192 ELEMENTARY SCIENCE 



up from the Gulf and bring these rains, it is necessary to 

 understand something about cyclones. 



A Cyclone Is a Wind Which Rotates. The thing about 

 which it rotates is an area on which the atmospheric pres- 

 sure is either higher or lower than it is on the surrounding 

 areas. When you hear the word cyclone you usually think 

 of a terrific storm, but most cyclones are not terrific at all. 

 The ones which are terrific are properly called tornadoes 

 instead of cyclones. You have often seen cyclones, at 

 least in miniature. On hot days you have seen little as- 

 cending whirlwinds of dust that rise, and move along, and 

 get you all dusty if they happen to come your way. These 

 are miniature cyclones. 



Now the cause of this miniature cyclone is this. Some 

 of the air, nearest the earth, has become heated (mostly 

 by conduction) until it is decidedly lighter than the air 

 which overlies it. At some point where the heating is 

 particularly strong, this lighter ah* begins to rise, and 

 breaks a passage through the cooler air above. Into this 

 passage more of the low-lying hot air rushes; soon it is 

 ascending from the ground in a whirling column which 

 carries dust up with it, and moves along erratically before 

 the breeze, becoming dissipated as its supply of hotter 

 air becomes exhausted, and the pressures become equalized. 



Cyclones similar in principle to these little whirlwinds 

 occur on a great scale. They may involve hundreds or 

 even thousands of square miles, and they may have much 

 to do with the distribution of rain. Such cyclones center 

 upon areas of low pressure; air is drawn inward toward its 

 center and there passes upward. So, wherever we see on 

 weather-maps centers (areas) of low pressure with higher 



