256 THE WEATHER 



The dew point is the temperature at which air is just saturated with 

 moisture. 



Frost is formed when the dew point is below C. It is not 

 frozen dew. 



Clouds are fogs "up in the air." A cloud is limited to the region in 

 which water vapor can be condensed more rapidly than the condensed 

 moisture can evaporate. 



Clouds are distinguished, according to their form, as cumulus, cirrus, 

 stratus, and nimbus clouds. 



Rain occurs when water mist collects in drops that reach the earth. 



Snow is formed when rain-forming conditions occur below C. 

 It is not frozen rain. 



Hail usually has ice and snow in alternate layers. 



Rainfall is measured by a rain gauge. 



The mean annual rainfall of the United States varies from a few 

 inches in the deserts up to 100 inches in the state of Washington. 



The circulation that forms the winds includes horizontal currents 

 at higher levels as well as at the earth, and downward as well as upward 

 currents. 



Winds blow in curved paths toward regions of low pressure. The 

 velocity of winds is measured by the anemometer. 



Regular winds are trade winds and land and sea breezes. 



Cyclones are great whirlwinds in the air. They bring our "cold 

 waves," but are usually not destructive. In the northern hemisphere 

 they revolve in a direction opposite to that of the hands of a clock. 



The curved path of the air in cyclones is due to its motion in straight 

 lines, together with the rotation of the earth. 



A thunderstorm has a center of descending air (high pressure), which 

 draws into it ascending, moist air. 



Tornadoes are small cyclones of great violence. 



Waterspouts are tornadoes over bodies of water. 



Hurricanes and typhoons are violent cyclones formed in the tropics. 



Meteorology is the science of the weather. 



The Weather Service of the United States collects the data regard- 

 ing the weather, and from them constructs daily weather maps. From 

 these it forecasts cold waves, floods, and storms, so that their harmful 

 results may be avoided as far as possible. 



