ATMOSPHERIC EDDIES 9 



Barometer and Wind Indications 



(W. M. Wilson) 



The mercurial barometer is the instrument used for all observations 

 when great accuracy is required, but an aneroid barometer is more 

 convenient, less liable to injury, and will answer all practical purposes. 



Attention need not be given to the legends fair, changeable, stormy, 

 etc., that usually appear on the face of the instrument, because 

 changes in pressure are much more important indications of approach- 

 ing weather than the actual pressure at a given time. 



To forecast the weather accurately, the force and direction of the 

 wind should always receive equal consideration with the changes of 

 pressure as indicated by the barometer. 



The following general statements may aid in showing the relation of 

 wind, pressure, and weather : 



The atmosphere may be compared to an ocean of air that rests 

 upon the earth just as the water rests upon the bed of the oceans. 

 There are great currents of air in the atmosphere, just as there are 

 great currents or rivers of water in the oceans. 



Storms are eddies in the atmosphere, and float along in the currents 

 or rivers of air very much like the eddies often seen floating on the 

 surface of a river. 



All of the United States and Canada, except the southern part of 

 Florida, lies at the bottom of a great river of air that flows from west 

 to east around the world with the north-pole at the center. It is 

 called the circumpolar whirl. And as the storms in this latitude 

 are eddies in the north-circumpolar whirl, they float along from west 

 to east in the current of this river of air. 



The air always whirls about the center of every storm-eddy in the 

 same direction counter-clock-wise in the northern hemisphere and 

 clock-wise in the southern hemisphere. Therefore, if a storm-eddy in 

 the latitude of the United States is approaching, the winds will first 

 be from a southerly direction, and when the center of the storm has 

 passed, the wind will come from a northerly direction. 



If the center of the storm passes north of the observer, the wind will 

 change from S.E. to S., then to S.W., and finally to W. or N.W. as 

 the storm passes on its way eastward. 



If the center of the storm passes south of the observer, the wind 



