178 GENERAL SCIENCE 



and sometimes remain over a certain area for several 

 days. Nimbus clouds have no especial form, and the 

 term is sometimes applied to any storm cloud. 



Rainfall. The term "rainfall" includes all forms of 

 precipitation rain, snow, hail, and sleet. Rain occurs 

 when the moisture condenses in drops large enough to 

 fall to the earth. Snow is formed when the moisture of 

 the air is condensed at temperatures below the freezing 



FIG. 149. Stratus Clouds. 



point, C. Sometimes moisture condenses as snow in 

 the higher atmosphere and then melts as it falls through 

 the warmer, lower atmosphere, reaching the earth as 

 rain. Hail occurs chiefly during thunderstorms in 

 summer, when the raindrops are frozen on their way to 

 the earth. There are several theories to account for the 

 formation of hailstones in concentric layers, but it is quite 

 probable that the ascending air currents of high thunder- 

 storm clouds carry the raindrops up into a region of quite 

 low temperature and that as they fall they pass through 



