The heat index combines air temperature and relative humidity into an apparent temperature: how hot it feels rather than how hot it is. It is the number behind the National Weather Service heat advisories.
It is useful and it is widely misused. The heat index describes a person standing in shade with light wind, doing nothing in particular. If that is not your situation, it understates your risk, sometimes badly.
Method. This is the National Weather Service heat index, the Rothfusz regression, with the documented low-humidity and high-humidity adjustments. It assumes shade and light wind. OSHA advises adding roughly 15°F for full sunlight, which is the “in full sun” figure above. For anyone exerting themselves outdoors, use WBGT instead: it accounts for sun and wind directly rather than assuming them away.
National Weather Service heat index categories
| Heat index | Category | Likely effect on the body |
|---|---|---|
| 80 to 90°F | Caution | Fatigue possible with prolonged exposure and physical activity. |
| 90 to 103°F | Extreme caution | Heat cramps and heat exhaustion possible. Heat stroke possible with prolonged exertion. |
| 103 to 125°F | Danger | Heat cramps and heat exhaustion likely. Heat stroke probable with continued activity. |
| 125°F and above | Extreme danger | Heat stroke highly likely. |
These categories assume shade. In full sun, add roughly 15°F before reading the table, which is the adjustment OSHA publishes in its own heat guidance.
What the heat index cannot see
Direct sunlight
The published figure assumes shade. Step into full sun and the real load jumps about 15°F while the number does not move.
Wind speed
It assumes light wind. A calm afternoon is substantially more dangerous than a breezy one at identical temperature and humidity.
Your surface
Artificial turf and asphalt can run 40 to 60°F above air temperature and radiate upward into everyone on them.
Exertion
It describes someone at rest. Working or training generates metabolic heat the index knows nothing about.
When to use WBGT instead
If people are exerting themselves outdoors, in equipment, or under any policy that specifies a measure, the heat index is the wrong tool and WBGT is the right one. WBGT includes solar radiation and wind directly instead of assuming them away, which is why ISO 7243, the ACGIH threshold limit values, NATA and a growing list of state athletic associations all specify it.
The two scales are not comparable. A heat index of 105°F and a WBGT of 88°F can describe the same afternoon; the first sounds worse and the second is the one that stops practice. See WBGT vs heat index.
The heat index remains the better choice for general public messaging, for people at rest or at low activity, and indoors where there is no solar load.
The same conditions, both ways
| Conditions | Heat index | WBGT in full sun | Which should decide |
|---|---|---|---|
| 100°F, 20% humidity, breezy | about 99°F | about 83°F | WBGT. Dry moving air lets sweat work; the heat index overstates the risk to someone exerting. |
| 85°F, 90% humidity | about 105°F | about 89°F | Both flag it. WBGT puts it in red, where hard training should stop. |
| 90°F, 70% humidity, calm | about 106°F | about 91°F | WBGT. The heat index cannot see that the wind dropped, which is what pushed this to black flag. |
Skip the shade assumption entirely
The app computes WBGT from live weather for your exact location, including solar radiation and wind, so there is no shade assumption to correct for and no 15 degrees to add in your head.
- 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
- Pro: turf surface mode for synthetic fields
- Pro: overhead cloud adjustment from what you can see
- 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.
Warning signs of heat illness
Heat stress shows up before it becomes an emergency. Anyone on site should speak up immediately on any of these:
When in doubt, cool first and call for help. Collapse or confusion in the heat should be treated as possible exertional heat stroke: begin cold-water immersion immediately and call 911. Cooling comes before transport.
Sources
- Occupational Safety and Health Administration. OSHA Technical Manual, Section III Chapter 4: Heat Stress.
- National Institute for Occupational Safety and Health. Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments. DHHS (NIOSH) Publication No. 2016-106, February 2016.
Common questions
How is the heat index calculated?
The National Weather Service uses the Rothfusz regression, a multi-term equation in air temperature and relative humidity, with an adjustment subtracted at very low humidity and added at very high humidity. Below about 80°F a simpler Steadman form is used instead.
Does the heat index account for sunlight?
No. It assumes shade with light wind. OSHA advises adding roughly 15°F to the heat index when people are working in full sunlight, which the calculator above shows alongside the shade figure.
What heat index is dangerous?
The National Weather Service treats 80 to 90°F as caution, 90 to 103°F as extreme caution, 103 to 125°F as danger and above 125°F as extreme danger. Those bands assume shade and rest, so adjust upward for sun and exertion.
Is the heat index the same as WBGT?
No. The heat index uses only temperature and humidity and assumes shade. WBGT adds solar radiation and wind and is the measure written into occupational and athletic standards. The two run on different scales and should never be compared directly.
What is the difference between heat index and wet bulb temperature?
The heat index is an apparent temperature that reads above air temperature. Wet bulb temperature is a physical temperature that reads below it, measuring how much cooling evaporation can achieve. See wet bulb temperature.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.