The wet bulb temperature is the temperature reading given by a mercury thermometer whose bulb is wrapped with a wet cloth. The wet-bulb temperature is generally a measure of atmospheric temperature when it has been cooled by evaporating moisture.
Source: Hardy, Colin C., Ottmar, Roger D., Peterson, Janice L., Core, John E., and Seamon, Paula. 2001. Smoke Management Guide for Prescribed and Wildland Fire 2001 Edition. Produced by the National Wildfire coordinating Group Fire Use Working Team.