WATER 



21 



ocean loses much of its moisture in passing the coastal mountains. 

 The warmer the air becomes the more water vapor it can hold. 

 The sun's heat falling on the moist earth and the water causes 

 evaporation. On dry, hot days the air is like a great sponge and 

 drinks up moisture from 

 the earth, plants, and 

 everything with which it 

 comes in contact. When 

 there is much moisture 

 present in the air and the 

 temperature is high, we 

 say the weather is heavy 

 and sultry. The warm, 

 humid air can absorb but 

 little moisture, and hence 

 drops of perspiration col- 

 lect on the body. In dry, 

 arid regions there is little 

 moisture present, and the 

 rate at which evaporation 

 goes on is rapid and so 

 perspiration does not col- 

 lect on the surface of our bodies. Whenever evaporation takes 

 place there is always a lowering of temperature. In Arizona 

 water is often placed in porous earthen vessels, which are hung 

 up where the breeze or a draft of air can strike them, and the 

 evaporation going on from the surface of these vessels is sufficient 

 to cool the water until it is almost like ice water. 



Another point we should remember in considering the humid- 

 ity of the air is that, when the sun's heat is removed, the air 

 grows cool, and its ability to hold moisture decreases; and as the 

 temperature continues to fall, a point is soon reached where the 

 vapor will condense on the grass and the leaves of the plants and 

 also on roofs and stones. This is called dew, and the tempera- 

 ture at which it begins to form is called the dew-point. Do 

 not jump at the conclusion that all the drops of water you see 

 sparkling on the trees and grass have come from the air, for many 

 of them have worked their way from the ground through the plant 



RAINFAJLX. 



Little or no rain 

 Light rains 



Moderately heavy rains 

 Hcavu rains 



