vations showing an easterly wind, an observa- 

 tion with a westerly wind may appear. A re- 

 versal in wind direction can easily occur in the 

 vector addition of the measured apparent wind 

 and the speed and direction of the ship. Incor- 

 rect vector addition also results in wrong wind 

 speeds and directions which are not as readily 

 detected as a 180° error in wind direction. 



Effects of Data Inadequacies 

 on the Heat Exchange Results 



The effects of data inadequacies in the trade 

 wind zone on the estimates of heat exchange 

 processes are best illustrated by examples. 

 Most critical are uncertainties in the cloudi- 

 ness, since it affects the primary process in 

 the net heat exchange across the sea surface, 

 the heat of radiation from sun and sky. 



The cloudiness (table B) shows remarkable 

 consistency both in spatial and temporal distri- 

 bution. This consistency may, however, be 

 misleading. The average obtained from only a 

 few cloud observations per month that range 

 from clear sky to solid overcast quickly con- 

 verges to a value representing a semiovercast 

 condition. With at least daily observations, 

 however, the monthly average may shift to a 

 higher or lower value. 



If an error of 0.1 is assumed for the cloudi- 

 ness, the cloud factor would be in error from 

 about 5 percent for low values in cloudiness to 

 over 20 percent for high values in cloudiness. 

 For example, at lat. 7° N., long. 142° W., July 

 1963, table B shows that C - 0.9 and Q(S) - 247 

 cal. cm7^day~'. An error of 0,1 in cloudiness 

 would change the heat of radiation from sun and 

 sky about 50 cal. cm7^day~'. A similar error 

 at lat. 22° N., long. 157° W., July 1963 with C = 

 0.4 and Q(S) - 529 cal. cmT^day"' would produce 

 a change of 30 to 40 cal. cmT^day"' in the heat 

 of radiation from sun and sky. 



The error in the monthly cloud estimate may 

 be larger than O.I, and other factors, such as 

 the type and thickness of clouds, that have not 

 been considered here may introduce errors of 

 similar magnitude. 



Values for the effective back radiation of ta- 

 ble B show that they fall within the limits of ob- 

 servation during the TWZO cruises (Charnell, 

 1967) and are small compared with the magni- 

 tude of the radiation from sun and sky. The ef- 

 fect of variations in effective back radiation on 

 the net heat exchange is also smaller than that 

 of variations in the radiation from sun and sky. 



Again, cloudiness enters into the effective 

 back radiation, since the long wave radiation 

 from the sky reduces the magnitude of the long 

 wave radiation which escapes into space from 

 the sea surface. An error of 0.1 in the cloudi- 

 ness introduces a relative error in the cloud 

 factor that ranges from 1 percent for low val- 

 ues of cloudiness to about 20 percent for high 

 values of cloudiness. In terms of the results 

 listed in table B, to use an extreme example, 

 the effective back radiation at lat. 7° N., long. 

 142° W., July 1963, for a solid overcast would 

 be 52 cal. cmT^ day"' instead of 65 cal. cmT^ 

 day"' for 0.9 cloudiness. 



In low latitudes the thickness of the humid, 

 tropical air layer may be as important as the 

 cloudiness. Charnell (1967) reported that the 

 variations of long wave radiation measured 

 during clear sky conditions were of the same 

 magnitude as the differences between clear sky 

 and overcast conditions. He pointed out that 

 radiosonde data indicate variations of thickness 

 of the tropical air mass of the order of 1,000 m. 

 within a few days. Such variations in the thick- 

 ness of the humid air layer may have an effect 

 on the long wave radiation similar to that of a 

 varying cloud cover. Although the factor con- 

 taining the vapor pressure in the effective back 

 radiation equation partially accounts for the hu- 

 mid air mass effect, in low latitudes the effect 

 of variations in the thickness of the humid air 

 mass should also be considered. 



The conduction of sensible heat is a small 

 quantity as compared with the other heat ex- 

 change processes. Uncertainties in this term 

 due to the shortcomings of the meteorological 

 observations have a negligible effect on the esti- 

 mate of net heat exchange across the sea sur- 

 face. 



Since the heat of evaporation is a large quan- 

 tity, errors in this process due to data uncer- 

 tainties may seriously affect the net heat ex- 

 change results. The heat of evaporation depends 

 on the sea-air vapor pressure difference and 

 the nonlinear function of the wind speed when a 

 variable drag coefficient is used. The effect on 

 the former due to erroneous sea-water temper- 

 ature or psychrometric measurements has al- 

 ready been considered. The latter is affected 

 by the manner of processing the meteorological 

 data. 



