218 SCIENCE REMAKING THE WORLD 



influence far into the interior of the country and over* 

 power the dry continental winds, the central states 

 and prairie region, the granary of the United States, 

 produce large crops. When the dry winds overpower 

 the humid southerly winds there are drouths and crop 

 failures. 



The southerly winds on their way from the Gulf of 

 Mexico do not meet any mechanical obstacles. Since 

 the Appalachian Mountains, running in a northeasterly 

 and southwesterly direction, do not hamper their pas- 

 sage, they are capable of penetrating far into the in- 

 terior of the country and, therefore, determine the 

 amount of precipitation even in such states as Min- 

 nesota, Nebraska, North Dakota, and South Dakota. 

 The moisture-laden winds from the Gulf, as soon as 

 they reach the land and encounter irregularities, are 

 cooled and begin to lose part of their moisture in the 

 form of precipitation. As long as the air currents are 

 saturated with moisture the slightest cooling or ir- 

 regularity of the land that causes them to rise will cause 

 precipitation. But as they move inland and become 

 drier, the remaining moisture is given off with difficulty 

 and precipitation decreases. The sooner the humid 

 air currents in the passage over land are drained of the 

 moisture, the shorter is the distance from the ocean 

 over which abundant precipitation falls; the longer the 

 moisture is retained in the air currents, the farther into 

 the interior will it be carried and the larger will be the 

 area over which precipitation will be distributed. 



If precipitation over land depended only on the 

 amount of water directly brought by the prevailing 



