SUMMER ASCENTS. 285 



(6) Between 2,000 to 4,000 metres every ascent showed a region in which the gradient 

 was less than in the regions above and below. The mean gradient in this region was •54°C. 

 per 100 metres. 



(c) Above the region described in (b) there was an increase in the gradient. 



{(I) In the highest ascent the gradient again decreased above 6,000 metres. 



These results show a gradient in the lower atmosphere as large as that found over land 

 in temperate regions during summer, and occasionally approached the adiabatic gradient in 

 dry air. Such a gradient indicates considerable forced motion in the atmosphere and as at 

 the times of the ascents there was practically no wind this motion could only have been 

 caused by convexion currents resulting from the sun shining on the snow surface. 



The decrease in temperature gradient which occurs between 2,000 and 4,000 metres is 

 probably due to the fact that in this region the air has to adjust itself to the different 

 conditions of pressure and temperature which exist at this height over the Ross Sea area and 

 over the high tableland to the west. The height of this tableland as found by Captain Scott 

 in November 1903 is approximately 2,500 metres and as the temperature gradient over 

 the high snow-covered plateau must be different from that in the free air at the same height 

 over a region at sea-level, some adjustment must take place which is shown by the reduced 

 temperature gradient. Above this region of adjustment normal temperature gradients are re- 

 established 



