221 



E = const. Aq u. 



The function depends to lesser degree on other quantities, such as the tempera- 

 ture difference and the two heights. 



As interfacial stress and evaporation are closely related phenomena, and as 

 Munk reports Van Dorn to have found a simple relation between stress and wind 

 we might expect evaporation to be a simple function of wind. Further new 

 evidence comes from the studies at Lake Hefner in Oklahoma. I have been 

 privileged to hear about the results, as yet unpublished, from G.Earl Harbeck, 

 Jr. (Geological Survey), E.R. Anderson, and J.J. Marciano at U. S. Navy Elec- 

 tronics Laboratory, San Diego. Comparison of measured daily evaporation 

 from Lake Hefner with meteorological measurements supports the simple linear 

 function. 



The Lake Hefner formula, however, should not be assumed to apply to the 

 ocean. To find oceanic evaporation, two steps are required: (1) The difficult 

 step of making sufficient measurements of E, Aq, u, and other quantities to de- 

 termine the evaporation formula. (2) The easier step of using this formula to 

 calculate evaporation from Aq, u, etc. 



The only method so far suggested for measuring evaporation from the 

 ocean is that of measuring the eddy flux of aqueous vapor a short distance above 

 the interface. The unit- area flux is ^/'vzq'^ j^y' ' where/*^is air density and 

 V2 is the upward component of velocity. This flux has been successfully meas- 

 ured over land by Swinbank (1951) by use of a hot-wire anemometer and thermo- 

 couple thermometers. Probably his method cannot be used over the ocean with- 

 out modificiation, but presumably a means can be found. This is the place 

 where instrumentation is needed. 



With regard to the second step, existing observations of wind, humidity, 

 and interfacial temperature, although very numerous, may not be adequately 

 accurate or suitable except in special cases. The weather-ship observations, 

 made regularly at fixed positions, can be expected to provide the most useful 

 data. Perhaps a special program will be required to obtain suitable observa- 

 tions from research ships and other selected ships. (An attempt to include such 

 observations in the bathythermograph program was early abandoned). 



Although the way seems clear, the study of evaporation is now inactive. 

 The reason for this inactivity must be that no great need is felt for knowing the 

 evaporation from the ocean. 



