220 



WEATHER AND CLIMATE 



Planetary Wind Belts. As the air at the equator re- 

 ceives a large amount of heat, it becomes warm and light, 

 while that near the poles is cold and heavy. The air would 

 thus have a constant tendency to move along the surface 

 of the earth toward the equator and in an upper current 

 from the equator toward the poles, just as in the dishes 

 where water and oil were connected. But this direct 

 movement is affected by the rotation of the earth and by 



certain atmospheric con- 

 ditions, so that between 

 25 and 35 both north 

 and south of the equator 

 there is an area of high 

 pressure. 



From these areas of 

 high pressure the surface 

 currents move both to- 

 ward the equator and 

 toward the poles. On 

 account of the earth's 

 rotation the directions of 

 these movements are not 



north and south but in the northern hemisphere northeast 

 and southwest. Winds of this kind must occur on every 

 revolving planet having an atmosphere ; hence these winds 

 are called planetary winds. 



As the rotation of the earth and the heating of the air 

 near the equator are conditions that do not change, among 

 the most permanent things about our planet are the belts 

 into which the wind circulation is divided. The change 

 in the position of the heat equator, the belt of highest 

 temperature, due to the apparent movement of the sun 



FIGURE 89. WIND BELTS OP THE 

 EARTH 



