Theory of the Winds. 25 



pressure area over the Antarctic Continent. North of the 

 high pressure belt the winds blow in accordance with the 

 surface temperature gradient ; but south of it they blow 

 against the surface temperature gradient: and instead of the 

 cold Antarctic Continent being an area of high pressure it is 

 an area of very low pressure. 



If the winds of the Southern Hemisphere followed the 

 surface temperature gradient, we should expect to find an 

 upper current from the north and east, and an under current 

 from the south and east. The lack of velocity of these east 

 winds, as compared with the velocity of the earth, would 

 produce an area of high pressure at the pole. Under these 

 circumstances we should have lower winds moving from the 

 poles to the equator, and upper winds blowing from the 

 equator to the poles. This is the condition Thomson con- 

 templated, but he added a lower thin current, resulting from 

 the friction of the earth, and blowing from the high pressure 

 belt towards the pole with directions from west to east. 



In the Northern Hemisphere, owing to the presence of 

 great land areas, and the northerly extension of the Pacific 

 and Atlantic Oceans up to the Arctic Circle, the pressure 

 and temperature curves, as we have seen, differ remarkably 

 from those of the Southern Hemisphere. In the Northern 

 Hemisphere there is a high pressure belt at about 30°N.lat. ; 

 but it is much broken in the summer by the heating up of 

 Asia and northern Africa. A steady westerly wind round 

 the North Pole, which would produce a very low pressure 

 there, is thus prevented, and we have instead, both during 

 winter and summer, two low pressure areas, one over the 

 North Pacific and the other over the North Atlantic. In the 

 summer these low pressure areas fill up slightly. Around 

 these two low pressure areas the winds circulate, especially 

 in the winter, and mitigate very considerably the winter 

 climate of the extreme north. They also, by carrying the 

 winds over the polar area, keep the pressures somewhat high 

 there. From these two low pressure areas the pressure 

 steadily increases as we approach the high pressure ridge in 

 30° N. hit., and by producing south-westerly winds in middle 

 latitudes on the east sides of the Pacific and Atlantic Oceans 

 it mitigates the climates of western Europe and the west side 

 of North America. On the western sides, however, we get 

 the cold winds from the arctic area in the winter. 



Indeed, in the Northern Hemisphere the winds do not 

 depart so markedly from what we might expect from the 

 temperature gradients as they do in the Southern Hemisphere. 

 The action, however, in both hemispheres of some force, other 



