WINDS AND STORMS '143 



ceding and during thunderstorms. This type of storm 

 is frequent in the United States. It occurs in cyclones, 

 but usually some distance from the center and toward 

 the south. 



A thunderstorm generally occurs in the warm season 

 and often follows a period of intense heat. The first 

 indication of a thunderstorm in temperate latitudes is 

 a large dark cloud in the west. It moves eastward, 

 preceded by a sharp breeze. The sky becomes over- 

 spread and rain pours down. The rainfall is often 

 heavy and the drops are large. It lasts, however, but 

 a short time, usually less than an hour. The sky then 

 becomes clear, and the air is noticeably- cooler and 

 fresher. 



Frequently in a thunderstorm the sun appears when 

 some rain is still falling. A rainbow may then be 

 seen opposite the sun. As the sun's rays pass through 

 the drops of water, the white light of which the ray 

 is composed is broken up into a number of different 

 colored rays. 



The lightning which accompanies a thunderstorm is 

 due to the fact that electricity is produced when water 

 condenses rapidly in the air. Each drop of water 

 becomes charged with electricity, which is then dis- 

 charged between clouds, between parts of clouds, or 

 between the earth and the clouds. 



The flashes of lightning cause vibrations in the air 



which produce a noise, which w~e call thunder. Sound 



travels much more slowly than light ; consequently we 



