How wet-bulb temperature calculator works
The wet-bulb temperature is what a thermometer wrapped in a wet wick reads as water evaporates off it. Evaporation pulls heat from the bulb, so the wet bulb sits at or below the air temperature — and the gap between them measures how much evaporative cooling the air still allows. It is the single best one-number summary of heat stress, because bodies cool by sweating, and sweat is exactly the evaporative process the wet bulb simulates.
The calculator uses the Stull (2011) approximation — accurate to roughly ±0.3 °C for relative humidity 5–99% and air temperature −20 to +50 °C: Tw = T·atan(0.151977·√(RH + 8.313659)) + atan(T + RH) − atan(RH − 1.676331) + 0.00391838·RH^1.5·atan(0.023101·RH) − 4.686035, with T in °C and RH in percent. Enter air temperature in °C or °F plus humidity and the tool returns the wet bulb in both scales.
The result is placed on a heat-stress band: below 25 °C wet bulb is manageable for most activity, 25–28 °C demands caution during exertion, 28–31 °C is very dangerous without cooling, and 35 °C is the theoretical survivability limit for a resting human in the shade — above it, evaporation can no longer net-cool the body and core temperature rises regardless. All math runs in your browser.