Every number on this page describes a notional adult: healthy, dressed for the season, walking rather than working. It is a weather index, not a physiological one, and the gap between those two things is where people get hurt.
You. Age, fitness, acclimatisation, body size, medication and clothing all move the real answer more than a few km/h of wind does. Someone who has spent a fortnight in the heat tolerates it markedly better than someone who flew in yesterday, and no term in any of these formulas knows that.
WBGT. Wet-bulb globe temperature is what sports governing bodies and workplace heat regulations actually use, because it measures radiant load and air movement directly rather than inferring them. If you need a threshold to make a decision against, it is WBGT you want, not heat index.
The assumptions inside each formula. Wind chill assumes an exposed adult face at 1.5 m in the shade, walking at 1.4 m/s, and wind measured at the standard 10 m — which is roughly 1.5× what you feel at head height. Heat index assumes shade and a light breeze. Apparent temperature assumes shade unless the sun toggle is on.
The far ends. Below about −50 °C and above about 50 °C all three are extrapolating past the data they were fitted to. The numbers keep coming; their meaning does not.
Wind chill and heat index are checked against the published NWS charts in feelslike_model.py — seven wind-chill cells and six heat-index cells, all reproduced exactly. That model also caught two things in this page: the last Rothfusz coefficient had been typed as 1.99788×10⁻⁶ where the regression publishes 1.99×10⁻⁶, worth up to 1.1 °F at the extreme corner, and the sun toggle was adding a flat +15 °F to the heat index rather than modelling radiation.
Wind chill stops at 10 °C. Heat index does not begin until 27. Between them sits most of the British year, and until now this page reported that wind and humidity did nothing there. They do: at 12 °C, still and dry feels like 9 °C and windy and damp feels like 5.
“Feels like −13” does not mean the air is at −13. It means exposed skin loses heat as fast as it would in still air at −13. Your kettle will not freeze any quicker, and a thermometer in the wind still reads the air temperature. Only living things feel wind chill.
It stops you cooling. Sweat works by evaporating, and evaporation slows as the air fills with water. At 35 °C and 20% humidity you are fine; at 35 and 80% you are in danger — same air, same thermometer. That is why the heat index climbs so steeply up the humidity axis and so gently along the temperature one.
All three formulas take wind at the standard 10 m measuring height, which is roughly 1.5× what you feel walking. A forecast of 30 km/h is nearer 20 at head height in the open, and far less between buildings. If you are entering what a weather app told you, that is the 10 m figure and it is the right one to use.
"Feels like" isn't one formula — it's two, switched automatically depending on conditions. Below about 10°C with meaningful wind, the wind chill formula estimates how much faster moving air strips heat from exposed skin. Above about 27°C, the heat index formula estimates how humidity blocks sweat from evaporating, the body's main cooling mechanism. In between, wind and humidity have only a minor effect, and "feels like" is just the air temperature.
0°C with a 24 km/h wind feels like about −6°C — noticeably colder than the thermometer, though still short of serious frostbite risk over a short exposure. 32°C at 70% humidity feels like roughly 40°C, squarely in the NWS "Danger" heat category, even though the thermometer reading alone looks merely hot.
NWS testing found humidity changed the result by less than a degree, so the official 2001 formula leaves it out entirely to keep the calculation simple and reliable — wind chill is driven almost entirely by temperature and wind speed.
No. Wind chill can never cool human skin below the actual air temperature — it only describes how much faster heat is lost. If the air temperature itself is above 0°C, frostbite from cold exposure alone isn't possible, whatever the wind chill number says.
The core Rothfusz regression is a statistical fit, not a physical law, and it drifts at the extremes — the NWS adds small correction terms for very low humidity (which lowers the index, since sweat evaporates efficiently) and very high humidity at moderate heat (which raises it further).
For heat index, yes, and it's built in above — toggle "direct sun exposure" and this calculator adds the NWS full-sun adjustment (up to 15°F / 8°C over the shaded value) directly. There's no equivalent sun adjustment for wind chill, since the formula is about heat loss to moving air, not solar gain.
Because two entirely different formulas are stitched together at their boundaries — the line is flat (feels-like equals actual) through the mild middle range, then bends onto the wind-chill curve below 10°C or the heat-index curve above 27°C. The kinks in the chart are exactly where NWS switches formula.
The NWS formula is only defined for wind above 3 mph — below that, it doesn't publish a value at all, it just reports the air temperature unchanged. Taken literally that's a step: drag the wind slider from 2.9 to 3.1 mph and the number would jump several degrees in an instant, which doesn't match how wind actually behaves. This calculator tapers smoothly from no effect at 0 mph up to the formula's own value at 3 mph and beyond, so the number always moves continuously as you drag the slider — a display smoothing choice on top of the formula, not a change to it.
The temperature at which air would become saturated (100% RH) if cooled at constant moisture content. This tool estimates it with the Magnus–Tetens approximation from air temperature and relative humidity — useful comfort context, separate from the NWS wind-chill and heat-index formulas.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.