WEATHER CHANGES 



171 



cyclone is also much less in summer than in winter, as the 

 difference in pressure between the low and high areas is 

 much less. The changes in temperature as the storms 

 pass are greater in winter 

 than in summer since the 

 regions from which the 

 northerly and southerly 

 winds flow in toward the 

 center of low pressure 

 vary more in their tem- 

 peratures. 



85. Sudden Weather 

 Changes. In middle lati- 

 tudes there often occur, 

 particularly in winter, 

 sudden changes in the 



WATERSPOUT SEEN OFF THE COAST 

 OF NEW ENGLANP. 



temperature of 20 or 

 more in a few hours. In 

 our own country, if the temperature falls 20 or more in 

 24 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 (Fig. 82) displayed 

 by the Weather Bureau to indicate the ap- 

 proach 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 

 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 

 oi'ange belt that for years afterward the "freeze" was the 

 date from which events were reckoned. 



If the northwesterly wind which brings on the cold 



Fig. 82. 



