ANTARCTIC ADVENTURE AND RESEARCH 



lively rapid vertical pressure change caused by the cold 

 dense air. The descending air is warmed giving clear 

 cloudless skies ... as one penetrates the Antarctic. ( See 

 Figure 30.) 



The clear skies in their turn facilitate radiation, as does 

 the small absolute humidity of the air. In consequence 

 the air and the snow surface become abnormally cold and 

 there is a great tendency to the formation of temperature 



BAfiS AT 3000 'T. 



Fig. 30. — Probable isobars at sea level (right) and 



AT 3,000 meters (left). 



Broken lines A. B, C, D show successive positions of pressure 

 waves. (After G. C. Simpson.) 



inversion, especially in the lower atmosphere. On these 

 normal fine weather conditions are superposed a series of 

 pressure waves which travel more or less radially out- 

 wards from the center of the continent. These waves 

 alter the surface pressure conditions, and cause air motion 

 which is frequently accompanied by forced ascending cur- 

 rents. The abnormally cold surface air is forced upwards 

 in these currents, rapidly cooled in the ascent, and the 

 water contained is precipitated as snow, which when com- 



196 



