Wet Bulb Temperature Calculator

How it works

T_w = T·atan[0.151977 (RH + 8.313659)^½] + atan(T + RH) − atan(RH − 1.676331) + 0.00391838 RH^(3/2) atan(0.023101 RH) − 4.686035 · T in °C, RH in % (angles in radians) · depression = T − T_w

The wet-bulb temperature is the lowest temperature air can be cooled to by evaporating water into it — what a thermometer wrapped in a wet cloth reads. Roland Stull (2011) fitted this single empirical equation to psychrometric data at sea-level pressure; it is accurate to within about ±1 °C for 5–99% humidity and −20 to 50 °C. Because human sweat cools the same way, the wet-bulb temperature limits how well the body can shed heat: research suggests that a sustained 31 °C is already dangerous for healthy young adults, and 35 °C is the theoretical survivability limit.

Worked example

At 30 °C and 50% relative humidity, Stull’s formula gives a wet-bulb temperature of 22.3 °C, a wet-bulb depression of 7.7 °C, and a dew point of 18.4 °C — low to moderate heat stress. At 20 °C and 50% it gives 13.7 °C, the check value in Stull’s paper.

Sources: Stull, R. (2011) Wet-Bulb Temperature from Relative Humidity and Air Temperature, J. Appl. Meteor. Climatol. 50, 2267–2269 · Vecellio et al. (2022) Evaluating the 35 °C wet-bulb temperature adaptability threshold, J. Appl. Physiol.

Frequently asked questions