Section 15. Evaporation and Condensation 



At the interface between the air and water, or between air and ice, the water molecules are 

 constantly transformed from liquid or solid phase into gaseous phase and vice versa. 



If the number of molecules transmitting into the gaseous phase is greater than the number of 

 molecules transmitting, simultaneously, into the liquid or solid phases, we have evaporation. 

 Otherwise we have condensation. 



It was stated that the further from the state of saturation the atmosphere contiguous to the 

 water surface, the greater is the speed of evaporation. Analogously, the more saturated the at- 

 mosphere with water vapor, the greater is the speed of condensation. Both evaporation and con- 

 densation are intensified by wind, because new air masses are involved in the process. 



The air over the sea is almost always in motion. The wind brings air masses of various 

 temperatures and moisture contents over ocean areas with various surface temperatures, and the 

 air masses are now heated, now cooled. In the first case, the moisture deficit will increase and 

 evaporation will be intensified; in the second case, on the contrary, the deficit will decrease and 

 the condensation and the formation of fog will be intensified. The greater the difference between 

 the temperatures of the surface layers of the sea, the more clearly are the mentioned phenomena 

 pronounced. 



The areas where the cold and warm sea currents come into contact are characterized by in- 

 tensified evaporation at winds blowing from the cold current toward the warm current, by intense 

 condensation and fog* at winds blowing from the warm sea toward the cold current. 



These phenomena become still more typical in areas covered by the arctic ice where alter- 

 nate the underlying surfaces of ice and water, the temperatures of which are different. When 

 navigating in scattered to broken ice at weak winds, now clearing, now fogging replace each other 

 in connection with smaller or greater amounts of ice encountered. 



A similar alternate clearing and fogging with weak winds is also observed in the fog of an ice- 

 free sea, but the origin of this phenomenon is different. The point is that the sea fog, generally, 

 does not propagate high above the sea surface. Owing to the differences of wind speeds above the 

 fog, waves are formed on the surface of the fog that are completely identical to the Helmholtz 

 waves observed at the lower and upper surfaces of clouds. Naturally, a rarefaction of clouds is 

 observed at the base of these waves and a compaction of clouds at the crest. It is also natural that 

 the crests of these fog waves are located approximately in a direction perpendicular to the wind. 

 Intense intermixing occurring at strong winds eliminates this phenomenon. 



The evaporation and condensation processes continue at very low air temperatures; however, 

 the absolute moisture is, in this case, so small that only a slight mist, and not fog, is formed at 

 cooling and condensation. 



*The greatest producer of fog is the cold Labrador current in the area where it comes in con- 

 tact with the warm waters of the gulf stream. In this respect is also known the area of Bear Island 

 where the water of the cold Bear Island current comes in contact with the warm water of the 

 Spitsbergen and Nordkapp currents . 



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