TROPICAL CLIMATE 7 



mountains of small islands and being forced to rise in order 

 to pass over the mountains. The water is thus in a sense 

 squeezed out of the atmosphere in passing over the high points 

 of islands lying in the trade winds. It may thus often occur 

 that during the prevalence of clear weather on the windward 

 side of an island, an almost constant precipitation of rain oc- 

 curs high up on the mountain side and this rain is frequently 

 blown over upon the lee shoulder of the mountains, sometimes 

 reaching almost to the sea on the lee side of the island. Such 

 rains are brought about merely by the presence of the moun- 

 tains and occur in an area in which no storm conditions in the 

 ordinary sense exist. The higher the mountain the heavier 

 the rainfall caused by its presence. In the case of mountains 

 of no more than 4,000 or 5,000 feet elevation, the presence 

 of a forest growth upon the upper ridges exercises a great 

 influence in increasing precipitation. A part of the explana- 

 tion of this fact is to be found in the radiation of heat from 

 forests and the consequent cooling of the trees and increase of 

 precipitation as a result. 



At any time of the day or night, therefore, at least during 

 the prevalence of the trade-wind season, the atmosphere may 

 be robbed of a portion of its moisture by coming in contact 

 with the mountains and being forced to rise to a height of 

 5,000 feet or more before passing on in the general course of 

 the trade wind. This peculiar cause of rain storms brings 

 about the frequent occurrence of light showers in a perfectly 

 clear sky, the rain being precipitated from the atmosphere at 

 the tops of the mountains and being blown down over the lee 

 side of the island by the trade winds. This condition is often 

 referred to by the natives as liquid sunshine and gives rise to 

 the almost daily occurrence of brilliant rainbows and the fre- 

 quent occurrence of lunar rainbows. 



The effect of tropical climate upon plants is manifested in 

 various ways. Some plants which are annuals in cold climates 

 become perennials in the Tropics. Similarly, some plants which 



