THE WEATHEK 



53 



The winds of this belt are so much disturbed by the pas- 

 sage of the cyclones that it is difficult to see any regular 

 eastward movement until one recalls that the cyclones them- 

 selves 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. It is in the western 

 sky that we usually see the first 

 indication of the coming storm ; 

 the first lightning flashes are us- 

 ually 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 phe- 

 nomena, extends entirely around 

 the earth in this latitude (fig. 33). 

 Mountain ranges which cross the 

 path of the westerly winds have 

 much heavier rainfall on the west- 

 ern than on the eastern side, 

 because the winds, as they ascend 

 the western side, are cooled and 

 some of the moisture condenses into clouds and rain, while 

 on the eastern side the air is warming as it descends and 

 becomes relatively dry. A good example of this will be found 

 by comparing the rainfall on the west slope of the Cascade 

 Mountains in Washington and Oregon with the rainfall on 

 the east side of the same mountains (fig. 34). 



In the tropical regions we have the belt of trade winds on 

 each side of the equator. The great heat of the equatorial 

 regions causes the air to expand and thus become less dense, 

 and it is pushed upward in the region of greatest heat, 

 the heat equator, which is near the true equator. In this 



FIG. 33. Diagram of the gen- 

 eral circulation of air upon 

 the earth 



These general currents are always 



more or less modified by local 



conditions 



