Wet Bulb Temperature Calculator

Wet bulb temperature (Tw) is the lowest temperature achievable by evaporative cooling. It is measured by wrapping a wet wick around a thermometer and measuring the cooling from evaporation. When relative humidity is 100%, wet bulb temperature equals air temperature. When RH is lower, evaporation cools the thermometer below the air temperature. Wet bulb temperature is critical for heat stress assessment, winter precipitation type, and industrial cooling.

Stull (2011) published an empirical formula for wet bulb temperature: Tw = T * atan(0.151977*(RH+8.313659)^0.5) + atan(T+RH) - atan(RH-1.676331) + 0.00391838*RH^1.5 * atan(0.023101*RH) - 4.686035, where T is in Celsius and RH is in percent. This approximation is accurate to within 0.35 C for T from -20 to 50 C and RH from 5 to 99%.

The wet bulb globe temperature (WBGT) used for heat stress assessment incorporates wet bulb temperature because it measures the cooling possible through sweat evaporation. When wet bulb temperature exceeds 35 C (95 F), evaporative cooling becomes insufficient to maintain safe body temperature even in the shade, representing a survivability limit for humans exposed for several hours.

Winter precipitation type (snow, sleet, freezing rain, or rain) depends critically on wet bulb temperature through the column. When wet bulb temperatures are above 0 C throughout the column, rain falls. When wet bulb temperatures are at or below 0 C near the surface, snow or ice pellets fall. Forecasters use wet bulb temperature profiles from soundings to predict precipitation type.

Both are lower than the air temperature and indicate atmospheric moisture. The dewpoint is the temperature at which air becomes saturated at constant pressure. The wet bulb temperature is the temperature measured by an evaporating thermometer; it is always between the air temperature and the dewpoint (wet bulb equals dewpoint only when RH = 100%). Wet bulb temperature is used for psychrometric calculations.