THE WEATHER 41 



origin near the West Indies. They usually move westward 

 toward the coast of the southern United States and then grad- 

 ually curve northward and northeastward and pass out across 

 the Atlantic Ocean with decreasing severity (fig. 28). It is 

 very 'common for them to pass almost parallel to our eastern 

 coast and to do a great deal of damage to shipping. Occasion- 

 ally one of them goes far enough west to enter the Gulf of 

 Mexico. In the year 1900 the city of Galveston was destroyed 

 by a hurricane which had taken this unusual course. 



These storms are of very much greater severity than the 

 cyclones and often do a great deal of damage in the West 

 Indies and occasionally upon the mainland. 



39. Thunderstorms. In the warmer parts of the year the 

 layers of air next to the surface of the earth may become 

 much warmer than the air higher up, and this difference may 

 become so great that the lower layers are distinctly lighter 

 than the upper layers. If this is the case, the lower air begins 

 to rise through the upper air in ascending currents somewhat 

 like the ascending currents of air over a heated stove. At a 

 height of several thousand feet the ascending air may be suf- 

 ficiently cooled to cause the condensation of some of the water 

 and the formation of the dome-shaped white clouds which 

 are such familiar objects in summer skies. They are called 

 cumulus clouds (fig. 17). 



If the conditions are such as to cause very vigorous move- 

 ments of the air, and if there is a good deal of water in the 

 lower air, the clouds may grow until they become heavy and 

 dark and rain begins to fall. The fall of rain is accompanied by 

 lightning and thunder, as well as by considerable wind. After 

 it has passed, the temperature is cooler because the warm air 

 has flowed away upward and has been replaced by cooler air 

 from above. 



Thunderstorms usually occur in the southerly part of a 

 cyclone during warm weather and in the warmer part of the 

 day, but they may occur even hi the winter. Thunderstorms 



