vni A tmospheric Phenomena 121 



denser air from the neighbouring temperate zones flows to- 

 ward the equator along the surface to take the place of the 

 ascending air, and is in turn heated and forced to rise. The 

 polar regions receive little heat from the Sun at any time, and 

 in the long dark winters radiate heat away into space. The 

 air over them consequently becomes chilled, grows denser, 

 .and descends toward the surface. Thus by equatorial 

 heating and polar cooling the air is constantly being raised 

 at the equator, carried in the upper regions north and 

 south to the poles, brought down there to the surface and 

 drawn back toward the equator. The upper current blows 

 spirally as a wind from the west-south-west in the northern 

 hemisphere, and from the west-north-west in the southern 

 hemisphere (as explained by Ferrel's Law), while the winds 

 from the poles would blow from north-east in the northern 

 hemisphere and from south-east in the southern. 



177. Ferrel's Theory of Circulation. The result of 

 this arrangement, according to Professor Ferrel, is that 

 in the upper layers of the atmosphere the pressure is 

 highest above the equator and lowest over the poles. 

 But the rush of air at a lower level from the poles toward 

 the equator tends to carry the mass of the atmosphere in 

 that direction, while the movement of the upper air toward 

 the poles tends but more feebly to carry the mass of the 

 atmosphere in the opposite direction. The two tendencies 

 balance each other between latitudes 20 and 30 north and 

 south, and the pressure of the lower strata of the atmosphere 

 is thus greatly increased in the neighbourhood of the tropics. 

 This is shown in Fig. 22 by the boundary line of the por- 

 tion of the atmosphere shown being drawn nearest the 

 surface at the equator and poles, farthest from it at the 

 tropics. The arrangement of pressure at the surface is thus 

 Two belts of air at high pressure girdle the Earth a little 

 poleward of the northern and southern tropics, a ring of air 

 at lower pressure lies along the equator, and great regions 

 where the atmospheric pressure is low surround the north 

 pole and the south pole. The tropical zones of high 

 pressure give rise to surface winds toward the equator, 

 strengthening the north-east and south-east winds of the 



