Variations in Relative Humidity With Height

Relative Humidity

During the day near the surface, particularly with clear skies, both the temperature and absolute humidity usually decrease with height. These two variables have opposite effects on the relative humidity. Which effect is dominant depends upon the dryness of the surface. The relative humidity usually increases with height over normal surfaces because the effect of the decrease in temperature is greater than that of the decrease in absolute humidity. Over a moist surface, however, the effect of the decrease in absolute humidity may overbalance that of temperature decrease, and the relative humidity in the surface layer will decrease with height.

At night, the change of temperature with height usually predominates, and the relative humidity will decrease with height through the lowest layers.

Above the lowest layers, the relative humidity generally increases with height in the day through much of the lower troposphere. Convection alone would account for this increase. As air is lifted, the temperature decreases 5.5° F per 1,000 feet, and the dew point decreases at about 1° F per 1,000 feet. Therefore, the dew point and the temperature become 4.5° F closer per 1,000 feet, and the relative humidity increases until saturation is reached.

A subsiding layer of air in the troposphere warms by the adiabatic process and forms a subsidence inversion. The relative humidity will decrease upward through the temperature inversion at the base of the subsiding layer. The marine inversion along the west coast, for example, is a subsidence inversion. The marine air below has low temperatures and high humidities, and the adiabatically heated subsiding air mass above has higher temperatures and lower humidities. This pronounced change in temperature and humidity is evident along the slopes of coastal mountains when the marine inversion is present.