4(5 MR JOHN AITKEN ON DEW. 



were taken, the air was warmer than the radiating surfaces, while in the 

 second the air was colder than the exposed surfaces, except towards evening. 

 A number of other readings were taken under different conditions, but with the 

 same results ; the snow was always colder than the blacker surface during day, 

 while other observations made at night show their radiating powers are then very 

 similar. 



Part of the cooling produced by the snow surface shown in the above tables 

 would be due to evaporation from the snow. The amount due to this cause 

 was not great, as the difference between the wet and dry bulb thermometers 

 was small at the time. I regret, however, that the readings of these instruments 

 have been lost, so I write from memory, It will, however, be observed that the 

 difference in the radiating powers of the two surfaces continued during the 

 whole time the readings were taken on the 19th January, though the weather 

 changed to snow, and the air at that time would be nearly saturated. The 

 readings given in the tables therefore give the total cooling effect of the snow, 

 which is produced by two causes — radiation and evaporation. 



This small absorbing power of snow for heat, reflected and radiated from 

 the sky during the day, must have a most important effect on the atmosphere, 

 causing its temperature to be much lower when the ground is covered with snow 

 than when free from it. So that when a country becomes covered with snow 

 — other things being equal — it will be accompanied by a depression of the 

 mean temperature of the air ; and, further, as cold tends to produce a stable 

 condition of the atmosphere, not creating the current disturbances of heating, 

 it would appear that once a country has become covered with snow there will 

 be a tendency towards glacial conditions. 



But this poor absorbing power of snow is not the only way in which it 

 tends to produce a glacial climate. Snow, in addition to being a bad absorber 

 of the heat of the sky, is also a very poor conductor of heat. In illustration of 

 this, let me mention a few temperature observations made while the ground 

 was covered with snow during January last. On the 18th of that month 

 there was about b\ inches (140 mm.) of snow on the ground; the night was 

 clear, and radiation strong. At 8 p.m. the temperature of the surface of the 

 snow was 3°, and a minimum thermometer also on the snow showed that it had 

 been at 0° at an earlier hour. Taking a maximum thermometer, the index 

 was brought down below the freezing-point, and the bulb plunged through the 

 snow down to the grass. On examining it a short time afterwards the index 

 was at 32°. In confirmation of this reading, it may be mentioned that on 

 removing the snow the top of the soil was found to be unfrozen. These 

 observations showed that there was a difference of about 30° between the 

 temperature of the top and the bottom of the snow — that is, a distance of 

 •)}, inches. 



