Heat Index Calculator
Calculate the "feels like" temperature based on air temperature and relative humidity. The heat index indicates how hot it actually feels when humidity is factored in.
Heat cramps and heat exhaustion are likely. Heat stroke is probable with continued exposure.
Heat Index Safety Levels
Fatigue possible with prolonged exposure and physical activity.
Heat cramps and heat exhaustion possible. Limit outdoor activity.
Heat cramps and exhaustion likely. Heat stroke probable with continued exposure.
Heat stroke is highly likely with continued exposure. Reduce exposure and follow local heat guidance.
Understanding Heat Index
What is Heat Index?
Heat index, also known as "apparent temperature," is what the temperature feels like to the human body when relative humidity is combined with the air temperature. High humidity reduces the body's ability to cool itself through sweating.
Why Does Humidity Matter?
When humidity is high, sweat doesn't evaporate as efficiently from your skin. Since evaporation is your body's primary cooling mechanism, high humidity makes it harder to regulate body temperature, making the heat feel more intense.
Heat Safety Tips
Stay hydrated, wear lightweight and light-colored clothing, take frequent breaks in the shade or air conditioning, and avoid strenuous outdoor activities during peak heat hours (10am-4pm). Check on elderly neighbors and never leave children or pets in vehicles.
How is Heat Index Calculated?
This calculator follows the National Weather Service procedure: a simple formula first, averaged with the air temperature, and the full Rothfusz regression applied only when that average reaches 80°F. Below that the simple average is reported, since humidity has little effect on how the temperature feels.
Note: Heat index values are calculated for shady, light wind conditions. Exposure to full sunshine can increase heat index values by up to 15°F. Strong winds, especially with very hot, dry air, can also be hazardous.
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