RAIN 



4922 



RAIN 



RAINFALL MAPS 

 Over 40 Inchest 10 to 20 Inches 

 30 to 40" III 5 to 10 " 

 20 to 30" Q_essthan5" 



NORTH AMERICAN RAINFALL, SUMMER AND WINTER 



Africa. These variations are due to a number 

 of causes, the principal ones being the height 

 of a given region above the sea, its distance 

 away from the sea, and the character of the 

 land, whether barren or wooded. On slopes 

 that are exposed to ocean winds rainfall is gen- 

 erally abundant, while regions shut off from the 

 sea by high mountain ranges are almost rain- 

 less. 



Lands That Always Have Rains. The south- 

 ern slopes of the Himalaya Mountains in India 

 have the heaviest rainfall in the world, from 

 200 to 600 inches a year, because the warm 

 winds from the Indian Ocean, heavily laden 

 with moisture, lose nearly all of it when they 

 strike the cool mountain slopes. These moun- 

 tains are very high, and on their northern 

 slopes the annual rainfall is less than ten 

 inches, because practically no moisture is car- 

 ried across their tops. The vast Amazon re- 

 gion in South America, because it is heavily 

 wooded and does not become superheated as 

 does a barren region, has an abundant rainfall ; 

 the winds which mount the eastern slopes of 

 the Andes, still have moisture to give up, but 

 they cannot cross the towering peaks, therefore 

 the western slopes of these same mountains are 

 practically desert. The winds blowing from the 

 ocean, on the other hand, are too cold to bring 

 rain. 



Desert Lands. The Sahara Desert is an ex- 

 ample of a rainless region. As it is situated in 

 the tropics and is surrounded on all sides by 

 warm ocean waters, an abundant rainfall might 

 be expected. Yet it is one of the driest re- 

 gions in the world. The lack of rain is due 

 in part to the fact that the region is well sur- 

 rounded by mountains, on whose slopes the 

 ocean winds lose most of their moisture. On 

 striking the sandy area, the air is heated still 

 further instead of being cooled, and, conse- 

 quently, clouds cannot form nor can any rain 

 fall. Similar deserts are found in the south- 

 western part of the United States, and in West- 

 ern Australia. There are desert table-lands, 

 too, particularly in west-central Asia, which, 

 because of their altitude, are almost as rainless 

 as the polar regions. California illustrates per- 

 fectly the effect of the changing seasons on the 

 rainfall of a coast country near the tropics, 

 where alternating wet and dry seasons are the 

 rule. During the summer months the ocean 

 breezes are heated when they strike the land 

 and do not give up their moisture, but during 

 the winter months they are cooled and almost 

 daily rainfall results. The maps illustrating 

 the distribution of rainfall over the world 

 and the maps illustrating the prevailing winds 

 of the world and the ocean currents, if studied 

 together, should be very helpful. 



