Heat Index Calculator
Calculate the "feels like" temperature in hot, humid weather from air temperature and relative humidity using the NWS heat index.
How it works
Rothfusz regression: HI = f(T, RH) — a polynomial in temperature (°F) and relative humidity (%)
When the air is humid, sweat evaporates slowly, so your body sheds heat less efficiently and the air feels hotter than the thermometer reads. The National Weather Service heat index quantifies this with the Rothfusz regression, a polynomial fitted to human comfort studies. It is defined for shade and light wind; in direct sun the true "feels like" can be up to 8 °C higher still.
Worked example
At 32 °C (89.6 °F) and 70% humidity, the Rothfusz regression returns roughly 106 °F, or about 41 °C — some 9 °C hotter than the air temperature, placing conditions in the "danger" band where heat cramps and exhaustion become likely with exertion.
Sources: Heat index — Wikipedia
Frequently asked questions
Why does humidity make heat feel worse?
Your body cools itself by evaporating sweat. Humid air is already near saturation, so sweat evaporates slowly and carries away less heat. The result is that a humid 32 °C day stresses the body far more than a dry one at the same temperature.
Does the heat index account for sunshine?
No. The standard heat index assumes shade and a light breeze. Full sunshine can add as much as 8 °C to the effective temperature, so outdoor exposure in direct sun is more dangerous than the raw number suggests.
At what heat index is it dangerous?
The NWS flags "extreme caution" from about 32 °C (90 °F), "danger" from about 39 °C (103 °F) where heat exhaustion is likely, and "extreme danger" above about 52 °C (125 °F) where heat stroke is highly likely with continued exposure.
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This calculator is for educational purposes. Double-check important results before acting on them.