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 underpins the heat-stress guidance used by the U.S. military (TB MED 507), NIOSH's recommended exposure limits, the ACGIH threshold limit values and the international ISO 7243 standard, and it is the measure high-school athletic associations increasingly write into their heat policies. OSHA has no WBGT standard of its own: its Technical Manual references WBGT, and it acts on heat hazards through the General Duty Clause.
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
WetBulbTracker reads WBGT on this screening scale (in °C). These are our own general-purpose bands, not any governing body's thresholds — we show them so one color means one thing across the whole app:
| WBGT | Level |
|---|---|
| under 18 | Safe |
| 18 to under 23 | Caution |
| 23 to under 28 | Moderate |
| 28 to under 32 | High |
| 32 to under 35 | Extreme |
| 35+ | Lethal |
The thresholds that actually bind you are your own organization's. A school is held to the heat policy it has adopted, which is usually its state athletic association's. A worksite is held to its employer's heat plan, and a unit to TB MED 507. Those numbers differ from each other and from the bands above. We publish every state athletic association's policy we could source with a link to the association's own document, rather than restating it as ours, and we mark any we could not source as unconfirmed. Every state is summarized on one page: school sports heat policies by state. What is common to all of them is the principle: 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.