MR JOHN AITKEN ON DEW. 49 



that the bad conducting power of snow will be a cause of tlie lower tempera- 

 ture, yet we think that the observations recorded in this paper show that 

 radiation plays a most important part in producing this result. It has been 

 long well known that snow receives less heat from the direct rays of the sun 

 than the surface of a dark body like soil or rock, and we now also know that, 

 under any condition of sky yet tested, it absorbs but little of the sky radiation. 

 From this we see that as the surface of the snow, under all conditions of 

 sky, receives less radiant heat from without than the surface of the ground, 

 the air in contact with it will be more cooled than the air over bare soil or 

 rock. 



In the radiation temperatures above recorded, the snow was generally 

 about 4 degrees colder than the black surface during the day. We must not 

 from this suppose that these 4 degrees are the greatest difference that could 

 exist in the radiation temperatures of the two surfaces. These 4 degrees 

 simply represent the difference that was maintained under the conditions when 

 the tests were made. If the air circulation had been greater, the difference 

 would have been less, and both would have been nearer the temperature of the 

 air ; and if the circulation had been less, the difference would have been 

 greater. But while there may be a variation in the difference of the tempera- 

 tures of the two surfaces, there will be but little difference in their respective 

 cooling effects, as with a quick circulation a large amount of air will be cooled 

 a little, while, when the circulation is slow, a little air will be cooled a great 

 deal. Not only so, but on account of the temperature of space being so much 

 colderthan the surface of the snow, the temperature of the snow will tend to sink 

 about the same amount below the temperature of the air, even though the air be 

 greatly cooled ; so that, under the same conditions of radiation and atmospheric 

 circulation, however much the air may have been cooled, the two surfaces will 

 tend to maintain the same difference in temperature, and the snow will always 

 tend to cool the air more than a black surface, when the surfaces are colder 

 than the air, or to heat it less when the surfaces are warmer. 



It is therefore evident that this radiation effect will be one of the causes 

 tending to produce the lower temperature of the air while snow is on the 

 ground, and it seems probable that the snow will not only reduce the minima 

 temperatures, but also the maxima. It seems also probable that the bad 

 absorbing power of snow will have a most important influence in retarding the 

 approach of warm weather, as the earth under snow receives far less heat from 

 the sun with the approach of spring than if the ground had been free from 

 snow. This is, of course, altogether apart from the question of the retardation 

 of the approach of warm weather by the sun's heat being spent in melting the 

 snow, instead of warming the air. 



VOL. XXXIII. PART I. G 



