164 Woeikqf- — CroWs Hypotheses of Geological Climates. 



cause of the fogs, which have been so often noticed in high 

 latitudes in summer. He thinks the fogs are caused directly 

 by the melting of the snow, and that, by interposing a screen 

 between the sun and the snow, they are effectual in lessening 

 the amount melted. 



The melting of the snow by the sun has not by itself power 

 to cause fogs. On extensive continental regions of Europe, 

 Asia and America, snow lies in winter and is melted from 

 March to June, yet fogs are of exceedingly rare occurrence 

 during that time, and they are more frequent at night. They 

 are rather instrumental in preventing the loss of heat by ra- 

 diation than the heating influence of the sun's rays. In the 

 same regions fogs are frequent in autumn, during anti-cyclones. 

 The cause of these fogs is the same as that of the London fogs, 

 that is, the temperature of the river or lake water is much 

 higher than that of the air, and thus the vapor is soon con- 

 densed. Neither are fogs common in summer over mountain 

 glaciers, notwithstanding the great amount of melting, but fogs 

 are experienced on the sea, in the vicinity of melting ice, be- 

 cause here we have two masses of air of unequal temperature, 

 both nearly saturated, and their mixture must produce satura- 

 tion, that is — fogs. Thus it is easy to see that fogs are not 

 necessary followers of the melting of snow and ice per se / 

 some other conditions are necessary for them, and Dr. Croll is 

 right in saying about a glaciated country, at some distance from 

 the sea, "fogs prevent, to a great degree, the melting of the 

 snow and ice." 



Mr. A. E. Wallace* has mentioned that in northern Siberia 

 the powerful sun of June cannot melt the snow until warm 

 southerly winds bring in warm air. In so far as the beginning 

 of melting depends on this, I am quite of the same opinion; 

 but Dr. Croll is wrong in extending too much the influence of 

 these southerly winds, and in believing them to prevail in sum- 

 mer on the north coast of Siberia. On the contrary, cold winds 

 from the sea prevail in summer and certainly chill the air, 

 while southerly winds prevail in winter. Thus Dr. Croll's state- 

 ment of " matters would be still worse if these southerly winds, 

 instead of ceasing, were simply to change from June and July 

 to December and January, for then, instead of producing a 

 melting effect, they would greatly add to the snow-fall"f is re- 

 alizecl, as far as the winds are concerned ; but the snow-fall of 

 winter is exceedingly light, because the southerty winds come 

 from the colder interior of the continent, and besides they are 

 descending winds, and in descending become relatively dry. 

 We know these facts from the observations of Wrangell and 

 Anjou at Nijnekolymsk and Ustjansk, and they were con- 

 * Island Life. t p - 88 - 



