Relative Humidity

Relative humidity is the ratio of the actual water vapor pressure in air to the saturation vapor pressure at the same temperature, expressed as a percentage. Saturation vapor pressure rises sharply with temperature following the Clausius-Clapeyron relation, roughly 7 percent per kelvin, so the same vapor content gives lower relative humidity in warmer air. Over the open ocean near-surface relative humidity averages about 75 to 80 percent. The air-sea humidity difference, between the saturation value at the sea surface temperature and the ambient air humidity, drives evaporation and the latent heat flux in the bulk formula, so relative humidity is a primary control on ocean surface cooling.