REGULAR WINDS 245 



horizontal currents at the earth's surface are the only parts of this 

 circulation that we ordinarily notice, but there are always horizontal 

 currents at higher levels, flowing in the opposite direction; and there 

 are downward currents to balance the ascending ones. 



The force with which the wind will blow toward any 

 given place depends upon the difference in atmospheric 

 pressure between that place and the region about it. 

 Thus, if the barometer pressure at one place (A) on a 

 horizontal plain is 29.9 inches, and if two neighboring 

 places (B) and (C) have pressures of 29.8 inches and 29.7 

 inches, respectively, the velocity of the wind from A to C 

 will exceed that from A to B, or from B to C. We should 

 expect this to be so, since we know that water will flow 

 more rapidly down a steep incline than down a gentle one. 



We can gather these facts together in one law of the 

 winds: 



The winds blow because of the force of gravity, and always 

 from a region of high barometer pressure to one that is low. 

 The velocity of the wind is greatest where the change of 

 pressure is the most abrupt. 



The instrument by which we measure the velocity of the wind is the 

 anemometer (pronounced an-e-mom'-e-ter; the word is from anemos, 

 "wind," and meter, " measure"). It is a windmill with hollow, cup- 

 shaped sails; the speed with which it revolves is registered on a dial as 

 so many miles per hour. A light wind has a speed of less than 10 miles 

 an hour; it can just move the leaves of trees. A high wind, blowing at 

 25 to 40 miles an hour, sways the trees themselves. In a gale the wind 

 is over 40 miles an hour; in a hurricane or a tornado (cf. 278 and 279) 

 it is above 60 miles, and may reach 200 or more. 



275. Regular Winds. On the land, the unevenness of 

 the surface and the different degrees of heating and cool- 

 ing at different places produce variable winds; but the 



