228 WEATHER AND CLIMATE 



cooler into warmer regions and their capacity for moisture 

 is increasing. As the center of the cyclone passes, therefore, 

 the clouds generally begin to clear and the atmosphere begins 

 to cool. 



Sudden Weather Changes. In middle latitudes there 

 often occur, particularly in winter, sudden changes in the 

 temperature of 20 or more in a few hours. In our own 

 country, if the temperature falls 20 or more in twenty-four 

 hours, reaching a point lower than 32 F. in the north or lower 

 than 40 in the south it is known technically as a cold wave, 

 and there is a special flag (Figure 91) displayed by the 



Weather Bureau to indicate the approach of 



such a change. 



When these waves extend over the southern 



part of the country, they are very destructive 



to the orange groves and delicate crops and are 

 FIGURE 91 known as " freezes." A notable freeze of this 



kind occurred in 1886 and did tremendous 

 damage to the orange groves of Florida. So great was the 

 effect upon this important industry throughout the orange 

 belt that for years afterward the " freeze " was the date 

 from which events were reckoned. 



If the northwesterly wind which brings on the cold wave 

 is .accompanied by snow, it is called a blizzard, and on the 

 plains and prairies, where the wind has a clear sweep, it 

 is much dreaded. Cattle and men, when caught in it, fre- 

 quently perish. In southern Europe the coldest winds 

 are from the Siberian plains and are therefore northeasters. 

 In the United States the cold area is at the southwest and 

 rear of the cyclone, whereas in Europe it is at the north 

 and front. 



