Heat Index Calculator
The heat index, sometimes called the apparent temperature, combines air temperature and relative humidity to estimate how hot it actually feels to the human body. When humidity is high, sweat evaporates slowly, so your body cannot cool itself as efficiently and the effective temperature climbs well above the reading on the thermometer. This calculator uses the same Rothfusz regression that the United States National Weather Service publishes, including the low humidity and high humidity adjustments, so the numbers match official heat advisories. Enter the air temperature and the relative humidity, choose Fahrenheit or Celsius, and read the heat index along with a caution level. It is useful for planning outdoor work, exercise and events during summer heat waves. Everything is computed locally in your browser for instant, private results.
How to use Heat Index Calculator
- Enter the air temperature.
- Enter the relative humidity percentage.
- Read the heat index, how hot it feels.
- Compare it against the actual temperature to see the humidity effect.
Frequently asked questions
How is the heat index calculated?
It uses the National Weather Service Rothfusz regression, a multi term equation of air temperature and relative humidity that models how hot the air feels.
Why does humidity raise the felt temperature?
High humidity slows sweat evaporation, so your body cools less effectively and the air feels hotter than the thermometer reading.
When is the heat index formula valid?
The Rothfusz regression applies best above about 80 degrees Fahrenheit and 40 percent humidity; below that the felt temperature is close to the actual temperature.
What is Heat Index Calculator?
The heat index, sometimes called the apparent temperature, combines air temperature and relative humidity to estimate how hot it actually feels to the human body. When humidity is high, sweat evaporates slowly, so your body cannot cool itself as efficiently and the effective temperature climbs well above the reading on the thermometer.
Is Heat Index Calculator free to use?
Yes. Heat Index Calculator is completely free, with no sign-up and no usage limits.
Does Heat Index Calculator work in a web browser?
Yes. Heat Index Calculator runs in any modern web browser. There is nothing to download or install.
Is my data private with Heat Index Calculator?
Yes. Heat Index Calculator runs entirely on your device in your browser, so nothing you enter is uploaded to a server.
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[](https://yourhack.ai/heat-index-calculator)