Wet Bulb Temperature Calculator
Calculate heat stress and evaporation levels using Stull's Formula
What is Wet Bulb Temperature?
Wet bulb temperature is the lowest temperature that can be reached by evaporating water into the air at a constant pressure. Unlike the "dry bulb" temperature (what you see on a standard thermometer), the wet bulb temperature accounts for the cooling effect of evaporation. This makes it a critical metric for understanding human heat tolerance and atmospheric stability.
Why It Matters for Human Health
The human body cools itself primarily through the evaporation of sweat. When the wet bulb temperature is high, the air is so saturated with moisture that sweat cannot evaporate efficiently. Scientists generally agree that a sustained wet bulb temperature of 35°C (95°F) is the absolute limit of human survivability, even for healthy individuals in the shade with plenty of water.
How to Interpret Results
- Below 21°C: Low risk for most people.
- 21°C – 27°C: Caution recommended; physical activity may lead to fatigue.
- 27°C – 31°C: Extreme caution; high risk of heat exhaustion.
- 32°C+: Extreme danger; high risk of heatstroke.
The Formula Used
This calculator utilizes Stull's formula, an empirical equation accurate for standard sea-level pressure and relative humidities between 5% and 99%:
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
Calculation Example
If the air temperature is 30°C and the relative humidity is 60%:
- Input 30 into the Temperature field.
- Input 60 into the Humidity field.
- The calculation results in approximately 23.6°C.
- This indicates a moderate heat stress level where caution should be exercised during outdoor activity.