Temperature/Levels » Wet-Bulb Temperature (Tw) Definition
Definition: Wet-Bulb Temperature
The wet-bulb temperature (Tw) is the temperature to which a parcel of air at a constant pressure cools through the evaporation of water into it. At this temperature, the parcel becomes saturated.
In other words, take a parcel of air, not at saturation. Then, at constant pressure (with no vertical motion), evaporate water into the parcel. Evaporative cooling will occur until the parcel reaches saturation. The wet-bulb temperature is reached when the air parcel achieves saturation.
The wet-bulb temperature will always fall between the dewpoint and the temperature, unless the air is saturated. At saturation, the temperature, dewpoint, and wet-bulb temperature are equal.
In the real atmosphere Tw often provides a good estimate of what the surface temperature will become after the onset of precipitation and once conditions become saturated.