What Is Wet Bulb Temperature?
Wet bulb temperature (Tw) is the lowest temperature achievable through evaporative cooling. It is measured by wrapping a thermometer in a wet cloth and measuring as water evaporates. In dry air, evaporation is fast and Tw drops well below the dry bulb. At 100% humidity, Tw equals dry bulb temperature.
Wet bulb temperature is critical for heat stress assessment. When Tw approaches 35°C, the human body cannot cool itself through sweating, making sustained outdoor exposure potentially fatal. Climate change is increasing dangerous wet bulb events worldwide.
Approximation Formula
This Stull (2011) approximation is accurate to ±0.3°C for RH 5-99% and temperatures -20 to 50°C.
Danger Thresholds
| Wet Bulb (°C) | Risk | Action |
|---|---|---|
| < 25 | Low | Normal activity |
| 25-28 | Moderate | Limit strenuous exercise |
| 28-31 | High | Reduce outdoor exposure |
| 31-35 | Extreme | Dangerous for outdoor work |
| > 35 | Lethal | Fatal within hours |
Frequently Asked Questions
Why is 35°C wet bulb lethal?
Human core temperature is 37°C. When wet bulb exceeds 35°C, evaporative cooling from sweat cannot lower skin temperature below core temperature, causing inevitable hyperthermia and death within hours even for healthy resting individuals in shade.
How is wet bulb different from dew point?
Dew point is the temperature at which condensation begins. Wet bulb is the temperature of adiabatic evaporative cooling. Always: dew point ≤ wet bulb ≤ dry bulb. At 100% RH, all three are equal.
Where are the highest wet bulb temperatures?
The Persian Gulf, South Asia, and US Gulf Coast experience the highest values. These combine high temperatures with high humidity from warm oceans. The highest recorded wet bulb was 35°C in Pakistan and the UAE.