VERTICAL DISTRIBUTION OF TEMPERATURE. 43 



air takes place. A temperature gradient is thus set up vvhicli approaches the adiabatic 

 gradient if the mixing is sufficiently rapid. On the other hand, if the base is cooled, a 

 layer of cold air forms there which has no tendency to rise and warmer layers rest upon 

 it. In this case a temperature inversion results. In the Antarctic summer the snow surface 

 is warmed relative to the overlying air by the almost continuous solar radiation and con- 

 vexion currents produce a normal temj)erature gradient. In the winter there is little or no 

 sunshine, while rapid radiation takes place from the snow surface which cools the air in 

 its immediate neighbourhood. A cold layer of air is thus formed, which, under favourable 

 circumstances, may be many degrees colder than the air a few hundred metres above. 



These conditions obviously can only obtain duiing the absence of wind. If there is 

 a wind it produces vertical mixing of the air without the aid of convexion currents. The 

 effect of a mnd is different in summer and in winter. In the summer there is already a 

 large temperature gradient which is little affected by the wind. In the winter the wind 

 removes the cold surface layer and produces a normal temperature gradient. This process 

 will be considered more fully later ; it is mentioned here to point out that the temperature 

 gradient shown by the curves on figure 13 during the summer probably exists during all 

 kinds of weather, while that shown for the winter exists only during calm weather. 



The different vertical temperature gradients in summer and winter have two very impor- 

 tant consequences when we consider the temperature of the upper air. The fir.st can be 

 seen at once from the curves in figure 13. The balloon ascents made on August 13 and 

 December 25, Nos. 1 and 10, were made on one of the coldest and one of the warmest 

 days of the year. The temperature difference at the ground on these two days was 36°C. 

 The curves show, however, that the temperature difference at 3,000 metres was only 18°C. 

 The four winter curves are probably typical of the conditions during the coldest days of the 

 winter, while the curves 9 and 10 are typical of the warmest summer conditions. It is very 

 probable, therefore, that the temperature difference between the warmest and coldest days at 

 two to three thousand metres altitude is only about half of the corresponding difference at 

 the ground. 



What has just been shown to hold for different times of the year is true also for 

 different geographical positions during the winter. That is, during the winter the temperature 

 differences between different geographical positions are less in the upper air than on the 

 ground. 



According to the isotherms for July shown in figure 9 the temperature over the Ross 

 Sea just to the north of the Barrier edge is — 10°F. ( — 23°C.) while over the Barrier itself 

 it is — 35°F. ( — 37°C.) Thus there is a temperature difference near the ground of 14°C. 

 The temperature gradients over the two regions are, however, quite different. The high tem- 

 perature over the Ross 8ea is due to the warming of the air by the warm water which is 

 never separated from the overlying air by more than a thin coating of ice. Hence during 

 the winter the air over the Ross Sea is warmed from below and convexion currents are 

 produced which give a normal temperature gradient. Over the Barrier there are often much 

 greater temperature inversions than those found during August in McMurdo Sound, but as 

 there are also periods during winds when the gradient is normal we probably shall not be 

 far wrong in assuming that the average conditions over the Barrier in July are similar to 

 those shown by curves 1 to 4 of figure 13. Based on these considerations the average 

 temperatures up to 4,000 metres during July have been shown on the left of figure 14 

 for the Ross Sea and the Barrier. The gradient over the Ross Sea has been taken as 5°C. 

 per 1,000 metres for the first kilometre and above this height slightly less. Over the 

 Barrier an inversion of 4°C. is shown for the first 500 metres above which there is a 



