Wet Bulb Calculator
Estimate wet bulb temperature from air temperature and relative humidity.
What it does
Wet bulb temperature marks the floor for evaporative cooling. It is the temperature a parcel of air reaches when water evaporates into it until the air is saturated, so it can never exceed the dry bulb reading.
The number matters anywhere evaporation does work: cooling towers, evaporative coolers, and above all human bodies. Sweat only cools people while the air still accepts water vapor, which is why humid heat is far more dangerous than the same temperature in dry air.
The Stull 2011 formula used here is a published fit to psychrometric tables, accurate to roughly 0.3 C across normal weather conditions. It takes dry bulb temperature and relative humidity and delivers the wet bulb value directly, no sling psychrometer required.
The defaults show why humidity drives the story. At 90 F and 60 percent humidity the wet bulb sits at 78.8 F, close enough to body temperature to blunt sweating. Drop humidity to 20 percent and the same air would read well below 70 F wet bulb, comfortable for any outdoor work.
The hard limit for humans is near 35 C wet bulb, where even a resting body stops shedding heat. Wet bulb readings that high are rare but rising in heat waves, which is why weather services now publish them alongside the heat index. This page computes the default cases, a worked example, a reference table, and practice checks.
How to use
- Pick F or C to match the forecast.
- Enter the air temperature from the shade.
- Enter the relative humidity as a percent.
- Read the wet bulb temperature.
- Read the depression, the gap under the dry bulb.
- Compare against heat safety guidance for your activity.
Formula
- Tw
- Wet bulb temperature in degrees Celsius from the fit.
- T
- Dry bulb air temperature in degrees Celsius.
- RH
- Relative humidity as a percent, 5 to 99.
- atan
- Inverse tangent, the arctangent of each term.
The fit covers dry bulbs from -20 C to 50 C and humidity from 5 to 99 percent, with typical error under 0.3 C against full psychrometric calculations. Below freezing it estimates an ice bulb loosely, since the real process freezes on the wick. Metric and imperial modes run the same Celsius math and convert the answer, so the two systems agree exactly.
Worked example
Example: 90 F at 60 % humidity
Air temperature 90 F with 60 % relative humidity (defaults).
- Convert to Celsius: (90 - 32) x 5/9 = 32.2 C.
- Run the Stull fit: Tw = 26.0 C.
- Convert back: 26.0 x 9/5 + 32 = 78.8 F.
- Depression: 90 - 78.8 = 11.2 F below the dry bulb.
- Metric mirror: 32 C at 60 % gives 25.8 C, mostly unchanged.
Reference table
| Dry bulb | Humidity | Wet bulb |
|---|---|---|
| 20 C | 50 % | 13.7 C |
| 30 C | 40 % | 20.4 C |
| 32 C | 60 % | 25.8 C |
| 35 C | 70 % | 30.3 C |
| 40 C | 50 % | 30.9 C |
Limits
- Fit range: -20 C to 50 C and 5 to 99 percent humidity.
- Below freezing it is a loose ice bulb estimate.
- No wind, sun, or altitude effects; those change real evaporative cooling.
- A number for planning, not medical advice on heat safety.
Common mistakes
- Entering humidity as a fraction like 0.6 instead of 60.
- Using a sun baked surface temperature rather than shade air.
- Reading wet bulb as the dew point, which only matches at 100 percent.
- Comparing Fahrenheit and Celsius values without converting.
- Assuming a low wet bulb makes heavy work safe when sun and wind add load.
Try it yourself
Quick checks. Answers sit tucked under each question.
20 C at 50 % humidity?
13.7 C wet bulb, 6.3 C depression.
95 F at 40 % humidity?
76.1 F wet bulb, 18.9 F depression.
35 C at 70 % humidity?
30.3 C wet bulb, 4.7 C depression.
Questions
What is wet bulb temperature?
The lowest temperature air can reach by evaporation alone. Water evaporating into the air cools the remaining air toward this floor.
Why is 35 C wet bulb dangerous?
Human skin cools by evaporation. Once wet bulb reaches skin temperature, sweat cannot carry heat away and core temperature rises even at rest.
How is it different from dew point?
Dew point is where air saturates on cooling; wet bulb is where evaporative cooling bottoms out. They meet at 100 percent humidity and diverge as the air dries.
Can wet bulb exceed the dry bulb?
No. Evaporation can only bring air down to the wet bulb value, never above the actual air temperature.
Does wind change it?
Moving air changes how fast evaporation happens and how comfortable you feel, but the thermodynamic wet bulb value itself stays the same.
How accurate is the formula?
Stull reports typical errors under 0.3 C inside the fit range. Outside 5 to 99 percent humidity or the temperature limits, accuracy drops quickly.
Sources
- Stull 2011: wet bulb temperature from relative humidity and temperature
- National Weather Service: wet bulb calculator page