136 GENERAL SCIENCE 



90. Sources of Water. The sun is the great natural 

 energy producer for the earth. The heat coming directly 

 from the sun causes great quantities of water to evaporate 

 from the land, the rivers, the lakes, and the ocean. Why 

 does more water evaporate from the ocean than from 

 elsewhere? The water which is thus evaporated is 

 carried by the winds to various parts of the earth. Warm 

 air can carry more moisture than cool air. When cool 

 air is warmed its capacity for carrying water is increased. 

 When warm air is cooled its capacity for carrying water 

 is decreased. When winds from the ocean pass over a 

 high mountain, they are cooled so that they are unable 

 to carry the heavy load of moisture which they brought 

 from the ocean. The moisture condenses, forms clouds, 

 and falls as rain. 



When the warm winds coming from the south are grad- 

 ually cooled, their capacity for carrying moisture is de- 

 creased, so clouds are formed and rain is the result. 

 When a cold wind from the north meets a warm wind 

 from the south, clouds are formed suddenly and heavy 

 rain falls. Since air expands when it is heated, it is 

 forced upward by the cooler air surrounding the heated 

 area. The air rising rapidly carries a large amount of 

 moisture with it, but the air is cooled rapidly in the high 

 altitudes and clouds are formed. Such cloud formations 

 may sometimes produce what is known as a "cloud burst," 

 which is nothing more than a sudden condensation of the 

 moisture in the air and the descent of the moisture to 

 the earth in the form of rain. 



The continuous evaporation of water, the distribution 

 of the vapor by the wind from place to place, the con- 

 densation of the vapor, the formation of clouds, and the 

 falling of rain keep the water moving about over the 



