What is wet-bulb globe temperature (WBGT)?
Wet-bulb globe temperature (WBGT) is the most complete single measure of heat stress on the human body. Unlike the air temperature on a thermometer, it accounts for everything that actually determines whether heat is dangerous: temperature, humidity, wind, and direct sunlight.
It is the standard the U.S. military, OSHA, athletic associations, and occupational-safety bodies use to decide when it is no longer safe to train, compete, or work outdoors.
Why air temperature isn't enough
Your body sheds heat mainly by sweating — and sweat only cools you when it can evaporate. When humidity is high, evaporation stalls, and the same 95 °F that is a pleasant dry afternoon in one place becomes a medical emergency in another. Air temperature alone can't see that difference. WBGT can.
How WBGT is built
WBGT combines three measurements, weighted for outdoor conditions in the sun:
WBGT (outdoor) = 0.7 × natural wet-bulb + 0.2 × black-globe + 0.1 × air temp- Natural wet-bulb temperature captures humidity and the cooling power of evaporation — it carries the most weight because it reflects how well you can still cool yourself.
- Black-globe temperature captures radiant heat: direct sun and hot surfaces. This is why standing in the sun is far more dangerous than the same air temperature in the shade.
- Air (dry-bulb) temperature rounds it out.
Indoors or in full shade, the black-globe term drops out and WBGT is approximately 0.7 × wet-bulb + 0.3 × air temp.
Reading the danger levels
WBGT is interpreted on a danger scale (in °C):
| WBGT | Level |
|---|---|
| under 18 | Safe |
| 18–23 | Caution |
| 23–28 | Moderate |
| 28–32 | High |
| 32–35 | Extreme |
| 35+ | Lethal |
The exact thresholds an organization uses can vary, but the principle is the same: as WBGT rises, the safe amount of exertion drops sharply.
See it for your location
WetBulbTracker computes WBGT using the validated Liljegren (2008) energy-balance model and shows it live, anywhere on Earth — with sun-vs-shade, a 7-day forecast, and a personal "safe for me" read.