164 GENERAL SCIENCE 



to measure the velocity of the wind, the anemometer. 

 The speed with which it revolves is registered as a certain 

 number of miles per hour. 



For purposes of classification winds may be grouped 

 into terrestrial, cyclonic, and continental winds. 



Terrestrial or Planetary Winds. On all the planets 

 which have an atmosphere, there are winds which are due 

 to the fact that the planet rotates and that it is heated 

 from some external source. On the earth the strength 

 of these winds and the boundaries of the wind belts vary 

 with the seasons. The air is heated in the region of the heat 

 equator and moves up, thus producing a low pressure belt 

 at the equator and a high pressure belt near the tropics 

 (Figure 140). 



The low pressure belt caused by the ascending air is 

 known as the equatorial calms or the doldrums, and the 

 high pressure belts caused by the descending air are known 

 as horse latitudes. 



The winds which blow from the regions of the horse 

 latitudes or tropical calms toward the equator are called 

 trade winds, so named because of the regularity with 

 which they blow. In the days of sailing vessels they 

 were important factors in navigation. The trade winds 

 do not blow directly north and south but are deflected 

 toward the west by the rotation of the earth. 



The anti-trade winds are those currents of air which are 

 high above the trade winds and blow in the opposite 

 direction. -It is the descent of these winds which pro- 

 duces the tropical winds or horse latitudes. 



The prevailing westerlies are the winds which blow 

 from the horse latitudes toward the poles. Like the 

 trade winds their course is affected by the shape and 

 rotation of the earth ; they move northeast in the north- 



