872 



Handbook oj Nature-Study 



Not all of us may be able to solve the problem, but we may understand 

 something of the effect of the rotation of the earth on moving wind currents. 

 It is a well-known principle of physics that if a body be given a motion in 

 any direction, it will continue to move in a straight line by reason of its 

 inertia, without reference to north, south, east or west. A personal 

 experience of this principle may be gained in a street car while it is rounding 

 a curve. 



In this diagram, we have a view of the northern hemisphere. The direc- 

 tion of the rotation is indicated by the curved arrows outside the circle 

 representing the equator. Suppose that a wind starts from the equator, 

 moving along the meridian A directly toward the north pole. It is clear 

 that it cannot continue to move along the meridian, because the direction of 

 the meridian with reference to space, is continually changing, and the inertia 

 of the wind compels it to move in a straight line without reference to the 

 points of the compass. So when the meridian A has been moved to B by 



the rotation of the earth, the wind, al- 

 though it maintains its original direc- 

 tion, no longer points toward the pole 

 but to the right of the pole. Like- 

 wise, a wind starting from the pole 

 toward the equator also turns to the 

 right of the meridians and becomes a 

 northeast wind as it approaches the 

 equator. A wind moving east or west, 

 also turns to the right of the parallels 

 for the same reason. So a wind start- 

 ing out from the equator with the best 

 / possible intention of hitting the pole, 

 and all the while continuing in the 

 same straight line, will miss the pole by 



Diagram showing effect of earth's rotation man Y miles - and alwa j s OI } 



on the atmosphere. 



side in the northern and on the lett side 

 in the southern hemisphere. Thus, 

 the oblique movement of both the trade winds and the prevailing westerlies 

 is accounted for. 



It now remains to consider the cause of the unexpected low pressure 

 found at the poles, and the reason for the belts of high pressure at the 

 tropics. If we refer to Fig. 2, it is evident that not all the air that ascends 

 at the equator descends at the tropics, else there would be an absence of air 

 at the higher latitudes, which is manifestly not the case. On the other 

 hand, it is equally impossible that all the air ascending at the equator 

 should move to the poles, because the space it could occupy decreases 

 rapidly from a maximum at the equator to zero at the poles. Only a part 

 of the air that ascends at the equator is, therefore, involved in the trade 

 wind circulation and a part passes over the tropics and moves on toward 

 the low pressure at the poles. Furthermore, some of the air that descends 

 at the tropics moves along the surface toward the poles, obeying the law 

 that impels air to move from high pressure to low pressure. Now every 

 particle of air that passes over the tropics, every particle that moves north- 

 ward along the surface, turns to the right in the northern and to the left in 

 the southern hemisphere. All, therefore, miss the poles on the right side 

 in the northern and on the left side in the southern hemisphere. The result 



