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Heat Index Calculator

Calculate how hot it feels with humidity.

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Heat Index Calculator
Calculate how hot it feels with humidity.

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Understanding the 'Real Feel' Temperature

The heat index, also known as the 'apparent temperature', is a measure of how hot it really feels when relative humidity is combined with the air temperature. High humidity reduces the body's ability to cool itself through sweating, making it feel hotter than the actual temperature. This calculator uses the National Weather Service (NWS) formula to determine this important safety metric.

The Heat Index Formula Explained

The NWS uses a complex multivariate regression equation. A simplified version is: HI = -42.379 + 2.049T + 10.143R - 0.225TR - 6.838x10⁻³T² - 5.482x10⁻²R² + 1.229x10⁻³T²R + 8.528x10⁻⁴TR² - 1.99x10⁻⁶T²R² Where:

  • T is the temperature in degrees Fahrenheit.
  • R is the relative humidity in percent.

How to Use the Calculator

  1. Temperature (°F): Enter the ambient air temperature in Fahrenheit.
  2. Humidity (%): Enter the relative humidity as a percentage.
  3. Calculate: The tool will display the calculated heat index temperature.

Real-World Example

The temperature is 90°F and the relative humidity is 70%.

  • Plugging these values into the formula gives a heat index of approximately 106°F.
  • This means the heat stress on your body feels equivalent to being in 106°F air with no humidity.

Frequently Asked Questions (FAQ)

  • Why is the heat index important? It's a crucial public health tool. High heat index values indicate an increased risk of heat-related illnesses like heat exhaustion and heatstroke. The NWS issues advisories and warnings based on these values.
  • Does the heat index work for all temperatures? The formula is generally applied when the temperature is above 80°F (27°C) and the humidity is above 40%.
  • Does wind affect the heat index? The standard heat index calculation does not factor in wind. A breeze can provide some relief by helping sweat evaporate. However, in extreme heat, it may not be enough to prevent heat-related illness.