THE WEATHER 47 



46. Weather in anticyclones. The air which flows into a 

 cyclone usually rises when it reaches the center of the storm, 

 and thus the circulation is kept up. An anticyclone is an area 

 in which the cooler air of the upper regions is settling to the 

 surface and flows away in all directions as described. Since 

 this air at the surface of the earth has come from the cooler 

 regions above, it is getting warmer, and as it flows away from 

 the center of the " high " most of it is still going to warmer 

 regions. Since it is becoming warmer, it is relatively dry. 

 For these reasons the weather in a region covered by a high- 

 pressure area is usually cool, clear, and dry, with a west or 

 northwest wind. Most of us have no difficulty in recalling 

 the bracing air of the pleasant days which follow a period 

 of clouds and rain, and this is typical anticyclone weather. 



47. Hot and cold waves. A large low-pressure area some- 

 tunes draws so much air from the south into the northern 

 part of the country that the temperature of the north rises 

 unseasonably high, producing a period of very warm weather 

 which is usually called a hot wave, or a sirocco. In the west- 

 ern part of the Mississippi Valley the crops are sometimes 

 seriously damaged by hot winds coming off the deserts of the 

 southwestern part of the United States. 



If a high-pressure area remains for a while in the North- 

 west, the cold air which comes from it may spread over the 

 greater part of the country (fig. 29) ; and if the fall in tem- 

 perature is rapid and of considerable amount, it is called 

 a cold wave. The presence of low pressure over the Gulf of 

 Mexico or over the Atlantic Ocean near our southern coast 

 will usually assist in the progress of the cold wave. It occa- 

 sionally happens that the freezing temperatures reach the Gulf 

 coast and even extend far down the Florida peninsula. In 

 these cases there is great loss resulting from the freezing of 

 orange trees and the crops of early vegetables. On the great 

 plains of the West the cold wave may be accompanied by a 

 strong wind and considerable driving snow. In this case it is 



