Heat Index Calculator
The heat index is what the air feels like to a human body that cools itself by evaporating sweat. When humidity is high, that evaporation slows, and the same air temperature becomes far more dangerous.
Conditions
The heat index is defined for shade; direct sun adds up to 15 °F.
Guidance at this level
Why humidity changes everything
The body sheds heat mainly by evaporating sweat, and evaporation slows as the air approaches saturation. At 92 °F and 40 per cent humidity the heat index is about 93 — barely above the air temperature. At the same 92 °F and 75 per cent humidity it is about 112, which is firmly in the danger zone. The thermometer does not move; the risk does.
Dew point is the better single number
Relative humidity depends on temperature, so 70 per cent means very different things at 50 °F and 90 °F. Dew point is absolute: it is the temperature at which the air would saturate, and it maps directly onto how oppressive the air feels. Above 65 °F most people notice; above 70 it feels muggy; above 75 is genuinely oppressive.
The formula is not a simple one
The National Weather Service uses the Rothfusz regression, a nine-term polynomial in temperature and humidity, plus two correction terms — one for low humidity at high temperature, one for very high humidity — and a simpler average-based form below about 80 °F. Shortened versions found in many places drop the adjustments and can be several degrees out at the edges. This page implements all of it.
Frequently Asked Questions
What is the heat index?
Does direct sunlight change the heat index?
What dew point feels humid?
At what heat index is it dangerous?
Sources
Official publications only. Links open the original document in a new tab.
- National Weather Service Heat index The official heat index table and formula
- National Weather Service Heat safety Heat safety guidance by risk level
- National Weather Service Relative humidity and dew point calculator How dew point and humidity relate