232 THE WEATHER 



Aside from the necessary connection of some cloud forms 

 with rain or snow, cloudiness is important as a weather feature 

 because it obstructs sunshine, and in that way affects nearly 

 all other weather elements. In recording sunshine, a day is 

 called clear if not over %o clouded; ${Q to Jf i g called partly 

 cloudy, and %Q or over, cloudy. 



FIG. 164. Strato-Cumulus, broken, showing irregular upper surface. The 

 dark spots on the under surface are often caused by higher tops like " a " above. 

 Dark patches on strato-cumulus or stratus, are also caused by broken or 

 irregular clouds of a higher type such as alto-cumulus above, whose shadows 

 fall on the lower clouds. 



256. The Formation of Clouds. Clouds may be formed by 

 several processes. The most important method is by the 

 cooling that takes place in ascending currents of air that is 

 relatively warm and moist. This cooling is due chiefly to 

 the expansion of the rising air as the pressure of the surround- 

 ing air upon it grows less with its ascent. Such cooling is 

 called dynamic cooling, and is the principal cause of cumulus 

 clouds. 



257. The Cumulus Cloud ; Air Movements Forming It. The 

 cumulus (Figs. 155 and 159) is the familiar cloud of summer 

 days, with rounded top and flattened base and great variety 

 of size. The earth's surface is warmed by the sun and in turn 

 heats the lower air. By mid-forenoon of a clear summer day, 



