Heat Index Calculator - Humidex & Heat Illness Risk
Free online heat index calculator using the NOAA National Weather Service Rothfusz regression equation. Computes apparent temperature from ambient air temperature and relative humidity (or dew point) with heat cramps, exhaustion, and heat stroke risk classifications.
Heat Index Calculator & Humidex
Calculate "feels-like" apparent temperature, heat illness danger levels, and dew point.
Temperature & Humidity Inputs
Heat cramps and heat exhaustion likely. Heat stroke possible with prolonged activity.
NWS Heat Index Alert Levels
Heat Index & Humidex Dynamics: Evaporative Cooling, Sweat & Heat Illness
The Heat Index (also known as the apparent temperature) combines ambient air temperature with relative humidity to determine human perceived heat. When relative humidity is high, the evaporation rate of perspiration from the skin is severely reduced, hindering the body's internal thermo-regulation.
The Rothfusz Regression Equation
The National Weather Service computes Heat Index using the 9-variable Rothfusz polynomial regression derived from Robert G. Steadman's 1979 human biometeorology studies. The formula is calibrated for shaded conditions with a light 5-knot breeze.
Exposure to direct sunlight can increase heat index values by up to 15°F (8°C).
| Heat Index (°F / °C) | Alert Classification | Physiological Symptoms |
|---|---|---|
| 80°F - 90°F (27°C - 32°C) | Caution | Fatigue possible with prolonged physical exposure |
| 90°F - 103°F (32°C - 39°C) | Extreme Caution | Heat cramps and heat exhaustion possible |
| 103°F - 124°F (39°C - 51°C) | Danger | Heat cramps/exhaustion likely; heat stroke possible |
| 125°F+ (52°C+) | Extreme Danger | Heat stroke imminent with continued exposure |
Step-by-Step Instructions
Enter Ambient Air Temperature
Input dry-bulb air temperature in degrees Fahrenheit (°F) or Celsius (°C) (valid for temperatures at or above 80°F / 27°C).
Input Humidity (Relative Humidity or Dew Point)
Enter relative humidity percentage (% RH) or outdoor dew point temperature.
Calculate Apparent Temperature (Feels Like)
The calculator evaluates the full 9-term Steadman regression polynomial adopted by NOAA / National Weather Service.
Inspect Heat Danger Category & Precautions
Review health risk classification (Caution, Extreme Caution, Danger, Extreme Danger) and heat stroke / heat exhaustion prevention guidelines.
Key Features & Capabilities
- Official NOAA / NWS 9-term Rothfusz / Steadman multivariable regression polynomial calculation
- Calculates Apparent Temperature ('Feels Like') accounting for evaporative cooling inhibition at high relative humidity
- NOAA Heat Disorder Risk Classification: Caution (80°F-90°F), Extreme Caution (90°F-103°F), Danger (103°F-124°F), and Extreme Danger (>=125°F)
- Full unit synchronization: Fahrenheit (°F) and Celsius (°C) with relative humidity and dew point cross-conversion
- Direct sunlight adjustment: calculates solar radiation heat index increase (+15°F in direct unshaded sunlight)
- Instant real-time evaluation with zero server roundtrips
Frequently Asked Questions
What is Heat Index and how is it calculated?
The Heat Index (Apparent Temperature) measures how hot it feels to the human body when relative humidity is factored in with dry-bulb air temperature. It is calculated using the National Weather Service 9-term polynomial based on Robert Steadman's biometeorological human heat balance research.
Why does high humidity make summer temperatures feel so much hotter?
The human body cools itself primarily through perspiration (sweat evaporation). When the surrounding air has high relative humidity, the rate of sweat evaporation from the skin decreases significantly, preventing the body from dissipating heat and causing internal body temperature to rise.
What are the NOAA Heat Index danger levels and associated health risks?
1. Caution (80°F–90°F): Fatigue possible with prolonged exposure. 2. Extreme Caution (90°F–103°F): Heat cramps and heat exhaustion possible. 3. Danger (103°F–124°F): Heat cramps/exhaustion likely; heat stroke possible. 4. Extreme Danger (125°F+): Heat stroke highly likely with continued exposure.
Does direct sunlight increase the effective heat index?
Yes. Official Heat Index values are calculated for shaded, light-wind conditions. Exposure to full, direct sunlight can increase effective heat index values by up to 15°F (8°C).
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