What WBGT measures that the heat index does not
The heat index takes two inputs: temperature and humidity. WBGT takes four — temperature, humidity, wind and radiation — which is why occupational and sports rules are written against WBGT rather than the heat index.
The extra two matter more than they sound. Standing in full sun instead of shade can add the equivalent of several degrees. A 3 m/s breeze can remove them again. A heat index cannot see either, so it gives the same answer for a shaded, breezy porch and an asphalt car park at noon.
WBGT is a weighted average of three thermometers:
| Component | What it responds to | Outdoors | Indoors / shade |
|---|---|---|---|
| Natural wet bulb | Humidity, wind and sun on a wet wick left in the open | 0.7 | 0.7 |
| Black globe | Radiant heat — sun above, hot ground below | 0.2 | 0.3 |
| Dry bulb | Air temperature in shade | 0.1 | — |
The globe is a 150 mm matt black sphere. It gets far hotter than the air (the calculator above will usually show it 30–40°F above the thermometer in sun), and its weight of 0.2 is what puts sunshine inside the number.
Why this is an estimate, and what that costs you
No weather service publishes solar irradiance. Neither Apple's weather data nor a typical national feed includes it. So any app or website producing a WBGT from ordinary weather data is modelling the radiation terms, and this one is no exception.
Here that model is: Haurwitz clear-sky irradiance for the sun height you pick, attenuated for cloud cover by the Kasten–Czeplak relation, split into direct and diffuse beams by the Erbs correlation, plus downward longwave from Brutsaert's clear-sky emissivity with a cloud correction. The wick and globe energy balances then follow Liljegren's model in the analytic form published by Kong and Huber.
Two consequences are worth stating plainly:
- In hot, dry, low-wind conditions the method reads low, by up to 2°C. That is documented behaviour of the analytic form, and it is the desert-afternoon case. Treat the number as a floor there.
- A certified instrument measures what this estimates. If you need a record that stands up — an OSHA inspection, a Japanese workplace heat assessment, a sports governing body's protocol — buy the instrument. When you need the meter goes through that honestly.
Flag conditions
| Flag | WBGT | What it asks for |
|---|---|---|
| White | Below 78°F · 25.6°C | Normal activity, water available. |
| Green | 78–82°F · 25.6–27.8°C | Limit heavy exercise for anyone unacclimatised. |
| Yellow | 82–88°F · 27.8–31.1°C | Curtail strenuous exercise and activity in direct sun. |
| Red | 88–90°F · 31.1–32.2°C | Stop all but essential activity for unacclimatised people. |
| Black | 90°F and above · 32.2°C and above | Stop physical training and strenuous work. |
For the work-and-rest side of the same number — how many minutes an hour a crew should be working at a given WBGT and workload — use the work/rest calculator.
The same number has a different name in Japan
Japan runs its heat-safety system on WBGT under its own national name, published as observed and forecast values rather than estimated by apps, and since June 2025 workplace heat measures are a legal requirement at WBGT 28°C or air temperature 31°C. The bands are different too: the Japanese guideline scale runs its own four levels on WBGT directly rather than the five flag colours.
If that is the framework you need, the Japanese pages cover it: the Japanese guide to WBGT and the Japanese guide to those workplace rules.