The grid
Temperature runs along the top, relative humidity down the side, and the cell where they meet is the heat index. That is all the mechanics there is. What people get wrong is not the lookup but the interpretation.
Three things the grid quietly assumes:
- Shade, and a light breeze. Not a car park at noon.
- A person at rest or walking, in ordinary summer clothing.
- That you know the relative humidity, which changes through the day even when the air's moisture content does not. At dawn 70% and at 3pm 40% can be the same air.
Where the bands fall
The four categories are set on the heat index, not on the air temperature — a distinction that matters, since a 90°F day can be in any of three bands depending on the humidity.
| Band | Heat index °F | Heat index °C |
|---|---|---|
| Caution | 80–90 | 27–32 |
| Extreme Caution | 90–103 | 32–39 |
| Danger | 103–125 | 39–52 |
| Extreme Danger | 125 and above | 52 and above |
The empty region below 80°F is a real feature, not an omission. If an app shows you a risk category at 78°F, it has invented one.
The twelve cells where the chart is wrong
This is the part worth knowing if you are ever comparing two sources and finding a stubborn one- or two-degree gap.
The NWS equation page specifies two corrections. One subtracts in very dry air; the other adds in very humid air, specifically when relative humidity is above 85% and the temperature is between 80 and 87°F. The printed NOAA chart does not apply that second correction. The NWS's own online calculator does.
The disagreement, at its largest
- Conditions
- 80°F, 100% relative humidity
- Plain regression
- 84.2°F
- With the official correction
- 86.3°F
Twelve cells of a standard 80–110°F by 40–100% grid sit inside that range. Everywhere else, chart and equation agree.
How the heat index is calculated has both corrections written out, and the printable chart applies them.
Using a chart on a wall
Printed charts earn their place in exactly the situations where a phone does not: a site hut, a nursery corridor, a school gym, a kitchen. A few things make one useful rather than decorative.
- Post the actions, not just the numbers. A grid of three-digit numbers tells nobody what to do. The four band descriptions matter more than the cells.
- Say that it is a shade chart on the sheet itself, because the person reading it in August will not remember.
- Put the humidity source next to it. A chart is useless without a humidity reading, and few thermometers on a wall have one.
- Consider WBGT instead for sport and outdoor work. If your governing body's policy is written on WBGT, a heat index chart is the wrong instrument, however good it looks.