RAINFALL 145 



can no longer float, or, to say the same thing in other words, 

 the particles or little drops of water will come together and 

 form large drops which are too heavy to remain in the air. 

 Rain then takes place, and continues if the condensation con- 

 tinues. The study of the causes which produce rain is one in 

 which the United States Government is much interested, and 

 for which large sums of money are spent annually. Thanks 

 to these investigations, which are country-wide, we are able 

 to state definitely just what the conditions are preceding a 

 rainstorm. 



In the prevailing westerlies there are vast whirlpools, just 

 as there may be in flowing water. Underneath one of these 

 whirlpools there is less air pressing upon the earth than there 

 is anywhere near the whirlpool. We call this section a " low " 

 because a barometer, which indicates the pressure of the air, 

 would show a low pressure. Just as in a whirlpool in water 

 the neighboring water rushes in to fill up the whirlpool, so the 

 air rushes from all directions, over a space of thousands of 

 square miles, towards the center of one of these lows. If a 

 low passes to the north of us, the wind will come from the 

 south to fill it up. This south wind, on account of its warmth, 

 contains a great deal of water vapor. As it travels north it 

 becomes colder, about one degree Fahrenheit for each geo- 

 graphical degree, roughly, sixty-eight miles. The air also 

 rises as it comes north, and becomes cooler, one and six 

 tenths degrees Fahrenheit for every three hundred feet which 

 it ascends. Both these conditions rapidly lower the temper- 

 ature of the whole mass, and the point at which the air 

 can no longer hold water vapor is rapidly reached. This is 

 called the saturation point, and at this point fog and cloud 

 are produced. Any further cooling of the air produces rain. 

 The scientific name for a " low " is " cyclone." 



