148 CLOUD AND PRECIPITATION. 



Stratus (overcast). — The most frequent entry in the cloud column of the meteorological 

 register is ' 10 ' or ' overcast,' without any description of the kind of cloud. This is because 

 the sky was frequently covered with a uniform layer of cloud which had no distinguishing 

 features and the height of which could not be determined. This layer was often very thin, 

 so that the moon and stars could be seen through it. It was usually associated with 

 blizzards. In some cases, the layer would gradually spread over the sky from the south, but 

 much more often, a clear sky would become hazy, and then the haze would thicken into 

 cloud which would get thicker and apparently lower until the blizzard commenced. During 

 many blizzards the drift snow would be so thick that the clouds could not be seen, there 

 would only be a general darkening of the sky. The chief difference between the overcast 

 sky of the Antarctic blizzard and that accompanying prolonged rain of more temperate regions 

 was that in the latter case the clouds usually have features, the motion of which can be 

 followed, while in the former the layer was so uniform that it was quite impossible to deter- 

 mine the movement of the cloud. In fact, it is very questionable whether in the latter case 

 true clouds were present ; there was no appearance of vapour or cloud particles, but the whole 

 sky had the appearance as if the air were full of very fine ice crystals which fell as powdery 

 snow. The end of a blizzard appeared to arrive in two different ways, which may be 

 described as ceasing from the top and ceasing fi-om the bottom. In the former case, while 

 the wind still carried along with it much drift snow, the sky would get lighter and the 

 drift less, until finally there would be a low drift of snow swept up from the surface with a 

 clear sky above. In the other case, the wind and drift would stop, while the sk}' retained its 

 layer of uniform cloud. 



The stratus cloud which remained after a blizzard and the stratus cloud which had formed 

 without a blizzard resulting would disappear in the same way. Sometime the sky would 

 (Gradually get lighter and the whole cloud mass melt into a thin haze which slowly 

 disappeared, or, what was more frequently the case, the layer would take on the characteristic 

 features of an alto-stratus or cirro-stratus cloud, which would break up into patches of alto- 

 cumulus or cirro-cumulus clouds. 



There can be little doubt that the cloud formation and precipitation in a blizzard is 

 due to air forced to ascend in some way analogous to the upward draught in a barometric 

 depression. 



In chapters VI and VII we shall discuss in some detail the mechanics and thermo- 

 dynamics of the blizzard ; here I only wish to examine the blizzard in so far as the cloud is 

 concerned. We will therefore assume that the air is forced to ascend sufficiently for preci- 

 pitation of the water vapour into ice particles to take place. The whole phenomenon as 

 described above is then clear. As the air rises, condensation takes place in the upper atmos- 

 phere, whence snow falls. This snow cannot settle, as the wind sweeps it along, and the 

 whole atmosphere below the condensation level becomes filled with driven snow. This is the 

 blizzard at its height. If now the ascending current stops before the wind ceases, new snow 

 is no longer formed and the sky clears above and the snow gradually settles out of the air, 

 until only a little suiiace drift is left. On the other hand, the ascending ciu'rent and the 

 wind may both cease before the cloud has had time to clear away ; this leaves the cloud 

 of ice crystals at the height of the condensation layer, which then becomes an ordinary 

 stratus cloud to be absorbed in the way that all other layer clouds are absorbed, which 

 may be either by a gradual disappearance of the layer as a whole or by its breaking up 

 in parts. 



Cumulus. — Cumulus clouds are due to unequal heating of the air adjacent to the groimd, 

 which gives rise to local ascending currents. In McMurdo Sound cumulus clouds only occurred 



