418 BOUNDARIES OF THE SEA 



Close to the sea surface iv will be very high and take a value 

 that corresponds to the rate of impact of molecules on the surface. 

 As gradients near the surface are large, w will decrease rapidly as 

 the distance from the surface increases, but farther away from the 

 surface w will decrease more slowly because gradients are small; 

 it always decreases with distance. 



It was mentioned earlier that the rate of evaporation from ocean 

 surfaces can hardly be limited by the mixing conditions near the sea 

 surface. An estimate of w for the atmosphere at the height of a few 

 meters can therefore be made from the rate of evaporation, the 

 relative humidity, and the temperature of the sea surface. With 

 Wust's (1954) estimate of the rate of evaporation in trade wind 

 regions (130 cm-year~^), a relative humidity from ships' measure- 

 ments of 80% and a sea surface temperature of 25° C, the corre- 

 sponding transport velocity would be 0.9 cm-sec~^ This should be 

 the rate of air exchange between the sea surface and the obser\^a- 

 tion level, say 8 m above the surface. If we used instead the average 

 cloud base level where we know the water vapor density, we would 

 arrive at a somewhat lower value for the transport velocity — 

 probably 0.3 to 0.4 cm-sec~^ This would be due to a lower water 

 vapor density at that level compared with the density at shipboard 

 level. This value is of special interest as it can be applied to com- 

 putations of the rate of tritium transfer to the sea by turbulent 

 diffusion. The tritium concentration at cloud base level can be 

 estimated from rain water analyses, and its concentration at the 

 sea surface can be taken as zero. The turbulent flux is thus easily 

 computed, and will be about twice the flux of tritium in precipi- 

 tation. 



With a still higher level, say the top of the troposphere, w with 

 respect to the sea surface will probably drop to about 0.2 cmsec~^ 

 or less, as judged from the residence time estimated for various 

 isotopes produced by cosmic rays near this level. 



In returning to the transport velocity at shipboard level (0.9 

 cm-sec-^ it can be expected that this velocity also is going to 

 transport the horizontal momentum of the air, pii, to the sea sur- 

 face, the horizontal motion of which is generally neglected. With 

 6 m-sec"^ for the average wind velocity at the same level, the 



