CHAPTER IV. 



CLOUD AND PRECIPITATION. 



Of all meteorological elements cloud is the most difficult to discuss either by simple 

 description or by statistics. This is particularly the case in high Antarctic latitudes where 

 one constantly observes a thin haze all over the sky, which is often so thin as hardly to 

 be noticeable. The difficulty has then to be faced whether the cloud amount is or 10. 

 A factor which, frequently leaves no room for graduated estimate but must be classed as either 

 in the minimum or maximum class is obviously unsuited for statistical investigation. Again, 

 while in other factors the mean value is, as a rule, the most frequent value, this is not so 

 in the case of cloud amounts, for with cloud the most frequent cloud amoimts are those 

 at the end of the scale, and 10, and the mean value is often one of the least frequently 

 observed. Thus at Cape Evans the mean cloud amount was 6, and as it happened this was 

 the cloud number which was by far the least often entered as being the actual amount 

 present, and was only recorded on r6 per cent, of the whole observations. 



, Again, when describing the cloud forms great difficulty is encountered. Even low clouds 

 are frequently formed of ice cry.stals and therefore are really of the nature of cirrus clouds, 

 so that even an experienced observer often finds difficulty in deciding whether the cloud 

 should be described as stratus or cirro-stratus. 



It is obvious therefore that the same rigid analysis cannot be applied to the cloud 

 observations as to other meteorological factors. In the following paragraphs the kind of 

 cloud will be described in general terms without any statistical data, although the kind of 

 cloud was recorded whenever cloud observations were made. The reason for this is that 

 observations were taken by several members of the exjiedition who varied greatly amongst 

 themselves as to the names to be given to the clouds. Further, the main cloud conditions 

 are so simple that nothing would be gained by counting the number of times each cloud 

 form was recorded. After the description of the clouds, the annual and daily variation of cloud 

 amount will be considered. In this latter section no account will be taken of the kind of 

 cloud, so that a sky so thinly overcast that the stars could be dimly seen will receive the 

 same weight as a heavily overcast sky during a blizzard. 



Kind of Cloud. 



Fog. — Only on a very few occasions was fog observed at Cape Evans and then it was 

 generally associated with the open water in McMurdo Sound. When the temperature was low 

 and the wind high, a peculiar kind of surface mist or fog was seen over the water; this 

 generally went by the name of frost smoke. Once or twice this was blown over the station, 

 when it was seen to be composed of minute ice cry.stals which gave rise to the usual 

 system of rings and mock suns aroimd the sun. 



On the Barrier fog was often reported during the night and early morning. The cause 

 of this fog is discussed on page 268. 



