EFFECT OF EARTH'S ROTATION ON WINDS 219 



As the earth is about 25,000 miles in circumference and 

 turns on its axis once in 24 hours, a body situated at the 

 equator is carried from west to east at the rate of about 

 1000 miles per hour, whereas a body at the poles simply 

 turns around during a revolution. Thus as we go on the 

 surface from the poles toward the equator, each point has 

 an Increasing west to east velocity. 



A body of air, not being attached to the surface, will 

 have this west to east velocity imparted to it very slowly 

 by friction. Thus as it goes from higher to lower lati- 

 tudes, it will lag behind particles on the surface which have 

 this west to east velocity, and so will appear to have an east 

 to west motion ; just as to a person riding in a rapidly mov- 

 ing open car on a calm day there seems to be a strong 

 " breeze." (That the " breeze " is produced by the motion 

 of the car and not by movements of the atmosphere is 

 shown when the car comes to a standstill.) The north to 

 south movement of the air combined with its apparent east 

 to west movement will give a northeast-southwest direction 

 to the air current. 



On the other hand, suppose an air current is moving from 

 the direction of the equator toward the north pole. It has 

 greater velocity toward the east than the part of the earth's 

 surface it is approaching, and so instead of blowing due 

 north it takes a northeast course. It can be seen then that 

 whether an air current moves from the north pole toward 

 the equator or from the equator toward the north pole, it 

 will be deflected toward the right. 



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. 



