HOBBS' THEORY. 249 



(8) Air highly charged with moisture wnthin the flat central area of calms and precipita- 

 tion of snow or ice near the glacier surface. 



We will now consider this evidence and Hobbs' conclusions in so far as they apply to 



the Antarctic. We shall find it more convenient not to follow the' order in which the 



heads are given above, but at the beginning of each of the follomng paragraphs the numbers 

 of the heads which are considered in the paragi'aph are given. 



(1). At every place where prevaiHng wind directions have been observed at the edge of an 

 inland-ice mass there has been found a larger or smaller component in the outward direction. 

 This is not only true at sea-level, but the air is found to be flowing outwards from the 

 high plateaux both in Greenland and the Antarctic. Special attention is drawn to the winds 

 observed by David, Shackleton, Scott and Amundsen, all of whom on their ascents to the 

 Antarctic plateau encountered winds blowing down the valleys. 



(7) & (8). This outward motion necessitates anticyclonic pressure distribution, with descending 

 air currents in the centre, where the air will be relatively calm. The deposition of ice 

 to supply the permanent snow under these anticyclonic circumstances is described as 

 follows. The frequency with which cirrus clouds are observed over Greenland and the Antarctic 

 is pointed out. It is then considered that the ice of these clouds is brought down with the 

 descending air current and deposited as ice crystals on the surface of the plateau. Hobbs 

 says : — 



' There is however the probability that in general this snow and ice is adiabatically 

 melted and vaporized during its descent to the plateau, and subsequently frozen 

 as it mixes with the cold air above the plateau surface.'* 



This method of explaining Antarctic snow especially the largo quantities associated with 

 blizzards is quite unsatisfactory and the following remarks by Meinardus are very much to 

 the point : — 



' I will consider next a possibility, which however has very little probability. It is 

 that in the central area of an anticyclone the dynamical warming of the descending 

 air is reduced by radiation, which in the dry air of high latitudes may be taken 

 to be very great, and in the regions near the ground the cooUng by radiation 

 may be so great that the air is cooled below its dew point. In this case the small 

 amount of aqueous vapour in the air would be in part condensed as rime, which 

 would replace falling snow as the cause of the ice sheet over the Polar region. 

 Does this process take place to any appreciable extent ? In my opinion it hardly 

 does, for the descending air in the anticyclone is very poor in vapour so that 

 in order to reach the desired end very large masses of air would have to be 

 brought into contact with the sm-face and in addition would have to be cooled 

 very much in order to bring it under its exceedingly low dew point. Now in the 

 central part of an anticyclone the air is usually very calm, hence the renewal 

 of air necessary for appreciable condensation can only take place very slowly and 

 only small quantities of water can be given to the surface. If on the other hand 

 one assumes strong air motion, then the action of radiation and with it the cooling 

 below the dew point ceases to act, and evaporation of rime previously deposited 

 might even occur. On these grounds I greatly doubt whether the ice-covering 

 of the central Antarctic can be due to the formation of rime to any extent. 'f 



* hoc. cit., page 208. 



t Deutsche Sudpolar-Expedition, page 327. 



32 



