188 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



THE HEAT THUNDERSTORM 



Hot humid days with Uttle or no general horizontal movement of 

 air near the earth's surface are favorable to the formation of heat 

 thunderstorms. Such days are lilvcly to occur when horizontal pres- 



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sure gradients are weak and temperature is high over extended regions. 

 Typical meteorological conditions of this character are illustrated by 

 the weather map reproduced in figure 4 and described by Humphreys " 

 in Physics of the Air. 



" Humphreys, W. J., Physics of the Air, McQraw-Hill Book Co., Iiic.,|New York, 1929. 



