dopecalc

Heat Index Calculator

Calculate the heat index (feels-like temperature) using the NOAA formula that combines air temperature and relative humidity. Includes health risk categories.

About this Calculator

Calculate the heat index (feels-like temperature) using the NOAA formula that combines air temperature and relative humidity. Includes health risk categories.

Formula & Calculations

Formula

HeatIndex(°F) uses the Rothfusz regression of the Steadman model: a function of temperature T (°F) and relative humidity R (%).
Where:
  • T=Air temperature in Fahrenheit
  • R=Relative humidity as a percentage (0-100)
  • HeatIndex=The perceived (feels-like) temperature combining heat and humidity
  • Risk Category=Health risk level: Caution, Extreme Caution, Danger, or Extreme Danger

Assumptions

  • The NOAA heat index formula is valid for temperatures 80-115°F and humidity 0-100%.
  • Exposure is in shaded conditions; direct sunlight can increase the heat index by up to 15°F.
  • Risk categories are based on NOAA heat disorder guidelines.
  • The formula uses the Lans P. Rothfusz regression, with adjustments for extreme conditions.

Calculation Examples

Example 1

Inputs:Temperature = 90°F, Humidity = 60%
Result:Heat Index: 100°F, Risk: Extreme Caution

At 90°F with 60% humidity, the body's cooling through sweat evaporation is reduced, making it feel like 100°F. Heat cramps and heat exhaustion are possible with prolonged exposure.

Example 2

Inputs:Temperature = 100°F, Humidity = 70%
Result:Heat Index: 129°F, Risk: Extreme Danger

At this heat index level, heat stroke is highly likely. Outdoor activity should be avoided.

Frequently Asked Questions

What exactly is the heat index and why does humidity matter?

The heat index is a measure combining air temperature and relative humidity to approximate how hot the human body perceives the temperature. When humidity is high, sweat does not evaporate as efficiently, reducing the body's natural cooling mechanism. This makes the environment feel significantly hotter.

How accurate is the heat index formula?

The NOAA heat index formula is scientifically validated for temperatures 80-115°F and humidity up to 100%. However, individual perception varies based on factors like clothing, physical activity, wind, and sun exposure. Direct sunlight can add up to 15°F to the calculated value.