RESULTS OF CHANGING AIR PRESSURE 



63 



mean great pressure. Every inch of mercury pressure (on bar- 

 ometer) is equal to about .49 pound per square inch, so that in the 

 diagram weather map, next page, the difference in the pressure be- 

 tween Florida and Ohio would be about \ pound per square inch or 

 more than 1,000,000 tons per square mile. Between what other 

 places would there be such pressure? Suppose the difference in pres- 

 sure was only .5 of an inch, 2 inches, 1 inch? 



How much difference in the pressure per square foot would there 

 be between places having a difference of 2 inches mercury pres- 

 sure? How many pounds per square mile? 



These great eddies or swirls in the atmosphere are known as 

 " cyclones " and " anticyclones." The eddies, hundreds of miles 

 in diameter, move eastward bodily across the country at an average 

 speed of something like 500 to 1000 miles a day. While they are 

 near us, we generally have strong winds; as they move away, the 

 winds subside. Cyclonic areas are those in which the air moves 

 around toward a region of low pressure. Anticyclonic areas are 

 areas in which the air 

 moves outward from a re- 

 gion of high pressure. 

 (Refer to diagram.) In 

 our latitudes cyclones and 

 anticyclones succeed each 

 other every two or three 

 days. 



How the Cyclones 

 Affect the Direction of 

 Local Winds. The wind 

 in Michigan will blow 

 toward the low area which 



is marked on the weather map, Fig. 54a and b as a low. This wind 

 would be called the north wind. Florida would get a southeast 

 wind. New Jersey would get an east wind. 



If the low passed south of New Jersey the people of that State 

 would get a northeast wind. If the low went north of the State, 

 a southeast wind would blow, shifting as the low moved. 



How Low Affects Weather* As the winds come in from over 



FIG. 54. Photograph of a distant tornado. 



