Antonio Cocco Quezada
Detailed maps of mean annual and monthly rainfall distribution over the island of Hispaniola are presented, based upon the records of 162 stations and upon recent information. According to these maps, the mean annual and monthly rainfall distribution, as well as the variations in the sequence of mean monthly rainfall in different parts of the island, has the most complex pattern of any unit of the Greater Antilles. This is due largely to the mountainous relief which exerts a marked influence upon the areal distribution of rainfall. Mean annual rainfall varies from less than 20 inches on Leeward Lowlands and enclosed interior valleys, to more than 100 inches (millimeters) on elevated mountain slopes and favorably located areas at low elevation. The most significant features of mean monthly rainfall distribution are best illustrated by the February, May, July, September, and November rainfall maps. The February map shows a generally light rainfall over the entire island. This is characteristic of the winter period of minimum orographic trade wind rainfall. The heaviest rainfall, from four to six inches, is recorded on the windward coast and mountain slopes bordering the Atlantic Ocean. With but one exception, not even the higher elevation of the central, interior, and southern mountains can compensate for the decreased humidity of the trade winds after they have crossed the coastal mountains, and from less than one to four inches of rainfall are recorded. The May map, representative of the spring and early summer period of maximum convectional rainfall, shows a heavy rainfall for the island as a whole, especially over the interior and southern mountain regions where more than ten inches are recorded. Even the dry leeward lowlands and enclosed valleys of the interior record more than two inches of rainfall everywhere, and from 6 to 8 inches are recorded in the most elevated and favorably exposed places. The July map for the midsummer "less rainy" period indicates a general decrease of the convectional rainfall over the entire island. This is a result of lower humidity, increased wind velocity, higher pressure, and more stable upper air conditions due to the extension of the southwestern portion of the Azores-Bermuda high over the region. From less than one to more than ten inches of rainfall are recorded. July is the month of minimum rainfall on the northern coast and mountain slopes bordering the Atlantic Ocean, where summer (late June through August) is the less rainy season. At this time the winds blow from the east parallel to the coastal mountains and only one to four inches(mms)of rain falls. The September map illustrates the rainfall distribution for the late summer and autumn period of maximum convectional and tropical cyclonic rainfall. The heaviest rainfall, from six to more than ten inches, is recorded on the southern slopes of the island, where cyclones come ashore. The northern side of the island, exposed to the less rainy rear quadrants of the tropical cyclones, records only four to eight inches of rainfall. The November map shows the distribution for the late autumn and early winter period of maximum orographic trade wind and northern rainfall. In contrast to September, the heaviest rainfall, from eight to more than ten inches, is recorded on the northern slopes of the island where the trade winds and northerners from the Atlantic are forced to rise. A remarkably sharp rain shadow appears on the leeward slopes of the central mountains where from two to six inches of rainfall are recorded. The mountains of the south are high enough to cause further rainfall of from six to more than ten inches on their upper slopes.