Wet-Bulb Temperature Calculator (from Temperature and Humidity)
Wet-bulb temperature from the air temperature and relative humidity with Stull's formula, and how far below the air temperature it sits.
Results
The dashed line marks the value you entered.
What this tool does
The wet-bulb temperature is what a thermometer wrapped in a wet wick reads in moving air: evaporation cools it, so it is the lowest temperature that evaporation alone can reach, and it equals the air temperature only at 100 % humidity. Stull's 2011 formula is a curve fitted to the full psychrometric calculation; it holds for relative humidity from about 5 % to 99 % and air from −20 to 50 °C at normal sea-level pressure, typically to within about 1 °C, and it drifts at altitude. Cooling towers, evaporative coolers and heat-stress assessments all work from the wet-bulb value rather than from temperature or humidity alone.
Formula
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 (Stull 2011, T in °C, RH in %)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
t | Temperature | °C |
rh | Relative humidity | % |
TW | Wet-bulb temperature | °C |
DP | Wet-bulb depression (air minus wet-bulb) | °C |
VA | Is the formula valid for these values? | — |
Worked example
- Temperature20 °C
- Relative humidity50 %
- Wet-bulb temperature13.7 °C
- Wet-bulb depression (air minus wet-bulb)6.3 °C
- Is the formula valid for these values?Yes — inside the range it was fitted to
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.