MR JOHN AITKEN ON DEW. 47 



During the night the temperature of the air fell 5 degrees lower, so that the 

 surface of the snow would be kept about zero for most of the night, yet next 

 morning at 9.45 the bottom of the snow was still at 32°. The temperature of the 

 surface of the snow had risen at this hour only to 8°*5. These observations 

 were repeated on other occasions when the coating of snow was thinner ; there 

 was then less reduction in the temperature of the surface, but on all occasions 

 when the snow was a few inches deep, the surface of the soil remained at 

 32°. As the ground was frozen when the snow fell, it would appear that the 

 earth's heat slowly thawed it under the protection of the snow, and the 

 temperature of 32°, which was below the surface when the soil was frozen, 

 gradually rose to it, where it of course stopped, and the rising heat was spent 

 in melting the snow. 



The protection afforded by the bad conducting power of snow is evidenced 

 in our climate, by the amount of vegetation that takes place underneath it, on 

 those occasions when we have had snow on the ground for a length of time. 

 After the snow is gone, many plants are found to have grown, and some 

 advanced nearly to bloom under its protection. This same influence may, 

 however, be seen at work in a more marked manner in spring on the slopes of 

 the Alps and other lands covered with snow all winter. As the snow recedes, 

 and the surface of the earth is gradually laid bare, the vegetation is found to be 

 in an advanced state — many of the flowers, if not in bloom, are just ready to 

 open. 



This bad conducting power of snow, compared with soil and rock, will 

 evidently tend to lower the temperature of the air over snow-clad lands, as a 

 few inches of snow in our climate conserves the earth's heat, and prevents its 

 surface being cooled below 32°. The surface of the snow thus gets very much 

 colder than the bare surface of the earth would have been if no snow had been 

 on it, and the air is thus cooled much more over a snow surface than over one 

 of bare earth. 



To get evidence of the statement that the surface of a country covered 

 with snow is much colder than it would have been if free from snow, we have 

 only to examine the surface of the ground under the two conditions. The 

 surface of snow covering the ground receives so little heat from the earth 

 that it gets cooled by radiation to such an amount that it is almost always 

 during frost cooled below the dew-point, and is covered with a heavy deposit 

 of hoar-frost. During the late snow I have frequently noticed a thickness of 

 more than 12 mm. of beautiful ice crystals of this kind deposited on its surface. 

 But while the snow is cooled so much, and has this deposit on it, the surface 

 of the soil where it has been laid bare keeps quite free from this deposit, as it 

 receives sufficient heat from below to keep its temperature above the dew-point. 

 The air over snow-clad lands is thus always in contact with a highly-cooled 



