vin Atmospheric Phenomena 123 



blow poleward and equatorward, there is a calm. Since the 

 upper air, which contains little vapour, is always descending, 

 these regions are cloudless and the scene of enormous 

 evaporation. The Temperate zones of poleward surface 

 winds receive the hot vapour-laden tropical air and conduct 

 it to colder regions, where much of its vapour is condensed. 

 They are thus windy cool regions of moderate cloudiness 

 and rainfall. The polar regions of low pressure are 

 practically calm, and as most of the air descends from above 

 they are relatively dry. The tropical regions swept by the 

 equator-seeking winds are windy, hot, cloudless, but the 

 scene of great evaporation from the hot sea surface. The 

 narrow equatorial belt of low pressure into which the 

 equator-seeking winds blow from north and south is also 

 a region of calm. The air as it ascends here expands, 

 cools, and the enormous supply of vapour swept in from 

 the tropics condenses into the heaviest cloud, and falls 

 as deluges of never-ceasing rain. The heat liberated 

 by the condensation of so much vapour strengthens the 

 equatorial up-draught. The equatorial belt of low pressure 

 always lies nearly under the vertical Sun, consequently in 

 the northern summer ( 122, 123) it swings to the north, 

 and in the southern summer it swings to the south, dis- 

 placing the belts of tropical high pressure northward and 

 southward alternately. For reasons which cannot be 

 explained here, this displacement is comparatively slight, 

 extending over only five or six degrees of latitude. In 

 the North Atlantic, for example, the equatorial low 

 pressure belt never moves farther south than 5 N. All 

 parts of the Earth's surface that the equatorial rain-belt 

 traverses in its annual movement, experience a rainy season 

 as it lies over them, and a dry season all the rest of the 

 year, when swept by the equator-seeking winds. Near the 

 equator, where the narrow rain-belt crosses a tract of the 

 Earth both in its northward and in its southward swing, 

 there are two wet and two dry seasons in the year. The 

 theoretical circulation of the air and its resulting climates 

 are affected by two causes, unequal heating of the air by 

 land and sea surfaces ( 164), and the deflection of the 



