234 THE WEATHER 



siderably warmer than the surrounding air it will rise consid- 

 erably higher than the base and so build a tall cloud (Figs. 159 

 and 160). (3) The volume of the ascending current and (4) 

 the amount of water vapor in it also help determine the size of 

 the cumulus. Cumulus clouds sometimes form over a large 

 fire when none appear elsewhere. The heat of the fire aids 

 in sending the air current high enough to develop cloud. 

 Cumulus clouds over low islands in the sea are caused by 

 the greater heating of the land surface and the air over it. 

 The heating of a mountain slope and the upward deflection of 

 the wind blowing against the mountain side, commonly cause 

 earlier and larger cumuli over mountains. 



Cumuli are often fair weather clouds; at other times they 

 develop into showers. By watching their growth, one familiar 

 with clouds can usually see whether or not showers are prob- 

 able, and the paths they will follow. Showers will be studied 

 after briefly considering the formation of rain. 



259. Precipitation of Water Vapor ; Rain. Cumulus cloud 

 particles float in the air like fog. The air current rising 

 through the cloud aids in keeping the water droplets from 

 falling. But under the right conditions of temperature and 

 moisture, condensation goes so far that the lighter droplets 

 join together and many become heavy enough to fall to 

 earth. Rain drops reach the ground in various sizes ranging 

 sometimes up to J< in. or more in diameter. Their sizes may 

 be measured by catching the rain in an inch or so of flour and 

 measuring the pellets formed in the flour. Then a medicine 

 dropper may be used to let artificial drops fall into the flour, first 

 measuring their diameters. By comparing the sizes of the 

 flour pellets formed by the artificial drops with those formed 

 by the rain, a fair idea of the size of the rain drops may be 

 obtained. 



260. Precipitation; Snow. When the temperature in the 

 cloud is below freezing, snow forms instead of rain. Snow 

 flakes, when not too much broken by the wind, show a great 

 variety of beautiful forms. Those in Fig. 166 were caught 



