# Heat Index Calculator

- **URL:** https://codeasystem.com/calculators/climate/heat-index-calculator/
- **Category:** climate
- **Description:** Turn temperature and relative humidity into the NWS heat index; the “feels like” temperature; with health risk banding.
- **Primary output:** Feels-like temperature: 105.9°F

## Inputs
- Air temperature (name: `tempF`, type: number, example: 90)
- Relative humidity (name: `humidity`, type: number, example: 70)

## Outputs
- Feels-like temperature: 105.9°F
- Air temperature: 90°F
- Relative humidity: 70%
- Risk band: Danger; heat cramps and heat exhaustion likely
- Caution 80–90°F: fatigue possible with prolonged exposure
- Extreme caution 90–103°F: heat cramps and heat exhaustion possible
- Danger 103–124°F: heat cramps and heat exhaustion likely
- Extreme danger above 124°F: heat stroke highly likely

## Formula / methodology
```
HI = −42.379 + 2.049T + 10.143R − 0.2248TR − 0.00684T² − 0.05482R² + …
plus NWS adjustments for very dry (RH<13%) or very humid (RH>85%) air
```

The heat index comes from the Rothfusz multiple regression of Steadman’s apparent-temperature table, using the standard coefficients the National Weather Service publishes for temperatures of 80°F and above.

Two standard corrections follow: very dry air (below 13% RH) evaporates sweat faster, lowering the index; extremely humid mild heat (above 85% RH between 80–87°F) raises it slightly.

When even the simple screening formula falls below 80°F, humidification of the air cannot meaningfully impair cooling and the calculator returns the plain temperature with a note instead of an inflated figure.

## Assumptions & limitations
- Valid for air temperatures ≥ 80°F; below that the concept is not defined.
- Shade conditions and light winds assumed; full sun can add up to 15°F.
- Relative humidity must lie between 0% and 100%.
- Individual heat tolerance varies with acclimatisation, hydration, age and health.

## How to use
1. **Enter the air temperature**; Use the shade temperature in °F of at least 80°F. The heat index is not defined below that.
2. **Add relative humidity**; Weather apps and station observations report RH directly. Anything outside 0–100% is rejected.
3. **Act on the risk band**; Caution means pace yourself; extreme caution calls for shade and water; danger and above call for moving activity indoors or cancelling.

## Example
A steamy 90°F afternoon at 70% relative humidity feels closer to 106°F; firmly inside the “danger” band where heat cramps and exhaustion become likely with exertion.

Result for these inputs:

```
Feels-like temperature: 105.9°F
```

## About this calculator
### Why humidity makes heat dangerous

Humans cool themselves by evaporating sweat. When the air is already moisture-laden, evaporation slows and body heat accumulates; the same thermometer reading turns lethal as humidity climbs. The heat index quantifies that combination.

### Heat index versus “real feel” marketing

Some weather services blend sun angle, wind and other effects into proprietary feels-like numbers. This calculator implements the official NWS heat index: shade, light wind, humidity only; comparable across sources because it is standardized.

### Using the risk bands

Bands map to NWS guidance: caution (80–90°F) warrants pacing; extreme caution (90–103°F) demands hydration and shade; danger (103–124°F) brings likely cramps and exhaustion; above 124°F heat stroke is highly likely without intervention.

## FAQs
### Why won’t it compute below 80°F?

The NWS regression was fitted for hot conditions; below 80°F humidity barely impairs the body’s cooling and the index loses meaning. The calculator says so rather than returning a misleading number.

### Does sun exposure change the result?

Yes; the official index assumes shade. In direct sunlight, effective heat stress can run up to 15°F higher than the calculated value.

### What if humidity reads over 100%?

That is physically impossible for relative humidity, so inputs outside 0–100% are rejected. Supersaturated foggy air still reports at or near 100%.

### Is the heat index valid at very low humidity?

Only with the NWS dry-air adjustment, which this calculator applies automatically below 13% RH for temperatures between 80°F and 112°F.

## Related calculators
- [Wind Chill Calculator](https://codeasystem.com/calculators/climate/wind-chill-calculator/)
- [Temperature Converter](https://codeasystem.com/calculators/conversion/temperature-converter/)

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Last updated: 2026-08-23 · Version: 1.0.0 · [HTML version](https://codeasystem.com/calculators/climate/heat-index-calculator/)