On many oceanic tropical islands the trade winds blow from the east and as the air passes over the island it loses moisture to rain. The two hypotheses for this study are that the western part of the island is less humid and has a larger diurnal air temperature cycle. Using data available on the Internet, the two hypotheses were assayed 7 times on 6 different islands in the Pacific and Caribbean. The islands used in this study are Puerto Rico, Hispaniola, the Big Island of Hawaii, Maui, Oahu, and Kauai. The first hypothesis was tested only on Hispaniola and is true. The second hypothesis was tested on all the islands and is true on all the islands except on the Big Island of Hawaii and Maui. Using a p-value of 0.05, these hypotheses are as predicted 5 of the 7 times, which has a p-value of 6 x 10-6.
Observations show that on islands, the topography of the island can act as an obstacle or barrier to the wind causing an unequal distribution of humidity and diurnal air temperature cycle. This study establishes the difference between the east (windward) and the west (leeward) part of the islands. The east side benefit from the moisture of the trade winds, while the west slope receives dryer wind. This generates microclimates that also depend on elevation and large differences can occur across small horizontal distances.