122 Our Surroundings 



In freezing, the vapor crystallizes into many shapes and forms of 

 snowflakes which, under certain conditions, fall to the earth in 

 large quantities and cover it with a white blanket. Although it 

 sometimes fills the highways, making travel difficult, snow also 

 serves some useful ends. In falling, it entangles considerable air 

 which is a poor conductor of heat. Thus the mass of snow pro- 

 tects plants, especially winter wheat crops, from freezing during a 

 cold winter by preventing the escape of heat from the soil. It also 

 gradually melts, filtering into the soil, and in this way feeds 

 springs, thereby aiding in keeping a fairly uniform flow of water 

 in the streams, and conserving the water supply. 



Rainfall. We commonly think of rainfall as merely the 

 rain that falls; we do not consider the snow. Weather men, 

 however, include snow, hail, and sleet in rainfall, according to 

 the amount of water these will form when melted. In the study 

 of weather, no topic is of greater interest to man than rain, 

 due to the fact that he realizes its great value. If it were not for 

 the rainfall, our streams and lakes would cease to exist, our lands 

 would become deserts and our food supplies would fail. 



The atmosphere always contains some water vapor. This is 

 shown by the constant formation of clouds, fog, dew, snow and 

 frost as well as rain. Rain occurs when the water vapor in the 

 air condenses into water particles heavy enough to escape from 

 the clouds and fall to the earth. When air is cooled its capacity 

 to hold moisture is lessened. Vapor, then, condenses when cooled 

 by cold currents of air. These currents reduce the temperature 

 so that the air can no longer hold all of its moisture. The greatest 

 amount of moisture which the air can hold is called the point of 

 saturation. 



Breathe for a short time on a cold window pane and notice 

 that small drops of moisture appear on the glass. This is caused 

 by the condensation of the water vapor blown from your mouth. 

 In the same manner, on a larger scale, water vapor in the air 

 condenses when it comes in contact with a colder mass of 

 air. 



The humidity, or amount of moisture present in the air, affects 

 our bodily health and comfort. If air is too dry, it absorbs mois- 



