In deriving the surface latent heat flux with the bulk formula for the thermal forcing of some ocean circulation models, two approximations are commonly made to bypass the use of atmospheric humidity in the formula. The first assumes a constant relative humidity, and the second supposes that the sea-air humidity difference varies linearly with the saturation humidity at sea surface temperature. Using climatological fields derived from the Marine Deck and long time series from ocean weather stations, the errors introduced by these two assumptions are examined. In the extratropical oceans, large meridional and seasonal variations of relative humidity are found, and the seasonal cycle of sea-air humidity difference is out of phase with that of sea surface temperature. The air humidity reaches the minimum earlier than the sea surface temperature during winter cooling. The errors reach above 100 W/m2 over western boundary currents and 50 W/m2 over the tropical ocean. The two approximations also introduce erroneous seasonal and spatial variabilities with magnitudes over 50% of the observed variabilities. Meteorological reports from atolls in the tropical Pacific indicated that in the atmosphere the humidity anomalies were much more pronounced than the temperature anomalies as the center of deep convection was dislocated during the 1982-1983 El Ni¿o-Southern Oscillation episode. ¿ American Geophysical Union 1990