the niasimuni and minimuin rainfall years, one fessor Henry, it is notable that while in 1930 the most of which is blie fact that the smal1e.r the average or normal severe drought on record was common alike to the United rainfall, the greater is the variation from year to year, or States and to Puerto Rico, the year 1933, though second to state it conversely, the grea.t,erthe rainfall normal, the in severity in the island was predominantly wet in large less tendency for large variation from year to year. In- portions of the United States; in 17 divisions (State asmuch as the precipitation normnls for Puerto Rico are boundaries) it was one of the 10 wettest years of record 42 to 75 inc.hes, representing the south and east divisions, although in some divisions it was one of the driest. respectively, this factor would be of some weight in In 1907, which was the third driest year in the island, explaining the smaller ranges and departures from normal notably in the east portion, where the drought was 11 shown from year to year in the island rainfall. Compar- percent more severe than that of 1930, in terms of ing the average deficiency for all droughts in the United deficiency, rainfall in the continental United States was States with some of the dry years in the island, the not generally subnormal, but on the contrary, established follow-ing will illustrate the smaller variations in the records for excessive amounts which made this one of the Puerto Ric.0 departure.s. 10 wettest years in 10 divisions, though for 2 divisions it Prrcent was one of the 10 driest years. There seems, therefore, United States, average of all droughts _____________ 68 to be no well-defined relationship between the rainfall of Puert,o Rico, drought of: Puerto Rico and that of the United States, except perhaps 1930_____________-______________-_----_-__79 in years of wide-spread and severe drought. 1923_-__________-_______________________--80 The comparative figures for the island from 1899 to 1907____---________________---_--__-______85 1933, shown in percentage of the normal, are given in 1925and 1929_-__-______-_____-______-___--88 tables 1, 2, and 3. As will be noted, tables 1 and 3 give 1926-_---_____________-_-------_____-_____90 the percentages of the normal by years in relative order The comparatively small rainfall deficienc,y as related from 1 to 10. Thus under group 1 is listed the year of t,o the normal of course fails to give an adequate indica- least rainfall and percent of the normal for each of the tion of the conditions nialiing for drought in the Puerto four divisions, and for the island as a whole; under group Rico area. Distribution through the year is an impor- 2 the year having the second lightest rainfall for each tant item, since heavy rains at widely separated intervals division, etc. A selected list or group of stations was may be, in total, a near approximation of the normal, but chosen, as follows: North, 24 stations; south, 10 stations; with periods of weeks or months intervening subject to west, 8 stations; east, 4 stations. Data for this selected subnormal precipit,ation and often severe droughts. list are practically unbroken for the given 34 years, and Heavy run-offs, and a high rate of evaporation due to from them division normals were established. long duration of sunshine, tire factors which enter quite Following the 1930 drought the years 1931 and 1932 largely into the product,ion of droughts, in the island, were marked by excessive rainfall in the island. In 1931 oft’entimes when the percentage of departure from the the headiest rainfall in, 34 years was registered in the normal is not notably large. east portion and the second greatest in the south, while In the tabulation of continental rainfall by years, the the island as a whole received the second greatest on tendenq is n0te.d for a year of abnormally dry conditions record. In 1932 rainfall vaned from the fifth heaviest to be preceded by gradually diminishing rainfall and fol- in the east portion to sixth, seventh, and ninth in the lowed by several years of dryness. Such a trend may be west, south, and north, respectively. Comparison of the siinilarly observed in the chronological re.cord of Puerto percentage of the average precipitation in the 3 years of Rico ramfall, to the extent, at least, that there appear to deficient, and the 3 years of greatest, precipitation in be sequences or unit groups of years with subnormal rain- Puerto Rico, with a similar grouping for four divisions fall. A 5-year sequence of gradually decreasing rainfall comprising the continental United States, is shown in led up to the drought of 1923 while that of 1930 was pre- table 4 below. The United States group no. 1 includes c,eded by gradually decreasing precipitation in 1928 and the Pacific Coast and Plateau States; no. 2 the Plains 1929. Wet years appear frequently to come in pairs, as States and Missouri, Iowa, and Minnesota; no. 3 the in 1911 and 1912, 1915-16, 1927-28 and 1931-32. Unauthenticated | Downloaded 10/01/21 12:01 PM UTC AUGUST 1933 MONTHLY WEATHER REVIEW 223 TABLE1.-Years of deficient rainfall (percent of normal) in order of TABLE3.-Percentage rainfall departure roin normal by sections: relative dryness, Island of Puerto Rico, 1899 to 193.2 Piierto Rico, 1899-1992- dontinued __ - Year North East South West [sland -- - - Pereeni Perccnt Pereen, %cen Percent 1909......................................... 108 125 1.18 lo( 116 1910......................................... 101 98 74 91 95 1911......................................... Io6 107 113 103 107 hTorth .......... 73. 5 1930 76 1923 83 19?5 84 1907 E5 1818 R7 1912......................................... 89 96 1?4 114 loa East ........... i5.28 1923 I907 7.1 19% i8 1915 85 1930 85 1913......................................... 90 96 94 93 91 South.. ........ 42.0 1Y30 73 1910 74 1923 75 1914 ......................................... 93 94 91 106 89 West ............ 1 76.9 19301929 !$;;;; ss72 1021 88 1915 h9 1915 69 1915......................................... I17 55 90 $9 lo? Island: _________. 67.4 1930 I 79 1 1933 1 YO 1807 85 1939 85 192.5 88 1916......................................... 122 131 134 97 121 1917......................................... 104 89 81 97 07 I918 ......................................... 67 82 93 99 51 1918.. ....................................... 9s 89 ion 89 97 1g20......................................... 90 96 1M 92 (14 section 1 Mean Per- 1921......................................... 90 99 57 101 92 rl~ 1833......................................... 91 95 72 106 Sn Year Pear cent ::; ::; 1923......................................... 82 72 75 88 SO 1924.. ....................................... 1M -110 111 88 105 I- I- 1-1-1--1- 1Y25 ......................................... 8.i 88 80 106 R9 North 19?6 89 1913 90 igzn yo 1936 ......................................... E9 78 65 99 80 East 1918 O? 1914 Y4 1Y11S 94 1927......................................... 13' 117 104 114 130 1917 81 1921 6i lY2R SS 19. ......................................... 10: 109 132 13 I14 west 193 92 ign5 93 IY~O 92 - 1913 91 1921 92 193 94 1929......................................... 88 105 2 92 88 I I I1 I 1930 ......................................... -76 85 72 -87 -79 Note: Old Spanish records, Ean Juan, dating from 1868 show a dry period, approxi- 1931......................................... 127 -143 143 111 123 matin? the record for 65 years in 1Yi3, annual rainfall 68 percent of the normal and in 1893 1932. ........................................ 108 121 124 110 112 67 percent of normal. Cnnoranas record lS9fl-98 also hm record dry penod. 1693 68 per- cent. TABLE2.-Years of greater than normal rainfall (percent of norinalj TABLE4.-Percentage of the average precipitation in the S years of in order of relative depth, Island of Puerto Rico, 1899 to 19% deficient and the S years of greatest precipitation in the groups of States, no. 1 to 4, and in Puerto Rico I Least Greatest pp_______--___ Groups Range North ..........