Heat Index Chart Table
| Classification | Heat index | Effect on the body |
|---|---|---|
| Caution | 80°F - 90°F | Fatigue possible with prolonged exposure and/or physical activity |
| Extreme Caution | 90°F - 103°F | Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity |
| Danger | 103°F - 124°F | Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity |
| Extreme Danger | 125°F or higher | Heat stroke highly likely |
The heat index is what the air actually feels like once humidity slows the evaporation your body relies on - and the official NWS categories turn it into a risk ladder from Caution at 80°F to Extreme Danger at 125°F. Enter any air temperature and relative humidity above for the heat index, its band, and what that band means for your body.
The bands are verbatim from the National Weather Service, and the computation is the official WPC algorithm - the same regression the Weather Prediction Center publishes, including its dry-air and humid-coast adjustment terms.
How to use
- Enter air temperature (°F) and relative humidity; the chips load the humid-surprise case (85°F at 90%) and the dry-heat case (80°F at 40%).
- Read the table below for the four official bands; anything under an 80°F heat index is below the chart, where the regression was never fitted.
- Add up to 15°F in direct sun, per NWS - the index assumes shade.
Frequently asked questions
How much does humidity really add?
At 85°F and 90% humidity the heat index is 101.8°F - moisture alone added nearly 17°F. At 100°F with 65% humidity it reads 135.9°F, Extreme Danger territory before any sunshine adjustment.
Why does the calculator say "below chart" on mild days?
The NWS table and the regression behind it start at an 80°F heat index. At 80°F air with 40% humidity the computed value is 79.6°F - humidity is barely biting yet, and the official categories simply do not speak below that floor.
How accurate is the heat index formula?
NWS states the regression carries an error of about ±1.3°F. It was fitted to the full Steadman model rather than derived from first principles, which is also why extrapolating far beyond the chart’s corners degrades.
Is heat index the same as "feels like" on my weather app?
Usually yes - most apps show the heat index in warm months and wind chill in cold ones. It is not the dew point: the dew point measures absolute moisture while the heat index measures combined physiological load, so muggy 85°F can feel worse than dry 95°F.