278 PHYSIOGRAPHY 



Less than one-thirtieth of the people of the United States live in 

 the third of the country where the rainfall is less than 20 inches per 

 year. The best of soil is not fertile unless adequately watered. 

 Twenty inches of rain per year is generally considered to be the 

 minimum for general agricultural purposes, but something depends 

 on the temperature, something on the time of year when the rain 

 falls, and something on the soil. The warmer the climate, the more 

 the water needed. The total amount necessary is less if it falls 

 when the growing crops need it most. If rainfall could be dis- 

 tributed just as farmers would like to have it, 10 inches would 

 probably be enough in the middle latitudes of the United States. 

 Water or snow falling at times when plants are not growing is, 

 however, not worthless, for some of it remains underground, and 

 is reached by the roots of plants at a later time. 



Land which can be irrigated does not depend directly on rain 

 and snow; but -the water used in irrigation is derived from the 

 atmosphere, though in many cases the water falls far from the 

 place where it is used. 



The distribution of rainfall is influenced largely by the winds, 

 which carry much moisture from the places where it is evaporated 

 to the places where it is precipitated. Winds help to determine 

 where rain falls, how much falls, and at what times of the year. 

 To know what the rainfall (or snowfall) of any given region will be, 

 it is needful to know (1) what winds affect it, (2) the topography 

 of the surface over which the winds have already blown before reach- 

 ing it, and (3) the topographic situation and relations of the place 

 itself. 



Rainfall in the zone of the trades. In the trade-wind zones, 

 the winds are blowing from higher to lower latitudes, and there- 

 fore, on the average, from cooler to warmer latitudes. As the air 

 is warmed, it may take more moisture. So long as the trades blow 

 over the sea, therefore, they do not ordinarily give rain. Where 

 they blow over low land, which in this latitude is generally warmer 

 than the sea, they take moisture, but do not drop what they have. 

 It follows that on the sea, and on low lands, like the Sahara, the 

 trade-winds are "dry." A part of Australia lying in the belt of 

 southerly trades is also dry. 



