# Heat Index Calculator

> Calculate the heat index (apparent temperature) from air temperature and relative humidity using the NOAA Rothfusz regression.

**URL:** https://www.calculatoranswers.com/heat-index-calculator
**Category:** Home

## Quick Answer

Calculate the heat index ('feels like' temperature) from ambient air temperature and relative humidity. View safety classifications and risk warnings based on National Weather Service thresholds.

## Definition

The Heat Index (HI) or apparent temperature is a measure of how hot it feels to the human body when relative humidity is combined with the air temperature. High humidity slows the evaporation of sweat, making it harder for the body to cool itself. The Heat Index is calculated using the NOAA/NWS Rothfusz regression model and Steadman's thermoregulation equations. Weather services and safety organizations use the heat index to issue heat advisories and prevent heat-related illnesses like heat exhaustion or stroke.

## How to Use

1. **Enter air temperature** — Type the measured outdoor air temperature in degrees Fahrenheit or Celsius.
2. **Enter relative humidity** — Type the relative humidity as a percentage between 0% and 100%.
3. **Read heat index & safety risk** — View the calculated apparent temperature and review the corresponding NOAA risk category (Caution, Extreme Caution, Danger, or Extreme Danger).

## Disclaimer

This calculator provides estimates for educational and planning purposes. Always consult a qualified professional for specific advice on financial, medical, legal, or other regulated decisions.

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