FACTORS OF ENVIRONMENT. 477 



II. Climatic Factors. 



924. Rainfall, or precipitation. This factor has a close rela- 

 tion to the water factor, but still its effect in general is climatic. 

 Rainfall is one of the most important elements determining the 

 plant population of a region. The luxuriance of vegetation and 

 the number of individuals of a kind, other things being equal, 

 is in direct relation to the percentage of rainfall over different 

 parts of the earth's land surface. Rainfall depends on currents 

 of moisture-laden air from warm bodies of water sweeping over 

 cooler areas of water or land, or more rarely, on currents of cool 

 air passing over warm areas covered with a moisture-laden 

 atmosphere. According to scarcity or abundance of rainfall 

 the earth may be mapped into areas of rich vegetation, largely 

 forest areas; into plains or steppes; prairies; and deserts (see 

 Chapter XLIX). It will be well to note here briefly the rainfall 

 or precipitation in different parts of the earth. 



925. In North America, a narrow belt along the Pacific in Oregon and 

 Washington, the rainfall is more than 60 inches annually. In some places 

 in Washington State it is more than 100 inches. This is due to the mois- 

 ture-laden winds from the warm waters of the Pacific Ocean coming 

 in contact with the cooler air of the land. In Florida there are similar 

 abundant rains, owing to the warm winds from the tropical seas. For 

 the same reason the annual rainfall in the Gulf and Atlantic States is 

 in general more than 50 inches. The luxuriance of vegetation in some 

 parts of Florida and Washington is found where the rainfall is so great. 



926. The great arid region of the United States. There is a district 

 in Nevada, southern California, and Arizona where the annual rainfall is 

 less than 5 inches, while in most of Nevada and Utah, parts of Arizona, 

 New Mexico, Colorado, and Wyoming it is below 10 inches. The Sonora- 

 Nevada Desert is located in this region. A wider belt extends from middle 

 Oregon far into Nebraska and South Dakota, where the precipitation is 

 less than 20 inches. The warm moisture-laden winds of the Pacific lose 

 a large part of their moisture near the coast, move eastward over the 

 Rocky Mountains, and furnish comparatively little rainfall from middle 

 Oregon eastward. The Rocky Mountains, being so high and cold, 

 cause a greater precipitation of the small amount of moisture remain- 

 ing in the air than occurs on other plateaus (Gilbert and Brigham, Intro- 

 duction, Phys. Geog.). The northern Mississippi region receives a medium 

 amount of rain from the diminishing moisture-laden atmosphere from 



