The WBGT index is a heat stress index: a composite value built from several measurements and expressed in degrees, which is exactly the thing that confuses people. It carries a degree sign but it is not the temperature of anything. It is a number on a scale, and the scale only means something against a threshold table.
Call it a temperature and you will instinctively compare it to the air temperature, find it lower, and relax. That instinct is backwards. A WBGT index value of 88 on a day when the air is 93°F is not good news; it is a red flag condition.
The index at a glance
What the index actually weighs
Outdoors, the index is 0.7 × natural wet bulb + 0.2 × black globe + 0.1 × dry bulb. Those weights are not arbitrary and they are the most informative thing about the index.
Seventy percent of the value is evaporative. During exertion, sweat evaporation does almost all of the body's cooling, so the index is dominated by whether the air will accept that sweat. Twenty percent is radiant, which is why standing in sun rather than shade moves the number substantially. Air temperature alone, the figure everyone quotes, gets ten percent.
That is the whole argument for the index in one line: the thing most people check is the thing that matters least.
WBGT compared with other heat indices
Several indices try to describe heat stress. They are not interchangeable and they do not share a scale.
| Index | Inputs | Built for |
|---|---|---|
| WBGT | Temperature, humidity, wind, solar radiation | Occupational and athletic exposure limits in direct sun |
| Heat index | Temperature, humidity | General public warnings, assumes shade and light wind |
| Humidex | Temperature, dew point | Public comfort reporting, mainly in Canada |
| UTCI | Temperature, humidity, wind, radiation, metabolic and clothing model | Biometeorological research, more detailed than WBGT |
| Wet bulb temperature | Temperature, humidity | A physical measurement and one WBGT input, not a standalone exposure limit |
WBGT survives in regulation because it hits a practical sweet spot: enough physics to be defensible, simple enough that a device on a tripod can report it and a supervisor can act on it.
The range the index runs over
In practice WBGT values that matter run from the low 70s to the mid 90s Fahrenheit, roughly 22 to 35°C. Below about 80°F almost nobody is at meaningful risk from the environment alone. Above about 92°F, very little organized outdoor exertion should be happening at all.
That compression is worth sitting with. The entire useful scale is about twenty degrees wide, and the difference between normal practice and cancel practice can be six degrees. Precision matters more here than it does with air temperature, which is another reason to measure or model the number rather than guess it.
WBGT compared with the heat index
Both are heat numbers, but they answer different questions, and swapping one for the other is the most common mistake in heat planning.
| Factor | Heat index | WBGT |
|---|---|---|
| Inputs | Air temperature and relative humidity | Air temperature, humidity, wind speed and solar radiation |
| Assumed conditions | Shade, light wind | Direct sun, measured wind |
| Typical use | General public heat warnings | Athletic, military and occupational safety thresholds |
| Reported as | An apparent or “feels like” temperature | A heat stress index with flag categories |
| Reads higher? | Usually higher than air temperature | Usually lower in number, but triggers action sooner |
That last row trips people up. A WBGT of 88°F is not milder than a heat index of 105°F; they are different scales. 88°F WBGT is a red flag condition where hard training should largely stop.
The index, live, wherever you are
Rather than reading an index value off a chart for a weather station across town, the app computes it for your coordinates from live data and tells you which category you are in.
- Real-time WBGT for your location or any city
- Hourly and 14-day WBGT forecasts to plan ahead
- Color-coded flag levels with mode-specific guidance
- Humidity, wind, sun and cloud cover behind every reading
WBGT is estimated from weather data with the peer-reviewed Liljegren model. Where a policy requires on-site measurement, use the app for planning and as a cross-check alongside your instrument.
Common questions
What is the WBGT index?
A heat stress index that combines air temperature, humidity, wind speed and solar radiation into one value, weighted 0.7 natural wet bulb, 0.2 black globe and 0.1 dry bulb outdoors. It is the basis of ISO 7243 and of athletic, military and occupational heat exposure limits.
Is the WBGT index the same as a temperature?
No. It is expressed in degrees but it is a composite index, not the temperature of the air or of anything else. Comparing it directly to air temperature is the most common way people misread it.
What is a normal WBGT index value?
Values below about 80°F are low risk for most people. The range that drives decisions runs roughly 80 to 92°F, which is where the five flag categories sit. Above that, organized outdoor exertion should generally stop.
What standard defines the WBGT index?
ISO 7243 defines the measurement internationally. ACGIH publishes threshold limit values based on it for workplaces, and NATA and state athletic associations publish athletic thresholds. The underlying index is the same; the limits differ.
Why is wind part of the index when it is not in the heat index?
Because wind strips the saturated layer of air off the skin and lets the next round of sweat evaporate. Losing the breeze on an otherwise identical afternoon can move WBGT by several degrees, which is enough to change a red flag into a black one.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
