THUNDERSTORMS 229 



When, instead of the strong, cold, northwest winds which 

 blow into the rear of a cyclonic area and in the colder seasons 

 may produce a cold wave, there is a prolonged movement 

 of highly heated air from the south into the front of the 

 low pressure, as sometimes occurs during the warm months, 

 the " hot spells of summer " are caused. The air is sultry, 

 exceedingly hot and oppressive. Sunstrokes and prostra- 

 tions from heat are common. The " hot winds " of Texas 

 and Kansas, the Santa Ana of lower California and the 

 siroccos of southern Italy are intensified examples of these 

 winds. All sudden weather changes of this kind are due to 

 atmospheric conditions related to areas of low pressure. 



Thunderstorms. Often on a hot, sultry summer after- 

 noon large cumulus clouds are seen to rise and spread out 

 till they cover the sky. The wind soon begins to blow 

 quite strongly toward the cloud-covered area, the clouds 

 moving in a direction opposite to the surface wind. As 

 the storm clouds approach, a violent blast of wind, often 

 called the thundersquall, blows out from the front of the 

 storm. Soon flashes of lightning appear and thunder is 

 heard. As the storm comes nearer, the rain begins to descend 

 and for a short time, usually about half an hour, it rains 

 heavily. Then the clouds roll away and the sky becomes 

 clear with perhaps a rainbow to heighten the' beauty of the 

 clearing landscape. 



Thunderstorms are caused by hot moist air rising over 

 certain areas and causing an updraft, which is increased 

 by the inflow and upward movement of air from the sur- 

 rounding regions. The condensation of the moisture in the 

 rising air quickly forms clouds, and these become charged 

 with electricity. As the electrical charge increases, dis- 



