148 FIRST YEAR SCIENCE 



an east to west motion, just as a person sitting in a train 

 that is just starting appears to be sitting still and the 

 objects outside seem to move in the opposite direction. 

 The combination of the north to south movement with 

 the apparent east to west movement gives a northeast 

 southwest direction to the air current. 



It can be proved mathematically that all freely moving 

 bodies on the earth's surface are deflected toward the right 

 in the northern hemisphere and toward the left in the 

 southern hemisphere. This statement is called Ferrel's law. 



73. Planetary Wind Belts. As the air at the equator re- 

 ceives a large amount of heat, it becomes warm and light, 

 while that near the poles is cold and heavy. The air would 

 thus have a constant tendency to move along the surface 

 of the earth toward the equator and in an upper current 

 from the equator toward the poles, just as in the dishe-s 

 where water and oil were connected. But this direct 

 movement is affected by the rotation of the earth and by 

 certain atmospheric conditions so that between 25 and 35 

 both north and south of the equator there is an area of 

 high pressure. These high-pressure areas can be seen on 

 the isobar maps of January and July. 



From these areas of high pressure the surface currents 

 move both toward the equator and toward the poles. On 

 account of the earth's rotation the directions of these 

 movements are not north and south but in the northern 

 hemisphere northeast and southwest. Winds of this kind 

 must occur on every revolving planet having an atmos- 

 phere; hence these winds are called planetary winds. 



As the rotation of the earth and the heating of the air 

 near the equator are conditions that do not change, among 

 the most permanent things about our planet are the belts 

 into which the wind circulation is divided. The change 

 in the position of the heat equator, the belt of highest 



