Weather Study 871 



are fulfilled by the high temperature and low pressure found at the equator. 

 Furthermore, the temperature decreases north and south from the equator, 

 and thus the belts of high pressure near the tropics may be taken to repre- 

 sent the conditions at the cool margins. The first and second principles of 

 a convectional circulation, viz., low pressure at the warm center and a high 

 pressure at the cool margins, are thus fulfilled. To satisfy the remaining 

 conditions, we should find ascending currents near the equator, upper cur- 

 rents flowing from the equator toward the tropical belts of high pressure, 

 descending currents at the tropics, and surface winds blowing from the 

 tropics toward the equator. Let us now examine the surface winds of the 

 world as illustrated by the diagram on page 867. 



On either side of the equator and blowing toward it, we find the famous 

 trade winds the most constant and steady winds of the world. Their 

 northern and southern margins coincide with the tropical belts of high pres- 

 sure. They blow from high pressure to low pressure and we cannot doubt 

 that they act in obedience to the fifth principle of convectional circulation. 

 From observation of the lofty, cirrus clouds in the trade wind belts, we have 

 abundant evidence of upper currents, flowing away from the equator toward 

 the tropical belts of high pressure ; thus the sixth principle is satisfied. The 

 torrential rains and violent thunderstorms, characteristic of the equatorial 

 regions, bear evidence to the rapid cooling of the ascending currents near the 

 equator ; while the clear, cool weather and light winds of the Horse Lati- 

 tudes clearly indicate the presence of descending currents at the tropics. 

 Thus, the six principles of a convectional circulation are satisfied, and the 

 evidence is conclusive that the trade winds form a part of a convectional 

 circulation between the tropical belts of high pressure and the equatorial 

 belt of low pressure. 



You have doubtless observed that the trade winds do not blow directly 

 toward the equator but are turned to the west so that they blow from the 

 northeast in the northern hemisphere, and from the southeast in the south- 

 ern. This peculiarity is not in strict accord with our ideas of a simple con- 

 vectional circulation and suggests, at least, the presence of some outside 

 influence. If we turn to Ferre's treatise on the winds, we find a demonstra- 

 tion of the following principle : a free moving body, such as air, in moving 

 over the surface of a rotating globe, such as the earth, describes a path on 

 the surface that turns to the right of the direction of motion in the northern 

 hemisphere and to the left in the southern. The curvature of the path 

 increases with the latitude, being zero at the equator and greatest at the 

 poles, and is independent of direction. With this in mind, if we take posi- 

 tion at the northern limit of the trade winds in the northern hemisphere and 

 face the equator, (see p. 867), we find that the winds moving toward the 

 equator turn to our right ; likewise, if we face the equator from the southern 

 limit of the southeast trades, we find them turning to our left. Observa- 

 tions of upper clouds in the trade wind belts show that the upper currents 

 also turn to the right in the northern hemisphere, and to the left in the 

 southern. It is, therefore, clear that the systematic turning of the trade 

 winds from the meridian is due to the rotation of the earth. The value of a 

 force at various latitudes and for various velocities that would cause a body 

 to turn away from a straight line, is purely a problem in mathematics, and 

 for the benefit of those versed in the science the formula is given. The 

 amount of such a force is expressed by 2 MVW sin D, where M is the mass, 

 V the velocity, W the angular rotation of the earth, and D the latitude. 



