1. What is the Heat Index?

Definition: The Heat Index (HI) is what the temperature feels like to the human body when relative humidity is combined with the air temperature.

Purpose: Developed by the National Weather Service (NWS), it helps assess heat risk and determine heat safety precautions.

2. How Does the Calculator Work?

The calculator uses the NWS Rothfusz regression formula:

Where:

• \( HI \) — Heat Index (°F)
• \( T \) — Temperature (°F)
• \( R \) — Relative Humidity (%)
• \( c_1 \) to \( c_9 \) — Regression coefficients

Note: The formula is only valid when temperature ≥80°F and relative humidity ≥40%.

3. Heat Index Risk Categories

Caution: 80-90°F - Fatigue possible with prolonged exposure
Extreme Caution: 90-103°F - Heat stroke, heat cramps, or heat exhaustion possible
Danger: 103-124°F - Heat cramps or heat exhaustion likely, heat stroke possible
Extreme Danger: 125°F+ - Heat stroke highly likely

4. Using the Calculator

Tips: Enter temperature in °F and relative humidity in %. Both values are required.

5. Frequently Asked Questions (FAQ)

Q1: Why does humidity affect how hot it feels?
A: High humidity reduces the body's ability to cool itself through sweat evaporation.

Q2: What's the difference between Heat Index and "Feels Like" temperature?
A: Heat Index only considers temperature and humidity, while "Feels Like" may include wind chill and other factors.

Q3: Why does the calculator say "N/A" for some inputs?
A: The NWS formula is only valid when temperature ≥80°F and relative humidity ≥40%.

Q4: How accurate is this calculator?
A: It uses the official NWS algorithm and matches their published heat index charts.

Q5: What precautions should I take during high heat index conditions?
A: Stay hydrated, limit outdoor activity, wear light clothing, and check on vulnerable individuals.