INFLUENCE OF SNOW-MASSES ON CLIMATE. 61 



A bed of snow covering the ground acts as a bad conductor, and 

 renders the exchange of temperature between the surface of the ground 

 and the lower stratum of the atmosphere slower than it would be if 

 the snow were absent. This is a result of the porous structure of snow, 

 the interspaces of which are filled with air. In this matter the con- 

 dition of the snow is of considerable importance ; the minute crystals 

 formed by cold are poorer conductors than the larger flakes ; but if 

 the snow by alternately thawing and freezing assumes the form called 

 neve, it becomes a much better conductor of heat. Farmers in coun- 

 tries that enjoy cold winters are well aware of the protective nature 

 of the snow-covering, and do not fear for their grain when it is 

 thick. 



The presence of snow thus assures a higher temperature to the up- 

 per layer of the soil than it would otherwise have, and its thickness is 

 an equally important factor with its structure. But its effect on the 

 air is different, for it separates the air from the warmth which without 

 its presence would escape from the ground. Snow also exercises an 

 important influence through its power of radiation, which is dependent 

 on its whiteness and the extent of its surface, but in which thickness 

 is not a factor. 



When air is rarefied, it contains, besides the vapor of water, only a 

 few scattering particles of dust or smoke. Even in the tropics, snow 

 on high mountains does not melt. Although a considerable amount 

 of solar heat is received by the upper surface of the snow, it is all sent 

 back into space, and the air, being very diathermanous, only retains a 

 very small part of it. In the plain countries of high latitudes the air 

 is not rarefied, but, when there is a large extent of snow, the other con- 

 ditions are similar to those which prevail at great elevations under the 

 tropics, particularly the absence of dust and the small quantity of 

 aqueous vapor. The latter condition is the result of cold, while the 

 former may arise from the fact that there is no dry bare ground near 

 from which dust can be swept up, or from the more or less complete 

 absence of organic life. In this case as in that of high mountains, 

 radiation into space goes on freely, and the solar rays are without 

 power to melt the snow so long as it preserves its light structure, and 

 its surface does not present any dark object to absorb the rays of the 

 sun and cause the snow around it to melt. Thus, we observe that it 

 melts on roofs and immediately around trees, while the clear snow in 

 the vicinity shows no signs of thawing. Snow will melt rapidly under 

 a piece of brown paper lying in a garden ; and a thin sprinkling of 

 dust on top of snow will produce a similar effect. 



We conclude, from these observations, that if a large extent of con- 

 tinent is covered with snow, that snow will not melt under the rays 

 of the sun. Yet we know that the winter snows in Northern Europe, 

 Asia, and America melt every spring and summer. How does this 

 come about ? Observations made in hiadi latitudes show that the 



