252 



GENERAL AIR CIRCULATION. 



Meimrduss Theory of the Air Circulation over the Antarctic .*—MemaTdviii starts his dis- 

 cussion from the generally accepted statements: (1) In regions of relatively low pressure there 

 exists an ascending air current which, if the consequent cooling is great enough, leads to the 

 condensation of water vapour first in the form of clouds and then of precipitation; (2) in 

 regions of high pressure, on the contrary, there is a descending current in which on account 

 of dynamical warming, not only is the condensation of water vapour prevented, but the air 

 becomes abnormally dry. His reasoning then proceeds as follows : 



Further one can lay down the law that, excluding very comphcated pressure systems, 

 the precipitation mil exceed the evaporation in regions having a cyclonic air circulation 

 while in regions with anticyclonic circulation the reverse will be the case. In regions where 

 there are changing cyclonic and anticyclonic systems the pressure type which has on the 

 yearly average the greatest frequency, intensity, or duration will decide whether precipitation 

 or evaporation will predominate. It is also possible to proceed in the reverse direction and 

 from the observed conditions of precipitation and evaporation deduce the prevailing type of 

 weather. Now it is known that there is a constant transfer of water chiefly in the form of 

 ice from the interior to the surrounding sea along the whole circumference of the Antarctic 

 and this necessitates, if the climate is not undergoing change, that more water in the form 

 of vapour must enter the Antarctic than leaves it. This consideration carries with it the 

 conclusion that the Antarctic, considered as a whole, resembles a region in which cyclonic 

 conditions predominate over anticyclonic. 



Having thus proved the necessity for cyclonic conditions over the Aaitarctic, Meinardus 

 proceeds to show how in spite of the increase of pressm-e at sea-level in high latitudes cyclonic 

 conditions prevail over a large area. His method is as follows : With the values of pressiu'e 

 and temperature found by the different expeditions, mean sea-level values for the whole of 

 the Antarctic are calculated for January, July and the year. With these average values at 

 sea-level and assuming a constant fall of temperature with height it is a matter of arith- 

 metic to calculate the pressui'e at different heights. The results of such a calcidation are 

 given in the following table : — 



Table 131. 

 From Meinardus. 



Colunms 2 to 5 give the assumed values of temperature and pressm-e at sea-level, the sea- 

 level pressure being considered constant throughout the year and to increase by 10 mm. 

 between 60 S. and the Pole The remaining columns contain the calculated pressure at 2,000 



* Jleinardus. Deutsche Sudpolar-Expedition, page 326. 



