Heat Index Deep Calculator

Sweat evaporation is the body's primary cooling mechanism, and high humidity slows that evaporation because the surrounding air is already close to saturated with moisture. Heat index converts that combined effect of temperature and humidity into a single "feels like" number that better represents the physiological heat stress your body actually experiences.

The full Rothfusz regression is a polynomial fit calibrated to conditions where heat stress is meaningful, and applying it at low temperatures can produce inaccurate results. Below 80°F, this calculator switches to a simpler linear approximation, which is standard National Weather Service practice, since heat index isn't a particularly meaningful figure at cooler temperatures anyway.

The Rothfusz regression alone can overstate or understate heat stress at extreme humidity levels. A downward correction applies when humidity is below 13% and temperature is 80-112°F, since very dry air lets sweat evaporate especially efficiently. An upward correction applies when humidity exceeds 85% at 80-87°F, since very saturated air severely limits evaporative cooling.

Heat illness risk accumulates with time spent in hot conditions, not just the instantaneous temperature and humidity reading, so someone spending eight hours at a dangerous heat index needs meaningfully more hydration guidance than someone stepping outside for ten minutes at the identical reading. This calculator multiplies a heat-index-based hourly rate by your planned exposure hours.

The categories follow the National Weather Service's published heat index chart: Caution (80-90°F) means fatigue is possible with prolonged exposure, Extreme Caution (90-103°F) brings possible heat cramps and exhaustion, Danger (103-125°F) makes heat cramps and exhaustion likely with heatstroke possible, and Extreme Danger (125°F+) means heatstroke is highly likely with continued exposure or activity.