Daily variation of cloud. 



157 



Autumn 



Tlie values for the ueasoiis and year have been plotted on figure 52. 



In all seasons except summer the maximum cloud amount is recorded at either 7 hours, 



11 hours or 15 hours and the minimum at either midnight or 3 a.m. In other words, except 



in summer, the maximum occurs 



during the day hours and the 



minimum during the night hours. 

 Unfortunately the measurement 



of cloud amount is ■ not made 



by instruments and tlie estimates 



depend partly on subjective factors. 



Any one who has tried to estimate 



the amount of cloud at night is 



aware of the difficulty of coming 



to a light decision. Even the casual 



observer must have often been 



surprised to see clouds clearly 



visible on what ho thought to be 



a cloudless sky when a distant 



lightning flash has lit up the hori- 

 zon. A long and careful survey 



of the siiy is necessary to detect 



clouds on a moonless night, and even 



then an experienced observer may 



miss some. Now in the Antarctic all 



the observers were not experienced, 



and also the conditions on a cold 



windy night are not conducive to 



a deliberate survey of the sky to 



detect small masses of cloud. On 



the other hand, when there is day- 

 light even an inexperienced ob- 

 server taking only a rapid glance 



at the sky can form a very good 



estimate of the amount of cloud. 



Thus the amount of daylight plays 



an important part in the estimate 



of the amount of cloud. If there was the same average amount of cloud throughout the 



day and night, the eflect of the daylight would be to produce a fictitious daily variation 



having a maximum during the hours of diylight and a minimum during the hours of dark- 

 ness, I.e., exactly the same variation as is found during all seasons in the Antarctic except 



summer, and we must therefore attempt to decide in how far the observed variation is 



affected by the variation of dayli"-ht. 



There are three periods to be considered. 

 {a) When there is sufficient daylight throughout the twenty-four hours for the clouds 

 to be clearly seen. For our purpose this may be taken as the whole period 

 between which the sun does not sink more than 10° below the horizon, i.e., in 

 McMurdo Sound th'i period from September 30 to March 14. Thus the months 

 October to February are included in this period, and March may also be added 

 as observations were taken at 1 p.m. local time which extends the jjeriod slightly. 



23 



23 



Fig. 52. Daily variation of cloud. 



