152 



CLOUD AND PRECIPITATION. 



Table 83. 



Cloud and Wind at Cape Evans. 



March, 1911, to August, 1912. 



It is a noteworthy fact that during calms and light winds, 0— .5 miles per hour, clear 

 skies and overcast skies occur with equal frequence and the mean cloud amount is practically 

 5. Moderate winds from the north have similar cloud conditions as those during calms. On 

 the other hand, the effect of strong northerly winds is remarkable, for during 64 per cent, 

 of the time they blow, the sky is practically cloudless, and the mean cloud amount is as 

 low as 30. The effect of southerly winds is quite clear. Even with moderate winds the sky 

 is overcast during half the time and clear only during one-fifth. With increasing southerly 

 wind strength, the contrast becomes greater, and when the wind strength is greater than 30 

 miles an hour, the sky is overcast during 74 per cent, of the time and clear only during 

 8 per cent., the mean cloud amount reaching the high figure of 8-4. 



Annual Variation of Cloud. 

 For reasons already given (page 149), the cloud amounts recorded on the Discovery 

 Expedition were too low and therefore have not been used in obtaining the mean cloud 

 amount for McMurdo Sound. There seems no reason, however, to doubt that the yearly 

 variation is more or less accurately given by them and therefore they will be used to obtain 

 this variation. Besides recording the actual amount of cloud, the observers recorded in their 

 weather remarks the state of the sky according to the four grades b, be, c, and o. The 

 former of these corresponds to the c'oud amount and 1 and the latter to 9 and 10, there- 

 fore they can be used in combination with the estimates of the frequence of these grades of 

 cloud made at Cape Evans. 



Table 84. 



Monthly variation of Cloud Amount (0 — 10) from mean of the year. 



1 904 



