598 H. MOHN. METEOROLOGY. [NORW. POL. EXP. 



the rays of the sun and atmosphere, and for radiation unto the sky, thus for 

 the heating or cooling causing convectional currents. In the latter case we 

 have a more or less smooth surface of the same quality throughout, reflecting 

 the heat-rays and consuming their power by evaporation, without being able 

 to produce differences of heating sufficiently strong to cause convectional 

 currents comparable in strength with those on land. 



The conditions under which the Fram was drifting across the arctic polar 

 sea are in many respects different from those of ordinary meteorological 

 stations. On a large scale, the surface of the earth is a level surface like 

 the sea, the horizon is free, and no mountains or hills disturb the uniformity 

 of the level. But on a small scale this surface is not at all level; it is rugged 

 in the extreme, and presents hillocks and furrows, elevations and depressions, 

 while the pressure-ridges form a network extending in every direction. And 

 the physical nature of the surface, like that of the sea, is of a particular 

 homogeneity, consisting as it does of frozen water in the form of snow or 

 ice, and only in the middle of summer partly covered with liquid water. 



The rugged surface of the ice acts as a resistance, causing the lowest 

 layers of the air to move more slowly than the higher layers. The condi- 

 tions for an increasing velocity of the wind with elevation were amply full- 

 filled in the case of the Fram. No direct measurements of the velocity of 

 the wind were made with the anemometer at different heights, but Professor 

 NANSEN'S experience, as he has told me, was that the wind at the crow's 

 nest, some 32 metres above the water-line (Vol. I. PI. Ill) was always much 

 stronger than at the height of a couple of metres above the surface of the ice. 



The conditions for creating ascensional and descensional motions in the 

 air above the polar ice are the same as on land. 



(1) The radiation from the sun or the sky heats the surface, but the 

 amount of this heating depends on the absorbing power of the surface. This 

 power varies in snow, ice and water. The absorbing power of the snow may 

 vary with its degree of freshness, compactness, or purity; that of the ice with 

 its degree of roughness, transparency, or purity; and that of the water with 

 its degree of agitation or saltness. The effect of the radiation also depends 

 upon the inclination to the horizon of the respective surface-element. The 

 sunny side receives more heat from the sun or the atmosphere than the 

 opposite or shady side. Under these circumstances, different parts or adjacent 



