564 Principles of Weather Forecasting. 



represented in Fig. 262. In the two tropical and two polar 

 zones the winds move toward the equator, while in the two 

 temperate zones they move away from the equator. 



Above the earth's surface the directions of the wind are 

 the reverse of those found below, that is, over the tropics 

 and in the polar regions the upper winds move toward the 

 poles, while over the temperate zones the upper winds are 

 toward the equator. 



717. Direction of Wind Modified by Form and Rotation of 

 the Earth. The shape of the earth and its rotation upon 

 its axis greatly modify the direction of winds. The rota- 

 tional velocity of the earth's surface at the equator is about 

 1,000 miles per hour toward the east. As the distance to- 

 ward the poles increases the eastward velocity decreases. 

 When therefore air moves toward the poles it travels east- 

 ward faster than the land it approaches and hence blows 

 from a westerly toward an easterly direction. 



On the other hand air moving toward the equator passes 

 over land traveling eastward more rapidly than it does, and 

 hence these winds fall behind and appear to blow from 

 some easterly toward some westerly direction. 



The surface winds in the tropics and polar regions are 

 northeast or southeast, according to which hemisphere 

 they are in, while the upper winds of the same zones have 

 the reverse direction. In the temperate zones the winds 

 are southwest or northwest at the surface and northeast 

 or southeast above, according as they are north or south 

 of the equator. 



718. Character of the Winds. Winds blowing toward 

 the equator or descending from the upper regions have a 

 tendency to be dry and to maintain a clear sky. On the 

 other hand winds moving toward the poles, or rising to 

 greater altitudes, tend to become more and more nearly sat- 

 urated with moisture and hence to produce cloudy skies 

 and precipitation. 



The reasons for these relations are found in the fact that 



