How to Use the Heat Index Calculator
Unlike South Korea, which has fixed national thresholds for "tropical night" (overnight low of 77°F+) and "heatwave" (daytime high of 91°F+), the United States has no single official heatwave temperature. Instead, the National Weather Service issues heat alerts based on the heat index — a combination of temperature and humidity — with thresholds that vary by region and local climate normals.
This calculator uses the Rothfusz regression, the formula NWS uses to estimate heat index above 80°F. Below 80°F, the heat index is treated as roughly equal to air temperature since humidity has little added effect. A commonly cited national reference point is a heat index of 90°F or higher, which falls into the "Extreme Caution" category.
The results are grouped into NWS-style categories: below 80°F is Below Caution, 80-89°F is Caution, 90-102°F is Extreme Caution, 103-124°F is Danger, and 125°F+ is Extreme Danger. Because local NWS offices set their own alert criteria, always check your regional forecast for official heat advisories rather than relying on this calculator alone.
Frequently Asked Questions
No single national threshold exists — NWS heat alert criteria vary by region and local climate. A heat index of 90°F or higher ("Extreme Caution") is commonly cited as a rough national benchmark.
Heat index combines air temperature and relative humidity to estimate how hot it actually feels, since high humidity slows sweat evaporation and reduces your body's ability to cool itself.