Wet bulb temperature is the lowest temperature that can be obtained by evaporating water into the air at constant pressure. The term got its name from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperature can be used along with the dry bulb temperature to calculate dew point or relative humidity.