Wet Bulb Calculator
Wet-bulb temperature is the lowest temperature air can reach through evaporative cooling alone. It combines heat and humidity into a single number that matters for human survival: at a wet-bulb of 35°C (95°F), the human body can no longer cool itself through sweat, and prolonged exposure is fatal even for healthy people in the shade.
How wet-bulb temperature works:
Imagine wrapping a wet cloth around a thermometer and fanning it. The evaporating water cools the thermometer below the air temperature. How much it cools depends on the humidity. In dry air, lots of evaporation happens and the wet-bulb drops well below the dry-bulb. In humid air, evaporation slows down and the two temperatures converge.
This calculator uses the Stull (2011) approximation, which is accurate to within 0.3°C for typical conditions.
Risk levels by wet-bulb temperature:
- Below 25°C (77°F): Low risk, normal activity
- 25-28°C (77-82°F): Moderate, take regular breaks
- 28-31°C (82-88°F): High risk, limit exertion
- 31-35°C (88-95°F): Very high, dangerous for outdoor work
- Above 35°C (95°F): Human survivability limit
The 35°C threshold:
At a wet-bulb of 35°C, even a perfectly healthy person sitting naked in the shade with unlimited water cannot maintain a stable core temperature. The body produces about 80W of metabolic heat at rest, and when the air's wet-bulb reaches skin temperature (about 35°C), sweat simply cannot evaporate fast enough to dissipate it.
Real-world context:
Wet-bulb temperatures above 35°C have historically been extremely rare, but they are becoming more frequent with climate change. Parts of the Persian Gulf, South Asia, and the Mississippi River valley have already recorded brief exceedances.