REPORT ON ATMOSPHERIC CIRCULATION. 51 



In the northern hemisphere the ocean maintains a higher temperature than the 

 land in regions open to its influence, as is seen not only in the higher latitudes to 

 which the isothermals push their way as they cross the Atlantic and Pacific, but in their 

 irregular courses over and near the Mediterranean, Black, Caspian, and Baltic Seas, 

 Hudson's Bay, the American Lakes, and all other large- sheets of salt 1 and fresh water. 

 The influence of the ocean and ocean currents in keeping up the temperature during the 

 winter months is most strikingly seen in the North Atlantic, where the isothermal of 35° 

 reaches a much higher latitude in mid-winter than anywhere else on the globe. The con- 

 serving influence of sheets of water on the temperature in all seasons is more strikingly 

 shown when the isothermals are drawn for single degrees on maps of a larger scale. 



In the southern hemisphere the highest isothermals are 90° in Australia and South 

 Africa, and 85° in South America. It is to be noted that in January, the summer of this 

 hemisphere, the lowest isothermal is 25° in the Antarctic Ocean to the east of South 

 Victoria ; whereas in July, the corresponding summer month of the northern hemisphere, 

 the lowest isothermal is only 35°, or 10° higher than in the Antarctic Ocean. The differ- 

 ence is due to the icebergs and icefields of Antarctic regions. In Antarctic and sub- 

 Antarctic regions the change of temperature through the months of the year is com- 

 paratively small,, the annual range being only about 10°. 



In this month the least variation of temperature occurs in the equatorial regions of 

 the Pacific, and in all seasons the variation there is small. 



In January the mean pressure of Central Asia rises to about 30 - 50 inches, which is 

 absolutely the highest mean pressure for any month anywhere over the globe. Now, 

 since the prevailing winds in this anticyclone, which virtually overspreads nearly the 

 whole of Asia and Europe, flow outwards in all directions, bringing S. and S.W. winds 

 over Russia and western Siberia, it follows that the temperature of these inland regions 

 is considerably higher than would otherwise be the case. On the other hand, since the 

 prevailing winds are N.W., N., and N.E. on the east and south of Asia, the temperature 

 of these regions is thus abnormally depressed. Indeed, so strong is this influence of 

 wind direction and ocean combined, that the isothermals run, roughly speaking, north 

 and south in the west of the Europeo-Asiatic continent, and do not assume an east and 

 west direction till about 70° or 80° long. E. 



Since in Siberia light airs and calms prevail, and the general drift of the atmo- 

 sphere is north-north-eastwards towards the higher latitudes of the Arctic regions, the 

 temperature continues rapidly to fall in that direction, with the result that the lowest 

 mean temperature is not coincident with the centre of greatest pressure to the south 

 of Lake Baikal, but occurs at Werkojansk, about thirty degrees of latitude to the N.N.E. 



The other anticy clonic regions are North America, in the centre of which pressure rises 

 to 30-20 inches ; two in the Pacific to the west of California and of Chile respectively ; 

 in the South Atlantic to the west of Cape Colony; and in the Indian Ocean to the west of 



