44 



ELEMENTS OF GENERAL SCIENCE 



41. General circulation of air on the earth. The various sorts 

 of winds and storms which we have been considering are com- 

 mon to the United States and to other countries that are in 

 the same latitude, but they do not occur in the tropics or in 

 some other parts of the globe. In fact, the surface of the earth 

 may be divided into zones, or belts, characterized by different 

 sorts of winds. The belt which we have been discussing is 

 called the belt of westerly winds, because the lower winds are 

 from a westerly direction more frequently than from any other. 



It is true also that the upper air 

 currents in this region are from 

 the west, as is shown by the move- 

 ment of the higher clouds. This 

 is always from the west. The winds 

 of this belt are so much disturbed 

 by the passage of the cyclones that 

 it is difficult to see any regular 

 eastward movement until one re- 

 calls that the cyclones themselves 

 move eastward. This eastward 

 movement is doubtless due to the 

 fact that they are merely great 

 whirls in the greater mass of 

 eastward-moving air. It will also 

 be recalled that thunderstorms and 

 tornadoes move eastward, as indeed is true of practically all 

 weather conditions. This general eastward movement is so 

 universal that we are accustomed to look 'at the western sky 

 to detect the approach of weather conditions. It is in the west- 

 ern sky that we see the first indication of the coming storm ; 

 the first -lightning flashes are usually seen in the west; and it 

 is in the west that the clearing of the sky begins after the storm. 

 This belt of westerly winds, with its cyclones and other 

 phenomena, extends entirely around the earth in this lati- 

 tude (fig. 31). Mountain ranges which cross the path of the 



FIG. 31. Diagram of the gen- 

 eral circulation of air upon the 

 earth 



These general currents are always 



more or less modified by local 



conditions 



