ME JOHN AITKEN ON DEW. 41 



was a little warmer than the black surface, a condition in which it remained 

 during the evening. 



The reason for the snow being colder than the black surface during the day 

 would seem to be, that both surfaces radiate and absorb " dark heat " about 

 equally well, both surfaces therefore throw off about the same amount of heat; 

 but while this is the case, their absorbing powers for the heat of the sun are 

 very different, and though the sun was not shining directly on the surfaces, 

 yet there is a considerable amount of its heat reflected to the surface of the 

 earth from the atmosphere overhead. Now a black surface absorbs most of 

 this reflected heat that falls upon it, while the snow absorbs very little. Hence, 

 while both surfaces are radiating about the same amount of heat, the black 

 surface is absorbing far more than the snow, and thus keeps warmer. As 

 the sun sinks, the amount of its heat reflected by our atmosphere gets less and 

 less, and the difference in the temperature on the two surfaces diminishes; and 

 when at last the sun is quite under the horizon the temperature of the two 

 surfaces becomes nearly equal. It will be, however, observed, that they never 

 become quite the same, the snow being generally about half a degree warmer 

 than the other. The whole of this difference is not, however, owing to differ- 

 ence in radiating powers ; the snow will tend to give a slightly higher reading 

 on account of its surface being rougher than that of the paint, thus causing it 

 to receive more heat from the passing air than the black surface. And, further, 

 from the conditions of the experiments, the readings being taken during a 

 falling temperature, and the snow not being a good conductor of heat, the 

 thermometer under it will take longer to fall than the one under the blackened 

 metal. From these conditions it seems probable that there is not much differ- 

 ence between the radiating and absorbing powers of snow and black paint at 

 night, while the difference is very considerable during the day. 



It has been suggested to me that this difference in the radiating and 

 absorbing powers of snow and black surface, such as soil, &c, will enable us to 

 explain a difficulty long felt, regarding the hour at which the diurnal variation 

 of temperature begins in countries covered with snow. Over those parts of the 

 surface of our globe, where there is no snow, the temperature of the air begins 

 to rise before sunrise, whereas in snow- clad regions this change does not take 

 place till the sun is above the horizon — the explanation would appear to be 

 that where the surface of the ground is dark it absorbs the heat of the sun, 

 and warms the air whenever the rays begin to shine into the air overhead; 

 but where covered with snow it is but little warmed by these early reflected 

 rays, and it is not till the sun gets higher and shines on the surface of the 

 earth that its effects begin to be felt. 



VOL. XXXIII. PART I. 



