454 R. M. Deeley — Polar Climates. 



the stratosphere, and on this account the great cyclonic belt of middle 

 latitudes is maintained. 



In the Northern Hemisphere the geographical conditions are the 

 reverse of those in the south. The North Polar area is a partially 

 ice-covered water area, the temperature of which is never excessively 

 low because the ice covering the sea is thin. The lowest temperatures 

 occur over Siberia at Yerkhoyansk. But this water-covered area is 

 largely surrounded by land, and the warm ocean currents have only 

 partial access to it. The surrounding land areas also result in the 

 production of low winter temperatures, and the formation of one great 

 cyclone such as that of the Southern Hemisphere is prevented by the 

 irregular arrangement of the land and water areas. However, over 

 the Pacific and Atlantic Oceans, where they approach the North Polar 

 area, two cyclonic centres are formed which remain powerful in the 

 winter. In the winter there are also high pressures over Asia, and 

 south-westerly winds prevail between them and the North Atlantic 

 depression. In the summer low pressures prevail over Asia, and the 

 gradients for south-westerly winds over Europe are less steep. 



The conditions existing in both hemispheres thus favour frigid 

 Polar conditions. But the present arrangement of the land masses in 

 the Northern Hemisphere is not such as existed in pre-Quaternary 

 times. Many of the sedimentary rocks of Central North America, 

 East Asia, and Mid-Europe are of ages ranging from Archaean to 

 Miocene times. Bailey- Willis 1 has constructed a series of maps 

 showing the areas in North America which were at various times 

 probably land or sea. They all show that, in the past, what is now 

 the area of the North American platform was more or less broken up 

 into islands, large and small, and that there were very persistent and 

 wide channels connecting the warm oceans with the Arctic Sea. 

 According to Martonne 2 there was a channel connecting the Arctic 

 Sea and the southern seas which passed over the area now occupied 

 by the Ural Mountains, and which remained open during the whole 

 of the Secondary and late Primary Eras. As far as the Northern 

 Hemisphere is concerned it seems pretty clear that for long ages the 

 openings from the south into the Arctic Sea were large and more 

 numerous than they are now. At present our knowledge of the 

 Southern Hemisphere conditions is less complete than is that concerning 

 the Northern Hemisphere ; but the flora and fauna of the Antarctic 

 Continental rocks show that a large portion of them must have been 

 below the sea-level at times, and that the climate must have been 

 temperate. Indeed, the whole of the evidence we have points to the 

 conclusion that in late Tertiary and post-Tertiary times the conti- 

 nental platforms became more land-covered than they had hitherto 

 been ; i.e. archipelagic conditions gave place in recent times to 

 continental conditions. 



Such an arrangement of the early land areas would of itself lead 

 to warmer conditions in high latitudes, and this would reduce the 

 strength of the temperature gradient in the troposphere. The 

 stratosphere temperature gradient would then meet with less 



1 Journal of Geology, vol. xvii, 1909. 



2 TraiU de Giographie Physique, 2nd ed., p. 595. 



