Is used in psychrometry and is the temperature recorded by a thermometer whose bulb has been covered with a wetted wick and whirled on a sling psychrometer. Taken with the dry bulb, it permits determination of relative humidity of the atmosphere.
Wet bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water into the air at a constant pressure. It is therefore measured by wrapping a wet wick around the bulb of a thermometer and the measured temperature corresponds to the wet bulb temperature. The dry bulb temperature is the ambient temperature. The difference between these two temperatures is a measure of the humidity of the air. The higher the difference in these temperatures, the lower is the humidity.