128 GENERAL SCIENCE 



AREAS OF HIGH AND Low PRESSURE 



The rotation of the earth is one of the great factors in 

 establishing the direction of the air currents of the earth as 

 a whole. At the equator the velocity eastward of the earth's 

 surface is about seventeen miles per minute (25,000 miles 

 in 24 hours) . The atmosphere resting on the land and water 

 surfaces shares in this eastward velocity. It is a rate much 

 greater than the drift of the upper air currents from the 

 equatorial region toward the poles. 



As a resultant of these two motions (velocities) northward 

 and eastward, the upper air currents of the northern hemi- 

 sphere instead of following a meridian due northward drift 

 toward the northeast, and perhaps somewhat more easterly 

 than northerly. 



By reason of the earth's rotation, too, the winds (surface 

 currents) in the northern hemisphere blowing toward the 

 equator pass over regions of greater and greater eastward 

 velocity as they approach the equator. This southward 

 moving air does not acquire this eastward motion of the 

 earth's surface readily enough to maintain a southerly 

 direction along a meridian, but is all the time being left 

 behind (deflected) to the westward. Its direction across the 

 earth's surface instead of being due south is southwesterly, 

 i.e., it becomes a northeast wind. 



As the causes which accomplish these results are con- 

 tinuously operative, the trade winds of the northern hemi- 

 sphere as well as like winds in the southern hemisphere are 

 quite constant both in direction and in velocity. This 

 movement is especially marked on the oceans where dis- 

 turbing factors do not enter in to overcome or divert them. 



In the middle latitudes, as in the United States, this 

 planetary circulation (air currents of the earth as a whole) 

 controls wind direction only in part. The course taken by the 



