MOVEMENTS OF THE ATMOSPHERE 



43 



the liiiL;li pressure belt in aliout lat. 35° S., while in July the center 

 shifts to 30° or less. (Fig. 7.) The shifting of the center of high 

 pressure is, therefore, in harmony with the apparent motion of the 

 sun. 



The general distribution of pressure areas is somewhat as fol- 

 lows : I, an equatorial belt of low pressure; 2, two belts of high 

 pressure in about latitudes 30° N. and 30° S., respectively ; and 3, 

 two high latitude areas of low pressure. In general there should 

 thus be a horizontal movement of air in the lower part of the atmos- 

 phere from the extra-tropical belts of high pressure, both poleward 



i i ( ( ( ( i ( i ( ' \ ( ( ( '\ ( ( ( '\ ^ 





Fig. 7. Diagram illustrating 

 the seasonal shitting of 

 the wind zones. (After 

 Davis.) 



Fig. 8. Generalized diagram illustrating 

 the deflection of the winds at the 

 bottom of the atmosphere, due to 

 rotation of the earth. (After Salis- 

 bury.) 



and equatorward. (Fig. 6.) This motion is modified by the rota- 

 tion of the earth, which causes a deflection of all currents to the 

 right in the northern hemisphere and to the left in the southern. 

 (Salisbury-83 : (5oj, Fig. 8.) As a result, the poleward winds in 

 each hemisphere are turned toward the east, and so become zvcsterly 

 winds, or southwest winds, in the northern hemisphere, and north- 

 west winds in the southern hemisphere. The winds blowing equa- 

 torward will be deflected to the west, and so become easterly xvinds, 

 northeast winds in the northern and southeast in the southern hemi- 

 sphere. These are the most constant winds and are known as the 

 trade zvinds. Where these currents meet at the equator, we have 

 the zone of equatorial calms, characterized chiefly by rising air 



