THE WEATHER 



49 



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. 



49. 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 



FIG. 30. The path of the Galveston hurricane 



Diagrammatic representation of the position of the storm on several successive 



days. Its intensity was much decreased after it left the Gulf of Mexico. After 



the United States Weather Bureau 



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 sufficiently 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. 

 These clouds are called cumulus clouds. 



