4 METEOROLOGY 



the surface of the lake was intersected with a network of cracks (see Fig. 54) 



and the fragments of ice between the cracks became slightly convex and tilted at 



various angles from the horizontal. 



Fortunately, this ablation can be checked by the observations made at the same 



time on the other stakes. 



Measurements on October 2 showed that, at Blue Lake, the ablation since 



July 16 had been about "42 of an inch, at Green Lake '25 of an inch, and at Clear 

 Lake "33 of an inch. It may be mentioned that the ice of Blue Lake and of Clear 

 Lake was practically fresh, while that of Green Lake was saline. 



Blue Lake, as being a large expanse, and having, at the point where the stake 

 was fixed, a smooth and but slightly cracked surface, may be taken as giving a 

 reliable figure for the ablation between July 16 and October 2 ; that is about "0054 

 of an inch per day — or 42 inches for seventy-eight days. Had the ablation at Coast 

 Lake continued at 1*9 inches (the amount measured on August 5) from June 20 to 

 October 2, 113 days, the rate of ablation would, practically, have been the same as 

 for Blue Lake, viz. "0053 inches per day. 



We may conclude that the ablation due chiefly to evaporation of ice-vapour and 

 water- vapour, was, in 1908, at Cape Royds, equal to about 2 inches from April 3 to 

 October 2 — a period of 150 days. 



This evaporation was probably, for the most part, due to generation of ice-vapour, 

 possibly in part to the formation of water-vapour ft-om microscopic thawing around 

 dust-jiarticles, at a time when the general temperature was still far below freezing-point. 

 This amount of ablation due to evaporation alone is truly surprising, especially 

 when one reflects that from April 23 until August 22 the sun was continually below 

 the horizon. Thus one can easily credit the evidence of ablation, already alluded to, 

 afforded by the distance to which the tips of the freshwater algse were found to 

 project above the sui-face of the lake-ice on April 3, viz. that about 2 inches of 

 ablation, due chiefly to evaporation, had taken place already between the beginning 

 of February and April 3.* This would make the total amount of ice-ablation about 

 4 inches from February to October. It remains to be seen what amount of ablation 

 due to evaporation takes place during October and Novembei^, December and 

 January. If another 3 inches disappears, then this will make the total evaporation 7 

 inches, which will about account for nearly all the precipitation in that part of the 

 Antarctic. The annual precipitation at Cape Royds, in 1908, was j^robably equal to 

 about 9^ inches of rain. The precipitation was entirely in the form of snow. It 

 must also be remembered that, in December and January, the ice and snow suffer a 

 considerable amount of ablation, not only through evaporation of ice and thaw water, 

 but also through the running oflf of the thaw water. 



* The alga being very flexible and flaccid during the summer thaw, its tips would not project above 

 the general level of the thaw water of the lakes as long as the water remained unfrozen. Thus the tips 

 would begin to be left in relief only after the date of the final freezing of the lake at the end of the summer. 



