Tropical communities and habitats vary from rain 

 forest to desert. The largest continuous mass of 

 tropical evergreen forest lies in the Amazon valley of 

 South America and extends from lower Mexico 

 across northern South America from the Pacific to 

 the Atlantic Ocean. It is interrupted by tropical de- 

 ciduous forest and savanna, as well as cloud forests in 

 the Andes Mountains. Elsewhere tropical vegetation 

 of various types covers extensive areas in central and 

 western Africa and almost the whole of the Oriental 

 Region. Tropical vegetation also occurs in Australia, 

 New Guinea, and the Pacific Islands. 



CLIMATE 



27 



Geographic 



Distribution of 



Communities: 



Tropical Biomes 



The conspicuous features of tropical cli- 

 mate are high, even temperatures throughout the 

 year ; uniform lengths of day and night ; and seasonal 

 variation in rainfall (Richards 1952). Mean monthly 

 temperatures do not drop below 18°C (64°F) and 

 may rise to 32 °C (90° F) or higher. Lowest mean 

 temperatures usually but not always occur during the 

 wet season, but the difference between monthly 

 means may be less than 1°C (1.8°F) and is seldom 

 more than 13°C (23°F). There may be a greater 

 range in temperature at different times of day and 

 night than in mean monthly temperature throughout 

 the year. Mean daily minimum and maximum tem- 

 peratures are seldom below 10°C (50°F) or above 

 43.3°C (110°F). As one ascends mountain slopes 

 there is, of course, a drop in temperature. 



On the equator, the length of day and night are 

 approximately 12 hours each throughout the year. 

 The seasonal variation increases away from the equa- 

 tor, both north and south, but in an opposite manner. 

 The shortest daylength in the Tropics is about 10.5 

 hours, the longest about 13.5 hours. In the rainy 

 season, the actual amount of sunshine is low, aver- 

 aging only five or six hours per day. 



In contrast to the uniformity of temperature and 

 the length of daylight is the considerable diversity in 

 rainfall and humidity in different parts of the tropics. 

 Deserts with insufficient moisture to support any 

 vegetation occur at one end of a climatic gradient, 

 while at the other end large areas exist where annual 

 precipitation is between 250 cm (100 in.) and 400 

 cm (160 in.). Rainfall is largely convectional and 

 results from the cooling of the air that rises from 

 heated land surfaces. Sudden showers are often ac- 

 companied by lightning and thunder and may bring 

 a sudden drop of as much as 4°C (7°F) in temper- 

 ature. These storms commonly occur in the after- 

 noon and may come regularly day after day. Because 

 of this influence of the sun, the rainy season typically 

 occurs when the sun is directly overhead. 



Adjacent to the equator, there is considerable 



340 



