56 



l/lO mm - is somewhat varying. This may be explained by variable surface 

 film effects. It is well-known that surface films, i.e. monomolecular 

 layers consisting of organic matter or originating from artificial contamina- 

 tion can be found on the sea surface rather often. The transport of water 

 through a compressed and, therefore, oriented monolayer is not an ordinary 

 diffusion process which involves a small energy barrier but is to be con- 

 sidered as a process in which the water molecule must pass along a molecular 

 pathway between long molecule chains, thus requiring a substantially higher 

 amount of energy (La Mer, I962). Consequently, a compressed monolayer 

 results in a retardation of evaporation. Laboratory studies have shown 

 (Haussler, 1955-56) that a cool skin does not occur in the presence of an 

 oil film, which prevents evaporation but does not impede the transfer of 

 heat. Thus it seems clear that the cool skin is actually due to evapora- 

 tion and that it will only exist on those parts of the sea surface which 

 are not covered by a monolayer. Basse's (1963) paper provides further 

 evidence for this . 



Quite recently it has been reported (Jarvis, I963) that monomolecular 

 films not only retard evaporation but will also change the temperature of 

 the sea surface, because of the effect of the film on the convective move - 

 ment of surface water. The surface is constrained by such a film, convection 

 is found to be inhibited with a consequent thickening of the surface layer 

 and a rise of the surface temperature which is said to be quite independent 

 of any reduction of evaporation. 



Field measurements indicating the reduction of evaporation by natural 

 surface films have been reported by Deardorff (1961). He compared the 

 evaporation rates of two pans floating at the sea surface . One of these pans 

 was filled with subsurface sea water vrhile special care was taJcen in filling 

 the second so that any surface film, which might have been present, would 

 have been retained. The result showed a distinct reduction of evaporation, 

 of the order of 20 percent, which was obviously caused by natural surface 

 films. This surface film effect adds another item to the substantial list 

 of difficulties encountered when trying to measure marine evaporation. 



Only very little is known about the effect that sea spray might exert 

 on evaporation, apart from that this influence must be present at higher 

 wind velocities. It is indeed very difficult to get any quantitative informa- 

 tion on this subject because field measurements of evaporation taken at the 

 sea surface can only be made with light winds where there is no appreciable 

 sea spray and, on the other hand, the problem seems hardly approachable by 

 means of theoretical studies. Perhaps, quantitative information can best 

 be expected from laboratory work. Relevant studies have been carried out 

 by Okuda and HayBmi (1959) in a wind- water channel of about 20 m in length. 

 They observed the vertical distribution of the horizontal transport by 

 sprayed water drops and found that the water transport by spray at 10 cm 

 height was negligible below 10 m/sec wind speed but increased rapidly in 

 the wind speed range of 12-13 m/sec. A strong decrease is found with 

 height, the water transport by spray at 30 cm being only about 10 percent 

 of its value observed at 10 cm height. The influence of sea spray on the 



