388 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Again, no observations of back radiation are 

 available and, as before, the principal error would 

 be due to an incorrect cloud factor for the trade 

 wind area. 



Next in the net heat exchange across the sea 

 surface is the evaporation. Its importance to the 

 net heat exchange can be gauged from the fact that 

 about 585 calories are used to evaporate 1 gram 

 of sea water. No observations for Qe are avail- 

 able, so that computed values must be used. These 

 are based on formulae described both by Sverdrup 

 and others (1942) and Jacobs (1951). They sug- 

 gested simplified formulae for use with average 

 climatic data to obtain the evaporation as a func- 

 tion of the sea and air vapor pressure difference 

 and the wind speed. Albrecht (1951) computed 

 the evaporation for the Indo-Pacific using the 

 formula from Sverdrup (1936) and the meteoro- 

 logical data of the revised, 1944, German edition 

 of McDonald (1938). In checking his results by 

 means of the total Indo-Pacific water budget, he 

 estimated the calculated evaporation to be 10 

 percent too low. Since most meteorological ob- 

 servations at sea are made from merchant ships, 

 Albrecht assumed heigh t-of-bridge (8 m.) for psy- 

 chrometric observations and masthead (20 m.) 

 for wind observations. By assuming both obser- 

 vations to be made at bridge height (8 m.), the 

 computed evaporation would be 10 percent higher. 



For computations of the net heat exchange in 

 this paper, revised evaporation charts, received 

 from Dr. Albrecht in a personal communication, 

 were used. Figure 13 shows the seasonal variation 

 in the heat used for evaporation at 20° N. and 



400 



400 



Figure 13. — Seasonal variation in the heat used for evap- 

 oration at 20° N. and 160° W. 



NORTH LATITUDE 



FiGUBB 14. — Meridional profiles of the heat used for 

 evaporation, 160° W. during April, August, and Decem- 

 ber. 



160° W. It peaks at about 330 cal. cm.-^ day-' in 

 April, with the low evaporation period extending 

 from Julj' to January at 220 to 250 cal. cm."^ 

 day-'. 



Meridional profiles of the heat used for evapora- 

 tion at 160° W. are shown in figure 14. In April, 

 during the maximum evaporation period at 20° N., 

 it is 260 cal. cm.-^ day-' at 10° N., 340 cal. cm.-^ 

 day-' at 18° N., and 210 cal. cm. -May-' at 30° N. 

 In August, during the minimum evaporation 

 period at 20° N., it is 220 cal. cm.-^ daj^-' at 

 10° N., 240 cal. cm.-2 day-' at 17° N., and 170 

 cal. cm. -2 day"' at 30° N. In December, the 

 evaporation is 240 cal. cm."^ day-' at 10° N., 

 220 cal. cm.-2 day-' at 22° N., and 260 cal. cm.-^ 

 day-' at 30° N. 



It should be noted that whereas maximum and 

 minimum periods of evaporation at 20° N. occur in 

 April and August, at 30° N. they occur in winter 

 and summer (December and June), respectively, 

 illustrative of different cUmatic areas. 



Riehl and others (1951) obtained an ahnost 

 exact heat balance in the atmosphere northeast 

 of Hawaii by using heat equivalents for evapora- 

 tion based on Jacobs' method of computation. 

 The values based on weather-ship observations 

 during July to October 1945 yielded evaporations 

 of 230 to 250 cal. cm.-^ day-' for 26° N., 149° W. 

 and Oahu, respectively. These are in good agree- 

 ment with Albrecht's results. Although Riehl's 

 values were obtained by essentially tlie same 

 method as Albrecht's, they do provide an indirect 



