rainfall every montli in most areas. Between latitudes 

 ,V' and 10°-15°, Xortii and South, tiie two periods 

 of the year when the sun is at its zenitii are far 

 enough apart so that tiiere are two rainy and two dry 

 seasons eacii year. At still higher latitudes there is 

 only one wet and one dry season. What constitutes 

 a dry season is arbitrary, but in the wetter parts of 

 the tropics, it is considered the period when the rain- 

 fall is less than 10 cm (4 in.) per month. Under 

 e.xtreme conditions no rainfall may fall during the 

 dry season, while in the rainy season some localities 

 may receive over 100 cm (40 in.) in a single month. 

 During the dry season tiie soil may become desic- 

 cated while during the wet season it may become 

 waterlogged. Grass fires frequently occur during the 

 dry season. The periodic monsoons of India and 

 southeastern Asia result from the outflow of dry 

 winds from a high pressure area that persists in 

 central Asia during the winter and the inflow of 

 moisture-laden winds from the surrounding oceans 

 toward a continental low pressure area during the 

 summer. 



In the wettest parts of the tropics, relative hu- 

 midity is always very high. It seldom drops below 

 60 per cent of saturation during the hottest part of 

 the day, and may average over 90 per cent for the 

 entire day. On tropical mountains, mean relative 

 humidity rises with increase in elevation until at 

 1000 m (3300 ft) in some localities there is almost 

 continuous fog and drizzle. 



Wiierc rainfall is .scant throughout the year ( I-^ig. 

 20-3), there occurs desert scrub and tropical thorn 

 forest belonging to the desert biome. Clima.x tropical 

 savanna occurs where rainfall ranges from 90 to 150 

 cm (36 to 60 in.), but there is a dry season that lasts 

 four or five months. Tropical deciduous forest re- 

 places savanna where the dry season is shorter and 

 less severe. Probably an annual total of 160 cm (64 

 in.) is the minimum that permits development of 

 tropical broad-leaved evergreen forest. Occasional 

 months may have as little as 6 cm (2.4 in.) but there 

 is no true dry season. In the so-called cloud forests 

 on the mountains, rainfall may not be particularly 

 high, but this is compensated for by almost continu- 

 ous fog and drizzle, condensation of moisture on all 

 the vegetation, and the very low rate of evaporation. 



VEGETATION 



Tropical vegetation has been described in 

 detail by Richards (1952). It is essentially a con- 

 tinuum from desert to savanna to tropical deciduous 

 forest to broad-leaved evergreen or rain forest. 

 There is no true lowland climatic grassland in the 

 Tropics except in relatively small areas (Pendleton 

 1949). Treeless grassland is the result of excessive 

 burning, cultivation, grazing, or unfavorable soil con- 

 ditions for the growth of trees. 



Tropical savanna is, however, extensive. Much 



Long 

 dry 

 season 



FIG. 27-1 Wet (Indicated by the 

 gray tint) and dry seasons in the 

 Tropics in relation to latitude 

 (from Richards 1952, after E. de 

 Martonne). 



Tropical biomes 341 



