What the wet-bulb temperature measures
Wrap a thermometer bulb in a wet cloth and blow air over it. Water evaporates, takes heat with it, and the reading drops until the air around the cloth is saturated. That reading is the wet-bulb temperature. In dry air it sits far below the air temperature; in saturated air the two are equal.
Your skin works the same way. Sweat cools you only if it can evaporate, so the wet-bulb temperature is a direct measure of how well the body can shed heat.
The formula
The calculator uses Stull’s 2011 empirical formula, with T in °C and RH in %:
Tw = T × atan(0.151977 × √(RH + 8.313659)) + atan(T + RH) − atan(RH − 1.676331) + 0.00391838 × RH^1.5 × atan(0.023101 × RH) − 4.686035
It is accurate to about 1 °C between 5% and 99% humidity and −20 °C and 50 °C at sea-level pressure. Fahrenheit inputs are converted to Celsius first.
Worked example
On a 95 °F afternoon with 50% relative humidity:
- 95 °F = 35 °C.
- Stull’s formula gives Tw ≈ 26.6 °C (79.9 °F).
- The wet-bulb depression, air temperature minus wet-bulb, is 15.1 °F: sweat still evaporates reasonably well.
At the same 95 °F with 80% humidity, the wet-bulb temperature climbs to about 32 °C (90 °F), which is dangerous for prolonged exertion.
Reading the result
| Wet-bulb temperature | What it means |
|---|---|
| Below 75 °F (24 °C) | Low heat stress for most people |
| 75–82 °F (24–28 °C) | Moderate heat stress during physical activity |
| 82–88 °F (28–31 °C) | High risk: limit heavy work, schedule breaks, drink water |
| 88–95 °F (31–35 °C) | Extreme: the body can no longer cool itself during effort |
| 95 °F (35 °C) and above | Beyond the theoretical limit of survival, even at rest in the shade |
The 35 °C limit comes from physiology models; laboratory studies from Penn State in 2022 found that young healthy adults lost control of their core temperature at wet-bulb values closer to 31 °C.
Wet-bulb versus WBGT and heat index
The heat index published by the National Weather Service expresses the same heat and humidity combination as a “feels like” air temperature. The wet-bulb globe temperature (WBGT), used by OSHA, the military and high school athletics, adds solar radiation and wind. In the shade, WBGT is close to 0.7 × wet-bulb + 0.3 × air temperature. For outdoor work or sport in the sun, use a WBGT reading when one is available.
Staying safe
When the wet-bulb temperature is high, move strenuous activity to the early morning, rest in shade, drink water regularly and watch for dizziness, nausea or confusion, which are warning signs of heat exhaustion and heat stroke.