268 GENERAL AIR CIRCULATION. 



temperature would reduce its density and its downwards motion would be stopped. Finally 

 its rate of descent would be adjusted so that at each height it has the temperature of the 

 surrounding air, this will necessitate the loss by radiation of the heat due to adiabatic com- 

 pression. In other words air could not descend through an atmosphere having the vertical 

 temperature gi-adient represented by the line G H without losing heat by radiation, and a 

 measure of the heat lost in the descent from say G to K is given by the horizontal distance 

 from K to the adiabatic line through G. When the air through its loss of heat due to 

 radiation arrives at H it \vil\ then descend much more slowly, bxit will finally reach the 

 ground at J, when its temperature will be — 35°C. Now during periods of calm and exces- 

 sive radiation the gromid may be 10°C. colder than the air just above it. If therefore 

 the air after reaching the groimd at J is cooled by contract another 10° its temperature 

 will be —45°. It will be noticed from the diagram that this temperature is to the left of 

 the saturation line G P and therefore the air actually in contact with the ground would be 

 saturated and a deposition of moisture on the ground would occur. This is the method 

 by which Hobbs supposes that the Antarctic receives its covering of ice. There can be little 

 doubt that such conditions are frequently met with, especially on the Barrier, and the 

 early morning fogs so often reported from the Barrier are due to the air near the ground 

 being cooled in this way below its saturation temperature. But as Meinardus points out in the 

 paragraph quoted above, the conditions are not those which would produce much precipita- 

 tion and the whole process could not occur during high winds or overcast weather. 



We will therefore not consider further this method of obtaining precipitation, but return 

 to the condition of the air represented by the point J. Now let us see what would be 

 the consequence of raising the air which has descended from G to J. Such elevation of the 

 air would result if a pressure distribution were imposed which set the sm-face air moving 

 faster than the air in front of it, the air from behmd would then be forced to rise over 

 the air in front. This is the case during blizzards. 



If this occurred very rapidly the cold surface layer would be raised under adiabatic 

 conditions and its pressure and tempsrature conditions as it rose would be represented by the 

 line J L which is parallel to the adiabatic lines. In these circumstances it would reach its 

 saturation line in L where condensation and precipitation would take place. This would give 

 a cloud layer from which snow would fall at a height of less than 1 kilometre although 

 this same air entered the Antarctic as a saturated current at 6 kilometres. In reality, however, 

 the cooling would not be adiabatic, but somewhat less. Let us assume that instead of cooling 

 at the adiabatic rate (a little more than 10°C. per kilometre) it cooled at the rate found 

 by the balloon ascents in the summer, i.e., at 6|°C. per kilometre. Its condition then during 

 ascent would be given by the line J N. In this case cloud and precipitation would occur 

 above the height of N which is well below 2 kilometres. 



We have thus shown that owing to the large mdiation within the Antarctic the air, 

 even after it has descended from the upper atmosphere, is in a state that a moderate amoimt 

 of forced ascent is sufficient to cause condensation of the contained moisture and so snowfall. 

 Thus although Meinardus was correct in saying that radiation alone is not sufficient to account 

 for the large precipitation he was wrong in not considering the effect of forced ascent on air 

 already cooled by radiation. 



A statement of the general air circulation over the Antarctic is now quite simple. Over 

 the snow-covered surface of the Antarctic whether at sea-level or at the height of the 

 plateau radiation is so strong that the air is abnormally cooled especially in the layers of 

 air immediately above the surface. This cooled air is heavier than the surrounding air and 



