274 PHYSIOGRAPHY 



hemisphere, and to the left in the southern. Other things also inter- 

 fere with the north and south directions of the winds, as we shall see. 



Effect of the extra-tropical belts of high pressure. Leaving 

 now the effect of unequal heating, let us note the effect of the high- 

 pressure belts just outside the tropics (Fig. 222). From these 

 belts the air flows to areas of lower pressure on either side, at the 

 bottom of the atmosphere. If it were not for the rotation of the 

 earth, the wind would blow north and south from each high-pressure 

 belt. But the rotation of the earth causes the winds to turn to the 

 right in the northern hemisphere, and to the left in the southern, as 

 shown in Fig. 231, which represents the prevailing direction of the 

 winds, at the bottom of the air, in low and middle latitudes. This 

 figure shows distinct zones of winds. 



Wind zones. The poleward winds from the high-pressure 

 belts are turned toward the east in both hemispheres, and so be- 

 come westerly winds (southwesterly in the northern hemisphere, 

 and northwesterly in the southern). The winds blowing from the 

 belts of high pressure toward the equator become easterly (north- 

 easterly in the northern hemisphere, and southeasterly in the 

 southern), and are known as trade-winds. The trade-winds are 

 remarkably constant, and have been known for ages by navigators, 

 who have often taken advantage of them. Sailing from the Canary 

 Islands to find Asia, Columbus came under the influence of the 

 trade-winds, which bore him steadily across the Atlantic. For 

 many years seamen from England followed this course. The zone 

 along the thermal equator where the northeasterly and south- 

 easterly trades meet, and where rising currents of air are stronger 

 than horizontal movements, is known as the zone of equatorial calms. 

 The position of this zone of calms shifts a little with the sun, its 

 center (p. 241) remaining near the thermal equator. 



The westerly winds of middle latitudes and the trades of low 

 latitudes are the prevailing winds at the bottom of the atmosphere. 

 They are sometimes called the planetary winds. 



Unequal heating of land and water. The unequal heating of 

 land and water interferes with the circulation indicated in Fig. 

 231. Land and sea breezes and monsoons have already been cited 

 (p. 249) as illustrations of the effects of this unequal heating. The 



