32 METEOROLOGY 



accumulated above Captain Scott's Depot A to the east of Minna BlufF, erected in 

 1902. This snow was found to be very hard and compact, and must have accumulated 

 at the rate of about 13 inches (0'33 m.) per year. When thawed down it was 

 found that these 13 inches were equal to 7^ inches (188 mm.) of rain. The 

 result, therefore, agrees fairly well with that obtained from the meteorological 

 observations at Cape Royds, The snowfall at Cape Royds is somewhat heavier than 

 that of the Great Ice Barrier, for the equivalent in snow of 7^ inches of rain near 

 Minna Bluff represents no doubt a great deal of drift snow as well as new-fallen snow. 

 At the same time allowance must be made in the case of the Minna Bluff estimate 

 for the removal of a certain amount of the snow by ablation. 



Ablation. The term "Ablation" as used by Drygalski,* means the general 

 lowering of the surface of ice, from whatever cause, except that of mechanical 

 movement. 



It may thus be due to (1) Generation of ice-vapour direct, without actually 

 the thawing of the ice surface ; (2) Generation of water- vapour from thaw water ; 

 (3) Mechanical abrasion of an ice-surface by drifting snow or by rock-dust impelled 

 by blizzard-winds. W. H. Hobbs f uses the term as synonymous with surface 

 melting ; Chamberlin and Salisbury ^ speak of the evaporation and of the melting 

 of ice, but do not use the term " ablation." It will be used by us to denote a general 

 lowering of the surface of the ice and snow from any of the three causes above 

 specified ; but as our experiments were conducted in the Antarctic, in autumn, winter, 

 and spring, at a temperature far below freezing-point, the most important factor to 

 be considered was, no doubt, in this case, ice evaporation. IIann§ emphasizes the 

 fact that the pressures of ice-vapour over ice and of water- vapour over water are 

 different at similar temperatures. His Table is given in the foot-note below. Hobbs 

 has pointed out (op. cit. pp. 162 and 176) the great importance of rock debris In 

 bringing about the ablation of snow.|| In view of the fact that fine dust derived 

 from rocks, mostly of dark colour, was widely distributed over the snow and ice at 

 Cape Royds, where our experiments were carried out, what may be termed 

 microscopic melting around minute dust-particles no doubt played an Important part 

 in the local ablation of ice and snow. 



* " Gronland-Expedition," " Der Gesellschaft fiir Erdkunde, zu Berlin," 1891-1893, Berlin, 1897 

 (Band i. s. 541), where he renders Ablation, " Schwund der Eisoberfliichen." 



t "Characteristics of existing Glaciers," New York, 1911, p. 162. 



J "Geology: Processes and their Results," London, 1908, p. 279. 



S " Ijehrbuch der Meteorologie," Leipzig, 1906, s. 162. He there gives the following interesting 

 Table : 



II He states that "for this purpose" (clearing the snow during the construction of the Bergen Railway 

 in Norway, completed in December 1909) "covering the snoiv surface with fine dirt proved more effective 

 than a corps of shoi>ellers, the sun, in this case, performing the work." 



