CHAPTEE XII. 



LOCAL WINDS. 



There are so many causes that will produce 

 air motion that it is often difficult to deter- 

 mine just what one is the chief factor in caus- 

 ing the direction of the wind at any particular 

 time. There are very many instances, how- 

 ever, where the cause can be traced without 

 difficulty; many of these have already been 

 mentioned and there are many more that 

 might be. Of course, as has been often stated, 

 there is only one remote cause for all winds, 

 and that is the sun, coupled with the move- 

 ments of the earth. But there are certain 

 local conditions that are continually modify- 

 ing the phenomena of air movement. The 

 velocity of winds as they occur from day to 

 day varies very greatly with the height above 

 the surface of the earth; ordinarily the ve- 

 locity at 1000 feet above the earth will be more 

 than three times greater than it is at 50 or 60 

 feet above, and even at 60 feet the velocity is 

 much greater than at the surface of the earth. 

 This is due partly to the retarding effect of 

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