CLOUD AMOUNT. 149 



very rarely in the summer and then they were generally associated with the open water or 

 the bare expanses of rock on the Western Mountains. Although so much sledging was done 

 during the summer over the Barrier, cumulus cloud was not once recorded in the sled^Lii" 

 diaries except when seen over the mountains. It would appear, therefore, that ascending 

 currents sufficiently strong to produce cumulus clouds are never formed over the snow sur- 

 faces in these high latitudes. 



Cumulo- stratus. — It sometimes hajipened that during the summer the cloud left after a 

 blizzard was sufficiently thick and low to break up into clouds which were described as 

 cumulo-stratus. These were summer clouds and were practically never recorded except in the 

 months from December to February. 



Alto-stratus, alto-cumulus, cirro-stratus, cirro-cumulus and cirrus. — These were the chief 

 clouds observed in the Ajitarctic after the stratus referred to above. As is well known, these 

 clouds are not due to ascending currents from the lower atmosphere, but are generally caused 

 either by the interaction of two currents of air, or by the raising in the atmosphere of a 

 layer of air as a whole, in which case cloud appears throughout the upper boundary. This 

 is a natural consequence of the absence of ascending currents, for the air settles into layers 

 which are difierenciated by their temperature or direction of movement. 



This description of the cloud forms may be summed up very shortly by saying that there 

 was an almost total absence of clouds due to convexion currents, while clouds due to the 

 stratification of the air into layers were abnormally developed. At the same time the most 

 frequent cloud was a complete uniform covering of the sky due to the forced ascent of air 

 during blizzards. 



Cloud Amount. 



Mean Cloud Amount. 



The mean amount of cloud for the two years 1911 and 1912 was 6'2 and 6-4 respec- 

 tively, giving 6'3 as the average. Tliis is considerably higher than the value found, 5-0, during 

 1902 and 1903 by the Discovery Expedition. The explanation appears to lie in the fact that 

 when the sky was thinly overcast with the thin haze mentioned above, the cloud amount 

 was entered by the observers on that expedition as 0. Thus against the entry of clear sky 

 one often finds in the remarks column such phrases as ' misty in sky ' (April 8, 1903), 

 ' Snow crystals falling, stars very misty ' (June 27, 1903), ' Faint halo, and slight tendency 

 to paraselene ' (.June 5, 1903), ' Moon misty ' ^April 26, 1902), ' A few stars visible ' (May 15, 

 1902), ' SUght precipitation ' (June 1, 1902), ' Stars visible but dim ' (June 7, 1902). 

 Also haloes and coronae are frequently noted when the cloud amount is given as ; thus on 

 July 21, 1902, a lunar corona; was recorded at every observation from 8 a.m. to 10 p.m., 

 yet during the whole time the sky was reported to be quite free from cloud. There appears, 

 therefore, no doubt that on the Discovery Expedition the sky was reported clear when we 

 should have reported it completely overcast. 



It is remarkable that the mean cloud amount for 1911 * at Cape Adare was 6'2, exactly 

 the same as at Cape Evans. It will also be shown later that the frequency of overcast and 

 clear skies was very nearly the same at the two stations. Also the Southern Cross Expedition 

 found the mean cloud to be 65 at Cape Adare. There appears to have been very little 

 difference between the cloud at Cape Evans and at Framheim. Observations were only taken 

 regularly at the latter station during April and fi'om September to January; during these 

 six months the mean cloud amount was 6"2, while during the same months at Cape Evans 

 it was 6' 4. 



* 10 months' observations and 2 months' interpolation. 



