Why Humidity Matters So Much for Heat Index
Summer heat index depends heavily on humidity, not just temperature. High humidity slows sweat evaporation, so your body can't shed heat as effectively — the same temperature feels far hotter as humidity climbs. This calculator applies the heat index formula used by the US National Weather Service.
Enter temperature and humidity to get the heat index along with a four-level heatstroke risk rating. Above 103°F, conditions move past cramps and exhaustion into a danger level with high heatstroke likelihood; above 125°F it's extreme danger requiring immediate shade or shelter. Checking this before outdoor activity, exercise, or work during a heat advisory helps you plan safely.
Because the formula is designed to capture perceived heat in hot conditions, it doesn't produce a meaningful value below 80°F and is excluded from calculation in that range. It's most useful when a heat wave is expected or already underway.
Frequently Asked Questions
High humidity slows sweat evaporation, so your body can't cool itself as effectively. As a result, the same temperature feels hotter as humidity rises.
A heat index above 103°F is classified as a danger level with high heatstroke likelihood. Above 125°F is extreme danger, requiring immediate shade or shelter.
The heat index formula was designed to capture perceived heat in hot conditions. Below 80°F it doesn't produce a meaningful value, so it's excluded from calculation.