Wet bulb temperature calculator
Enter the air temperature and relative humidity to estimate the wet-bulb temperature, the lowest temperature evaporation can cool a wet surface to. The calculator also shows the dew point and how far the wet bulb is below the air temperature.
Runs in your browser: nothing you enter is sent anywhere.
How to use
- Choose Celsius or Fahrenheit.
- Enter the air (dry-bulb) temperature and the relative humidity in percent.
- Read the wet-bulb temperature, the depression (air minus wet bulb) and the dew point.
Stull (2011), with T in °C and RH in percent:
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
Temperatures in °F are converted to °C first and the result back. The dew point comes from the Magnus formula, the same as in the dew point calculator.
The wet bulb always lies between the dew point and the air temperature.
Worked example
68 °F and 50 % humidity (the default values) give a wet bulb of 56.7 °F, 11.3 °F below the air, with a dew point of 48.7 °F.
In Celsius: 20 °C and 50 % give 13.7 °C. At 95 °F and 40 % the wet bulb is 76.1 °F; at 30 °C and 70 % the wet bulb is already 25.6 °C.
Tips
- Use a reading from the shade; a thermometer in sunlight shows more than the air temperature.
- The formula is an approximation of a psychrometric calculation, accurate enough for weather and comfort, not for laboratory work.
- Wet-bulb temperature is not the same as the WBGT heat-stress index, which also accounts for sun and wind.
Frequently asked questions
What is the wet-bulb temperature?
It is the temperature a thermometer wrapped in a wet cloth shows when air flows past it. Evaporation cools it below the air temperature; the drier the air, the bigger the difference. In saturated air (100 % humidity) the wet bulb equals the air temperature.
Which formula does the calculator use?
Roland Stull's 2011 empirical formula: Tw = T·atan(0.151977·(RH + 8.313659)^½) + atan(T + RH) − atan(RH − 1.676331) + 0.00391838·RH^1.5·atan(0.023101·RH) − 4.686035, with T in °C, RH in % and the arctangent in radians. It was fitted at sea-level pressure.
Where is the formula valid?
Stull fitted it for relative humidity from 5 % to 99 % and air temperature from −20 °C to 50 °C. Outside that range the calculator still shows a number but marks it; at high altitude the lower pressure also makes the true value somewhat different.
Why does the wet-bulb temperature matter for heat?
Sweat cools the body by evaporation, and evaporation cannot cool below the wet-bulb temperature. That is why hot and humid air is far more dangerous than hot and dry air at the same temperature.