Weather Study 



867 



toward the northwest, hence the name Southeast Trade Winds. 

 The dividing line, or bank, between the rivers in each hemisphere 

 belts the earth at about 3 5 north and 30 south of the equator. Why 

 does the air move and why does it move in such a regular, systematic 

 manner? To answer these questions we will rely upon gravity, the heat 

 from the sun and the effect of the rotation of the earth on moving wind 

 currents. 



Everyone knows that water flows down hill because of the force of 

 gravity. Gravity is nature's great peacemaker. It is always trying to 

 settle disturbances, even things up, smooth them over. If there were 

 no winds to bring rain to the land or to stir up the ocean, gravity would 



NORTH-EAST TRADES 



EQUATOR 



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SOUTH-EAST TRA D ES 



WINDS OF THE WORLD 



soon run all the water into the lakes and the seas, and then smooth them 

 out like sheets of glass; and if there were nothing to stir up the winds, 

 gravity would soon settle all differences in the atmosphere and the air would 

 become perfectly quiet. So gravity is kept busy trying to smooth out the 

 water which the wind stirs up, at the same time trying to quiet the winds 

 which are stirred up by the heat of the sun. 



Tyndall says that heat is a mode of motion, that when heat is imparted 

 to a substance the molecules of which it is composed are set into very rapid 

 vibration. They are continually trying to get away from each other and 

 usually succeed in getting more space, and thus increase the size or volume 

 of the substance, or, in other words, expand it. Iron, brass, copper, water 

 and many other substances expand under heat. Air is a gas and expands 

 very rapidly when heated. One cubic foot of cold air becomes two cubic 

 feet when heated. Now gravity pulls things down toward the center of 

 the earth in accordance with their weight-density, and a cubic foot of cold 

 air, being more dense and thus heavier than an equal volume of warm air, is 

 pulled down with greater force. We, therefore, say that warm air is lighter 

 than cold air, and if lighter it will rise. What it actually does is to press 

 equally in all directions and when a place is found where there is less 



