44 



HYDROLOGY 



TABLE XX 



EQUIVALENT MEASURES AND WEIGHTS OF WATER AT 4 CENTIGRADE, 39.2 



FAHRENHEIT 



2. RAINFALL 



Source of Water Supply. The ultimate source of our water 

 supply is the precipitation in the form of rain or snow which 

 reaches the earth. For the United States the chief source of this 

 is the evaporation from the Pacific Ocean which by westerly winds 

 is carried eastward in diminishing quantities. In the Mississippi 

 Valley the small supply of moisture still remaining is augmented 

 by a generous contribution from the Gulf of Mexico, from where 

 it is carried inland by southerly and southwesterly winds. East 

 of the Appalachian Mountains the precipitation is mainly derived 

 from the Atlantic Ocean. 



Rain is formed whenever the air is cooled below the point of 

 saturation. This cooling may be caused by the air currents 

 being forced upward, as when they strike mountain ranges, or 

 they may be intermingled with other colder air currents, or come 

 into contact with a cold land. 



Variation in Rainfall. The rainfall varies greatly in different 

 parts of the country and is governed quite largely by the geo- 

 graphic or topographic relations. It is usually given in either 

 total inches of rain per year or per month, while the daily maps of 

 the Weather Bureau show the variations from day to day. A 

 map of the United States giving the average annual rainfall in 

 inches for the different sections is shown in Fig. 11, the mean 

 annual precipitation for the whole country being 29.4 inches. 

 Table XXI also gives some typical values of rainfall in different 

 parts of the country. 



