What the heat index actually tells you
The heat index answers one question: with this much moisture in the air, how hot does it effectively get for a person in the shade? Sweating works by evaporation, and evaporation slows as the air fills with water vapour. At 35% humidity, 95°F is 95°F. At 70% humidity, the same 95°F behaves like 124°F, because the body's main cooling mechanism has largely stopped working.
That is the entire idea. The number is not a measurement of the air — it is a measurement of what the air does to a person.
Two conditions come with it, and both matter:
- Shade. The heat index is defined for shaded conditions with light wind. Standing in direct sun is worse, by up to 15°F according to the National Weather Service.
- A typical adult, at rest or light activity. Someone working, running, older, very young, or wearing protective clothing crosses each threshold earlier.
Why your weather app may show a different number
Most weather apps show an apparent temperature or "feels like" value. That is a different model. Apple's apparent temperature, for instance, also accounts for wind, and in cold conditions becomes wind chill — one continuous number across the whole year. The heat index is not that. It is a hot-weather-only quantity from temperature and humidity alone.
Both numbers can be correct and still disagree. If you want the quantity that heat safety guidance, sports rules and OSHA materials refer to, it is the heat index specifically.
Worked example: an ordinary July afternoon
- Air temperature
- 90°F (32.2°C)
- Relative humidity
- 60%
- Dew point
- 74°F — the air is holding a lot of water
Drop the humidity to 25% and leave the temperature at 90°F: the heat index becomes 88°F, slightly below the air temperature. Same thermometer reading, twelve degrees of difference in what it does to you.
What each band means
The four categories come from the National Weather Service. They describe conditions and precautions, not diagnoses.
| Band | Heat index | What it means |
|---|---|---|
| Caution | 80–90°F · 27–32°C | Fatigue is possible with prolonged exposure and activity. |
| Extreme Caution | 90–103°F · 32–39°C | Heat cramps and heat exhaustion are possible with prolonged exposure and activity. |
| Danger | 103–125°F · 39–52°C | Heat cramps and heat exhaustion are likely; heat stroke is possible with continued activity. |
| Extreme Danger | 125°F and above · 52°C and above | Heat stroke is highly likely with continued exposure. |
Below 80°F there is no band. That sounds pedantic and is not: an app that renders a needle in "Caution" at 78°F is inventing a category that no published source defines. This calculator says so explicitly instead.
Where the equation stops working
The heat index is a regression fitted to a physiological model, not a law of physics. It has edges:
- Below about 80°F the full regression is not used at all. The National Weather Service runs a simpler form first and only switches to the regression when that lands at 80°F or above.
- Below 13% humidity, between 80 and 112°F, an official correction subtracts from the result. Very dry air lets sweat evaporate faster than the base regression assumes.
- Above 85% humidity, between 80 and 87°F, another official correction adds to it.
- The printed NOAA chart omits that second correction. Twelve cells in the published table therefore read up to about 2°F cooler than the equation on the NWS's own equation page. This calculator follows the equation, which is also what the NWS's own online calculator does.
If you want the derivation rather than the caveats, see how the heat index is calculated. If you want the grid, the chart page explains how to read it and the printable version is a single sheet.
Which number should you use?
Four quantities come out of the same temperature and humidity, and they answer different questions.
- Heat index — how hot it effectively is for a person in shade. Use it for everyday decisions and for the published risk bands.
- Wet bulb — the lowest temperature evaporation can reach. Use it for the survivability question and as a hard physical limit. Calculate it here.
- Dew point — how much water the air holds, in degrees. Use it to compare days: it barely moves through the afternoon, while humidity swings wildly.
- WBGT — the heat-stress index that occupational and sports rules are written against, because it includes sun and wind. Estimate it here.