780 The Outline of Science 



8 



CYCLONIC DISTURBANCES 



Cyclones and Anticyclones 



Cyclones and anticyclones are names familiar to everyone 

 who pays attention to weather reports. They are terms applied 

 to those erratic changes which take place within the great general 

 circulation of the atmosphere which has been already described. 



Within the Tropics sudden sharp revolving storms, known 

 locally as "hurricanes," "whirlwinds," and "typhoons," sometimes 

 spring up and cause immense havoc. These storms usually 

 cover a limited area, and follow well-defined tracks. The 

 "tornado" of the United States often leaves behind it a track 

 about ten miles long and only a few hundred yards wide, but 

 within this track destruction is singularly complete. 



In our latitudes, however, the cyclonic disturbances are much 

 more frequent and are not nearly so severe ; but they cover a much 

 greater area, being sometimes 1,000 miles in diameter. 



The cause of these disturbances has been stated to be due to 

 the irregular heating of the land, and to the rapid ascent of 

 warmed air in the case of the tropical storms. But for those dis- 

 turbances in our latitudes, the meeting of two great air currents of 

 different temperature has been regarded as the chief cause by some 

 meteorologists; others believe they are due to changes that take 

 place in the temperature of the stratosphere the upper layer of 

 the atmosphere. 



Probably all these causes contribute in some measure towards 

 the production of the differences in pressure that give rise to the 

 violent winds. 



The Formation of Cyclones and Anticyclones 



Now air, as we have seen, tends to flow from high-pressure 

 areas towards low-pressure ones, just as brooks run down a hill 

 to seek the valley, but this air will, in the Northern Hemisphere, be 



