CONCLUSIONS. 269 



therefore the pressure increases from the exterior to the interior of the Polar area ; in other 

 words the pressure distribution is anticyclonic and the air motion is in general outwards. 

 Above each anticyclone a cyclone forms on account of the relatively rapid vertical pressure 

 change caused by the cold dense air. These cyclones convey air from higher latitudes over the 

 Polar region and supply the air which passes outwards near the surface. In the normal 

 steady state the air circulation takes place slowly and the descending air is warmed up 

 dynamically so dissolving cloud and giving clear cloudless skies, thus accoimting for the 

 decreasing cloud amounts observed as one penetrates the Antarctic (see page 151). 



The clear skies in their turn facilitate radiation as also does the small absolute humidity 

 of the air. In consequence the air and the snow surface become abnormally cold and there 

 is a gx-eat tendency to the formation of temperature inversion especially in the lower atmos- 

 phere. On these normal fine weather conditions are superposed a series of pressure waves 

 which travel more or less radially outwards from the centre of the continent. These waves 

 alter the surface pressure distribution and cause air motion which is fi'equently, and especially 

 over the west of the Barrier accompanied by forced ascending currents. 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 combined with the high surface winds produces 

 the typical Antarctic bhzzard. 



