Heat Index Calculator
Estimate the apparent temperature based on air temperature and relative humidity.
Heat Index Results
What is the Heat Index Calculator?
The Heat Index Calculator estimates how hot it feels when relative humidity is combined with the actual air temperature. High humidity reduces the body's ability to cool itself through sweating, making the perceived temperature higher.
How to Use
- Enter the air temperature in degrees Fahrenheit.
- Enter the relative humidity as a percentage.
- Click Calculate Heat Index to see the apparent temperature and danger level.
Frequently Asked Questions
What is the heat index?
The heat index is a measure of how hot it feels to the human body when relative humidity is factored in with the actual air temperature. It is also known as the apparent temperature.
Why is the heat index only valid above 80°F?
The Rothfusz regression formula used by the National Weather Service is designed for warm conditions. Below 80°F, the effects of humidity on perceived temperature are minimal and the formula becomes less accurate.
What should I do at extreme danger levels?
At extreme danger levels (heat index above 124°F), heat stroke is highly likely. Avoid outdoor activity, stay hydrated, seek air conditioning, and check on vulnerable individuals.
How accurate is this calculator?
This calculator uses the standard Rothfusz regression formula. Results are accurate for typical warm-weather conditions but may vary slightly from official NWS computations due to simplifications.