SUMMER ASCENTS. 



■277 



These conditions can best be explained by considering that the air in the upper atmos- 

 phere is being cooled by radiation and in consequence sinking. The fact that the temperature 

 gradient is much smaller than the adiabatic gradient indicates that the downward motion is 

 so small that a large amoimt of the heat generated by compression is lost by radiation. 

 Near the ground the radiation is so great and the downward motion of necessity so small 

 that the air is abnormally cooled. Thus a layer of cold dense air forms near to the ground 

 which the descendmg current cannot move and over which it must flow, thus accounting 

 for the difierent winds fouiad at the upper surface of the lower cold layer. 



Summer Ascents. 



Table 139. 



Record Y. 



November 12, 1911. 



Time 11-l.j. Erebus smoke from the W. 



A blizzard occurred throughout the 10th and continued until the morning of the 11th. 

 The wind dropped during the afternoon, and from 17 hours on the 11th mitil the time of 

 the ascent (11-15 on the r2th) a breeze between TS and 2-5 metres a second continued 

 from the S.E. During this period the temperature was fairly constant, being about +12°C. 

 Three hours before the ascent the temperatm-e commenced to rise, and when the ascent took 

 place it was -9''C. The sky was nearly overcast with alto-cumulus clouds, and it is 

 probable that the instrument went into, if not through, these. At first the balloon travelled 

 in the surface S.E. wind until the alto-cumulus clouds were reached, and then it moved with 

 them nearly due west. Soon after the balloon had ascended the surface wind increased and 

 th? weather became thick. Some snow fell and it appeared as if a bhzzard were startmg 

 but although the wind continued at about 20 miles an hour the sky cleared, and the blizzard 

 did not become marked until the next day. The mean temperature gradient up to 1,500 m. 

 was •72°C. par 100 metres. At this height the gradient became smaller for the next 1,000 

 metres, being only -55" from 1,500 metres to 2,500 metres. This probably was the region of 

 movement fi-om the west and of the alto-cumulus clouds. At 2,250 metres the temperature 

 commenced to fall more rapidly again, and from 2,250 to 2,750 metres the fall was at the 

 rate of •84°C. per 100 metres. It will be found that in most of the summer ascents there 

 is a similar region in which there is a smaller gradient than in the regions above and below. 



