166 GENERAL SCIENCE 



curved route. The winds coming from all sides from 

 similarly curved routes set the air at the center in a whirl. 

 These cyclonic whirls cover large areas, being frequently 

 1000 miles or more in diameter. They should not 

 be confused with the violent whirling storms which 

 are so commonly called cyclones. Such storms are 

 properly called tornadoes (see United States weather map 

 showing cyclonic (low) and anti-cyclonic (high) areas) 

 (Figure 141). 



In the northern hemisphere the direction of the rotation 

 of the air in a cyclonic whirl is opposite to that of the hands 

 of a clock. In the southern hemisphere the direction is 

 the same as that of the hands of a clock. 



The general direction in which the whole cyclonic area 

 moves is from west toward the east in the 1 United States. 

 Sometimes cyclones enter the United States from south- 

 western Canada and travel southeast to the Mississippi 

 valley and then northeast to the Atlantic Ocean. Some- 

 times they njove almost due east across the continent and 

 at other times they develop in the southwest and move in a 

 northeasterly direction. They travel at varying rates of 

 speed, averaging 600 to 700 miles a day. 



An anti-cyclone is an area of high pressure. It is the 

 opposite of a cyclone, the winds blowing away from the 

 center instead of toward the center. 



Hurricanes. Very violent storms, known as hurricanes 

 in the Atlantic Ocean and typhoons in the Pacific Ocean, 

 occasionally develop in the doldrums. They grow to be 

 several hundred miles in diameter and are accompanied 

 by violent winds, which whirl in great spirals around 

 an area of low pressure and an enormous fall of rain. 

 They usually come when the heat equator is farthest from 

 the geographical equator. They occur in the Atlantic 



