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Everyday Math Suite

Heat Index Calculator

Calculate the apparent felt temperature by combining air temperature and relative humidity, and review NOAA heat advisory danger zones.

1. Enter Heat parameters

Understanding the Heat Index

The Heat Index, occasionally called the apparent temperature, represents what the hot weather feels like to a human body. In humid conditions, sweat doesn't evaporate as fast as normal, causing bodily heat to accumulate. The index warns of dangerous heat fatigue conditions.

The NOAA Heat Index Hazards

The National Oceanic and Atmospheric Administration (NOAA) categorizes heat index dangers into five distinct levels:

  • Normal / Safe (< 80°F): Comfortable ambient conditions. Standard hydration rules apply.
  • Caution (80°F to 90°F): Fatigue is possible with prolonged physical exertion.
  • Extreme Caution (91°F to 103°F): Sunstroke, heat cramps, and heat exhaustion are possible.
  • Danger (104°F to 129°F): Heat cramps or heat exhaustion are highly likely. Heatstroke is possible.
  • Extreme Danger (>= 130°F): Heatstroke is highly likely. Avoid physical activity and seek cooling immediately.

Mathematical Algorithm

The NWS utilizes the Rothfusz regression equation, which is a multi-parameter fit model. It requires temperature to be in Fahrenheit and relative humidity in percentage. Standard regression adjustments are made for dry heat (RH < 13%) and high humidity (RH > 85%) conditions to ensure high statistical correlation with biological heat loss profiles.

Common Questions & Heat Index Insights

What is the difference between air temperature and heat index?

Air temperature is the actual reading from a thermometer in the shade. Heat index (or apparent temperature) combines air temperature with relative humidity to estimate how hot the air actually feels to the human body.

Why does relative humidity make it feel hotter?

Your body cools itself by evaporating sweat from your skin. When relative humidity is high, the surrounding air is already saturated with water vapor, which slows down sweat evaporation. As a result, heat is trapped in your body, making you feel much hotter.

At what threshold does the heat index start to apply?

The official National Weather Service heat index is only defined for air temperatures at or above 80°F (26.7°C) and relative humidity at or above 40%. Below these thresholds, humidity does not significantly elevate heat discomfort, so the heat index is equal to the air temperature.

Does direct sunlight affect the calculated heat index?

Yes. The standard heat index formula assumes you are in the shade with a light breeze. Exposure to direct, full sunlight can increase the apparent temperature by up to 15°F (8°C). If you are working in direct sun, you should take extra precautions.

What are the warning signs of heat exhaustion and heatstroke?

Heat exhaustion signs include heavy sweating, muscle cramps, weakness, dizziness, headache, and nausea. Heatstroke is a life-threatening emergency where sweating stops, body temperature rises above 103°F, and confusion or loss of consciousness occurs. Seek immediate medical help if heatstroke is suspected.

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