874 Handbook of Nature-Study 



unequal distance of the tropical belts of high pressure from the equator. 

 These questions may be considered together. 



It is to be remembered that the southern hemisphere is the water hemi- 

 sphere, and that the prevaihng westerhes, in ghding over the smooth water 

 surface, are but Httle retarded by friction and, therefore, attain a higher 

 velocity than the corresponding winds of the northern hemisphere, where 

 the rougher surface materially retards their movement. As a consequence, 

 the circumpolar whirl of the southern hemisphere is stronger, and develops a 

 greater centrifugal force, thus holding a larger quantity of air away from the 

 south pole and reducing the pressure to a greater degree than is brought 

 about by the weaker winds of the northern hemisphere. 



Since the circumpolar whirl of the southern hemisphere is the stronger 

 of the two, it withdraws the air to a greater distance from the pole than does 

 its weaker counterpart of the northern hemisphere, and piles it up in the 

 tropical belt of high pressure about five degrees nearer the equator than 

 does the weaker forces of the northern hemisphere. 



Storms 



Having gained a comprehensive view of the 

 general, planetary wind system, we may now 

 undertake the study of local disturbances that 

 arise within the general circulation and are 

 known as "storms." 



Storms are simply eddies in the atmosphere. 



They may be compared to the eddies that are 



often seen floating along with the current of a 



river or creek. In these eddies the water is 



seen to move rapidly around a central vertex, 



developing sufficient centrifugal force to hold 



Snow crystal. some of the water away from the center, thus 



Photomicrograph by forming a wcU marked depression, frequently 



w. A. Bentiey. of Considerable depth. The whole circulation 



of the eddy is quite independent of the current 



of the stream which carries it along its course, and while its general direction 



and velocity of movement coincide with that of the current, there are times 



when it will be seen to move quickly from side to side and again when it will 



remain nearly stationary for a time or take on a rapid movement. 



The eddies or storms in the atmosphere act in much the same way. 

 They are carried along by the general currents of the river of air in which 

 they exist. Their general direction coincides with the direction of the 

 current in which they are floating, and their rate of movement conforms in 

 a general way to its velocity; but like the eddies in the river, they do not 

 always move in straight lines nor at a uniform rate of speed. 



There is one important respect in which the eddies in the air difEer from 

 eddies in water. The water eddy may revolve in either direction, depend- 

 ing upon the direction in which the initial force was applied, but the storm 

 eddies in the atmosphere always revolve counter-clockwise in the northern 

 hemisphere, and clockwise in the southern. 



This is due to the deflecting force of the earth's rotation, which is fully 

 explained on page 872. 



