What is the heat index?

Heat index is the "feels like" temperature that combines air temperature with relative humidity. High humidity makes it harder for sweat to evaporate, making you feel hotter.

What heat index is dangerous?

Caution begins at 80°F heat index. 91-103°F requires extreme caution. 104-125°F is dangerous (heat exhaustion likely). Above 126°F is extreme danger (heat stroke highly likely).

How does humidity affect heat index?

Higher humidity makes the heat index higher. At 90°F with 50% humidity, it feels like 95°F. At 90°F with 90% humidity, it feels like 122°F!

Heat Index Calculator

Calculate the heat index (feels-like temperature) using the official NWS formula. Enter air temperature and relative humidity to see how hot it really feels. Get heat-related health risk categories and activity recommendations.

Features

NWS heat index formula
Health risk categories
Activity recommendations
Fahrenheit and Celsius support
Heat index reference chart

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