DISTRIBUTION OF RAINFALL 67 



tralia. South Africa, and southern South America present such 

 lowlands in the trade-wind zones, and hence are deserts. Indeed 

 most of the great deserts of the modern world lie in the belts of 

 lowland country swept by the trades outside the poleward limit of 

 the equatorial rain belt, or roughly between latitudes 20° and 30°. 

 On the leeward western coasts of Australia, South America, and 

 South Africa, these deserts extend directly to the edge of the sea. 

 If these winds are forced to rise to greater altitudes, as is the case 

 when a mountain range stretches across their path, they are cooled, 

 and the moisture is condensed and may fall as rain or snow. Thus 

 the windward sides of these mountains will be characterized by 

 heavy rainfall, as illustrated by the eastern slope of the Andes 

 Mountains. The southeastern trades of this region first lose a part 

 of their moisture over the comparatively low Sierras of the east 

 coast, where the rainfall is over 80 inches a year. Then, as they 

 sweep over the Brazilian plains, the rainfall is less (between 40 and 

 80 inches per annum), to be increased again to over 80 inches as 

 they rise in the Andes. After crossing the Andes, they descend, and 

 being warmed both by compression during the descent (see page 

 49, foehn). and by contact with the warm surface of the lower 

 region, they become drying wands, with the result that the nar- 

 row Pacific coastal strip, to leeward of the Andes, is a pronounced 

 desert from near the equator to about latitude 30° S. Cold ocean 

 waters and prevailing southerly (drying) winds along shore further 

 contribute to this result. The leeward sides of the mountains in the 

 path of the trade winds are thus a region of little precipitation, dry 

 savannas or deserts occurring, while the windward sides support a 

 luxuriant vegetation. If the eastern coast mountains were higher, 

 the winds after passing over them would become drying winds, and 

 the plains of Brazil would be arid, rainfall becoming abundant again 

 only as the winds rose over the Andes. Corresponding to such a 

 distribution of rainfall, one would expect densely wooded areas on 

 the eastern side and heavy torrent-laid deposits on the western bor- 

 ders of the two mountain chains, with semiarid or desert conditions 

 between them. The latter might be characterized either by eolian 

 erosion, or by deposition of wind-transported material, or by both. 

 Some of the lofty islands of the Hawaiian group which lie in the 

 northeast trade wind belt furnish other good examples of rainy 

 windward and dry lee slopes, the latter in some cases being of a 

 desert character. 



The equatorial belt of calms and variable zviiids, i. e., the dol- 

 drums, is a region of exceptionally abundant rainfall, being one of 

 the rainiest regions of the world, its average being about 100 inches 



