The heat index is the number on the news. WBGT is the number in the rulebook. They measure overlapping but different things, and swapping one for the other is the single most common error in heat safety planning.
The short version: the heat index tells a general audience how hot it will feel standing in the shade. WBGT tells someone responsible for people exerting themselves in the sun whether to change what those people are doing.
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.
Why WBGT reads lower but acts sooner
This is the part that catches everyone. On a typical hot afternoon the heat index might be 105°F while WBGT is 88°F. The instinct is that 88 sounds manageable. It is not: 88°F WBGT is a red flag condition where hard training should largely stop.
The two numbers sit on different scales. The heat index is expressed as an apparent temperature, so it is designed to read above the real one. WBGT is an index value that typically lands below air temperature because the heavily weighted wet bulb term is lower than the dry bulb reading. Comparing their magnitudes is like comparing a percentage to a dollar amount.
Always read a number against its own threshold table. Never against the other one.
The same four afternoons, both ways
Where the two numbers agree, and where they diverge badly.
| Conditions | Heat index | WBGT (full sun) | Which is the better guide? |
|---|---|---|---|
| 100°F, 20% humidity, breezy | about 99°F | about 83°F | WBGT. Dry windy air lets sweat work, and the heat index overstates the danger to someone exerting. |
| 85°F, 90% humidity | about 102°F | about 89°F | Both flag it, WBGT more sharply. This is a red flag condition that air temperature alone makes look mild. |
| 90°F, 70% humidity, calm | about 106°F | about 91°F | WBGT. The heat index cannot see that the wind has dropped, which is what pushed this into black flag. |
| 88°F, 60% humidity, full shade | about 98°F | about 82°F | WBGT. The heat index assumes shade, so it misses nothing here, but it also cannot credit you for being in it. |
Row one and row two together make the case. The heat index ranks the 100°F day as the more dangerous of the two. For anyone running, drilling or lifting outdoors, it is the safer one.
What the heat index cannot see
Direct sunlight
The heat index assumes shade. Step into full sun and the real heat load jumps, but the published figure does not move.
Wind speed
It assumes light wind. A calm afternoon is substantially more dangerous than a breezy one at the same temperature and humidity.
Your surface
Artificial turf and asphalt radiate heat upward into everyone standing on them. No heat index accounts for that.
What you are doing
It describes the environment for someone at rest. Exertion changes the problem entirely, which is why exposure limits use WBGT.
When the heat index is the right tool
It is not a bad number, it is a number for a different job. The heat index is well suited to public messaging, to decisions about people at rest or at low activity, and to indoor environments without a solar load, where it is simpler to obtain and tracks the risk adequately.
Use WBGT when people are exerting themselves outdoors, when equipment or protective clothing is involved, and whenever a policy, standard or governing body specifies it. Increasingly, that last category decides the question for you: ISO 7243, ACGIH, NATA and a growing list of state athletic associations all specify WBGT, not heat index.
Both numbers, one screen
The app leads with WBGT because that is what the guidelines use, and shows the underlying conditions alongside it so you can see exactly why the number landed where it did: humidity, wind, solar radiation and cloud cover.
- 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
Is WBGT better than the heat index?
For people exerting themselves outdoors, yes, because WBGT accounts for sun and wind and the heat index does not. For general public messaging about heat, the heat index is simpler and adequate. They are built for different audiences.
Why is WBGT lower than the heat index?
They are different scales. The heat index is an apparent temperature designed to read above the actual air temperature. WBGT is an index value weighted heavily toward the wet bulb reading, which sits below air temperature. The lower number often describes the more urgent situation.
Can I convert heat index to WBGT?
Not reliably. The heat index does not contain wind speed or solar radiation, which are exactly the inputs that separate a yellow flag from a black one. Any conversion has to assume values for them, and those assumptions can be off by several degrees.
Which does OSHA use?
OSHA's own heat materials reference the heat index for general screening because it is easy to obtain, but its guidance recognizes WBGT as the more accurate measure for direct sun and heavy work, and the ACGIH limits that inform enforcement are written in WBGT. See OSHA and WBGT.
What do high school athletics use?
Increasingly WBGT. NATA's position statement recommends it, and a growing number of state associations now require it, including Texas through the UIL policy. Some states still permit heat index as an alternative; check your own association.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
