How this instrument works
Heat index is what still air actually feels like on skin once humidity is folded in. The body sheds heat mainly by evaporating sweat, and evaporation depends on how much more water vapor the surrounding air can still absorb. At 95°F with low humidity, sweat evaporates freely and the skin stays close to the thermometer reading. At the same 95°F with the air already carrying 60% of the moisture it can hold, that cooling pathway is half-blocked — the body keeps heat it would otherwise shed, and the effective temperature climbs well above what the thermometer shows.
The number comes from the Rothfusz regression, a 1990 National Weather Service technical note by Lans Rothfusz that fits a polynomial in temperature and relative humidity to Robert Steadman's earlier, far more computationally demanding physiological heat-balance model. Rather than solving Steadman's equations directly for every forecast, the regression reproduces them to within about 1.3°F across the range meteorologists actually use, at the cost of coefficients — 2.04901523, −0.22475541, and the rest — that carry no tidy physical meaning of their own; they are curve-fit weights, not constants of nature.
The fit only holds where it was calibrated: air temperature at or above roughly 80°F together with relative humidity at or above 40%. Push either value much lower and the polynomial can return a heat index below the actual air temperature, which is not physically meaningful — the National Weather Service instead falls back to a simpler averaging formula outside that window. The regression also assumes shade and a light breeze; full sun can add another 15°F to how hot the air actually feels.
- Enter the plain thermometer reading in Air temperature, °F — not a forecast high, the current reading.
- Enter the current Relative humidity, % from a local station or hygrometer, not a forecast estimate.
- Check the intermediate Heat index, linear/quadratic terms value if you want to see the regression's first pass before the interaction terms are added.
- Read Heat Index, °F for the finished figure — the number the National Weather Service maps to its caution, extreme caution, danger, and extreme danger bands.
- Re-enter values as conditions change; both regression stages are plain polynomials, so the result updates instantly.
Worked example — 95°F at 60% humidity
Set Air temperature, °F to 95 and Relative humidity, % to 60 — a plausible July afternoon across much of the humid eastern United States. The first-pass regression, Heat index, linear/quadratic terms, works out to −779.2817417; that negative intermediate number looks alarming but it is simply an artifact of the polynomial fit, not a temperature of its own.
Adding the three interaction terms brings the total to Heat Index, °F = 113.0903083, which the National Weather Service rounds to 113°F. That sits inside the agency's 'Danger' band, 103–124°F, where heat cramps and heat exhaustion become likely with continued exposure — eighteen degrees hotter than the 95°F thermometer reading, purely because more than half the surrounding air is already carrying the water vapor that would otherwise carry sweat away.
Questions
Why is heat index higher than the actual air temperature?
Because humidity blocks evaporative cooling. Sweat cools skin only by evaporating, and warm air that already holds a lot of water vapor can absorb little more of it, so sweat lingers on the skin instead of evaporating away. Heat index restates the combined effect of temperature and humidity as a single equivalent temperature — what a thermometer would need to read, at low humidity, to feel the same on skin.
What is the valid range for the heat index formula?
Roughly 80°F and above with relative humidity at or above 40%. The Rothfusz regression was fit to Steadman's physiological model inside that window, and its accuracy degrades outside it; below about 80°F, or at very low humidity, the National Weather Service uses a simpler formula instead, since the full regression can otherwise return a heat index lower than the actual air temperature.
Does the heat index account for direct sun or wind?
No — it assumes shade and a light breeze, the same conditions Steadman's original physiological model specified. Direct sun can add up to 15°F to how hot conditions actually feel, and the National Weather Service advises adding that margin by hand. Wind helps at moderate humidity but stops helping once the surrounding air is nearly saturated, since there is little dry air left for it to bring in.
Why does the formula use so many decimal-place coefficients?
Because they come from a multiple regression fit, not a physical derivation. Lans Rothfusz fit a polynomial in temperature and humidity to thousands of outputs from Robert Steadman's 1979 heat-balance model, and coefficients like 2.04901523 and 0.00085282 are the least-squares weights that reproduced Steadman's results most closely — not measured physical constants.
What do the two output fields actually represent?
They are the regression's two stages. Heat index, linear/quadratic terms is the first six-term polynomial in temperature and humidity alone; Heat Index, °F adds three further terms that capture how temperature and humidity interact, producing the final published figure. Comparing the two shows that the interaction terms, not the base terms, do most of the work once humidity climbs high.
Is 80°F at 40% humidity dangerous?
Not especially — at that milder combination the heat index works out to about 80°F, close to the plain air temperature, because the regression is calibrated for conditions where heat and humidity compound each other more strongly. The heat index only pulls sharply away from the thermometer reading once both temperature and humidity climb into the range that actually prompts National Weather Service warnings.