The evaporation and condensation resulting from contact between very cold air and relatively 

 warm sea surface are no less typical phenomena. In arctic conditions, the fog blanket curls over 

 individual spaces and cracks in the ice at low temperatures, the curling being reminiscent of steam 

 over a plate of hot soup. This is explained by the fact that water vapor rising from the sea surface 

 becomes intermixed with the contiguous cold air and is condensed into clouds of fog which gradually 

 dispenses. 



According to Mitchell and Albers, for the formation of winter fog it is necessary that air 

 temperature be 10.6° below the temperature of fresh water and 14. 40° below the temperature of sea 

 water. In winter, however, the temperature difference in the arctic reaches 40°. 



A stable winter fog can occur only under certain conditions. It is necessary for the extremely 

 cold air to be covered above by warmer air (temperature inversion), which limits convection. In 

 such a case, slight evaporation causes supersaturation. But the simultaneous heating may be in- 

 sufficient for the formation of convertlve currents. Therefore, the formation of winter fog In the 

 sea can take place only over ice or Intensely cooled land* — from which masses of very cold air 

 may reach the sea. 



Such fog formations during severe frosts are observed not only over spaces and cracks in sea 

 ice but also over thin ice. In open portions of the sea, only summer fog is possible. This, in 

 contrast to the winter fog, is formed only over a colder sea surface or over ice. 



The unstable winter fog, as a result of strong convective currents formed over warmer sur- 

 faces, induces intense evaporation. The Intense evaporation of snow and ice in arctic regions, and 

 in winter also in temperate latitudes, is explained by the higher temperatures of the underlying 

 surface in comparison to the air temperatures. Thus, the moisture deficit appears to be the main 

 factor determining evaporation or condensation. The greater the moisture deficit, the more in- 

 tense the evaporation. The temperature of the underlying surface and the air affect the evaporation 

 also in indirect ways — namely, at high temperatures with the same quantity of water vapor in the 

 air, the moisture deficit increases. Of the greatest significance in this respect is the rise of 

 temperature of the underlying surface, which causes, in addition to an increase in the moisture 

 deficit, the formation of convective currents which take away the particles of evaporating water 

 from the surface. The latter aspect is of very great Importance. Indeed, the evaporation is ac- 

 companied by almost momentous formation of a thin saturated air stratum at the sea surface, which 

 is slightly cooled and therefore, despite the fact that the density of water vapor makes up only about 

 0.6 of the density of the air (this circumstance is not accounted for in the present theories on evap- 

 oration), it obstructs further evaporation. Convection eliminates this stratum, but because the 

 wind is of still greater significance, the evaporation is always Intensified with the intensification of 

 windc 



Reaching a certain force, the wind gains in the sea special significance for evaporation. In- 

 deed, as soon as the wind starts tearing off the crests of waves, minute particles of water are 

 thrown Into the air, which, by means of turbulent air movement, are raised to the layers that are 

 less saturated with moisture and evaporate. The salt crystals are carried into the atmosphere 

 and, owing to their hygroscopicity, become excellent condensational nuclei for the formation of 

 clouds and fog. 



*The Gulfs of Finland and Bothnia of the Baltic Sea and the White Sea, due to their small sizes 

 In comparison to the sizes of the adjacent land masses, are typified by their winter fog. Here it 

 sometimes happens that in winter very cold air masses reach the sea from the land and each influx 

 of these air masses Involves the formation of fog or foggy smoke. 



33 



