Wet-Bulb Temperature Calculator

Calculates the wet-bulb temperature at normal air pressure from air temperature and humidity. The wet bulb temperature is the maximum outside temperature cooled by evaporation. It is measured with a psychrometer, which, to put it simply, is a thermometer that is kept moist (e.g. wrapped with a wet cloth that has the air temperature) and which therefore shows lower temperatures due to the evaporation cold. The higher the humidity, the less water can still evaporate and the higher and closer to the air temperature is the wet bulb temperature. From above approximately 30 °Celsius, the wet temperature is dangerous for people because the body can no longer cool down. 35 °Celsius for six hours is considered deadly.

The wet bulb temperature cannot be calculated exactly, but there are approximate formulas. The formula used here from Stull, Roland 2011: Wet-Bulb Temperature from Relative Humidity and Air Temperature, Journal of Applied Meteorology and Climatology, 50 (11), 2267–2269 delivers good results for normal air pressure of 1013.25 hPa, humidity between 5 and 99 percent and temperatures between -20 and +50 degrees Celsius, except for extreme cold and low humidity combined.

The formula is:
T_w = T * atan(0.151977 * √H + 8.313659) + atan(T+H) - atan(H-1.676331) + 0.00391838*H^3/2 * atan(0.023101*H) - 4.686035

T_w = wet bulb temperature in °C
T = air temperature in °C
H = humidity in percent

Wet-bulb temperature is likely to become an increasingly important value as climate change progresses, and we will hear more about it. So far in historical times, conditions have been such that a life-threatening wet-bulb temperature has only been reached very rarely and in a few regions. For example, for a wet-bulb temperature of 35 °C, you need a temperature of 40 °C and a humidity of 71 percent. These are still extreme values (as of 2023).

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