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CALCPARK

Wind Chill Calculator

Feels-like temperature from air temperature and wind with the NWS / Environment Canada wind chill formula and frostbite bands, plus a heat index mode for hot, humid days.

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How to use it

  1. Pick the mode: wind chill for cold and windy weather, heat index for hot and humid weather.
  2. Choose °C or °F and, for wind chill, km/h, mph or m/s. Switching converts values you already typed.
  3. Enter the air temperature and the wind speed or relative humidity from the forecast.
  4. Read the feels-like temperature in both units, the risk band and the table of bands. Outside the formula’s range the tool says “not applicable” instead of inventing a number.

Example

Air −10 °C, wind 20 km/h. 200.16 = 1.615, so WCI = 13.12 + 0.6215 × (−10) − 11.37 × 1.615 + 0.3965 × (−10) × 1.615 = 13.12 − 6.22 − 18.36 − 6.40 = −17.9 °C ≈ −18 °C(0 °F). That is in the “low risk of frostbite” band; at −20 °C with the same wind the index is −30 °C and exposed skin can freeze in 10–30 minutes. Heat index: 32 °C (89.6 °F) at 70 % humidity gives about 41 °C (105 °F)with the Rothfusz regression – in the NWS “danger” band (103–124 °F), where heat cramps or exhaustion are likely with prolonged exposure.

Formulas, validity and sources

Wind chill (°C, km/h): WCI = 13.12 + 0.6215 T − 11.37 V^0.16 + 0.3965 T V^0.16
Wind chill (°F, mph):  WCT = 35.74 + 0.6215 T − 35.75 V^0.16 + 0.4275 T V^0.16
  valid for T ≤ 10 °C (50 °F) and V ≥ 4.8 km/h (3 mph)
Heat index (°F):       HI = −42.379 + 2.04901523 T + 10.14333127 R − 0.22475541 T R − 0.00683783 T² − 0.05481717 R²
                            + 0.00122874 T² R + 0.00085282 T R² − 0.00000199 T² R²
  R < 13 % and 80–112 °F: HI −= (13 − R)/4 × √((17 − |T − 95|)/17)
  R > 85 % and 80–87 °F:  HI += (R − 85)/10 × (87 − T)/5
  below 80 °F: HI = average of T and 0.5 × (T + 61 + 1.2 (T − 68) + 0.094 R)

The wind chill index was developed in 2001 by the Joint Action Group for Temperature Indices from wind-tunnel and human trials; it models heat loss from a bare face walking at 1.4 m/s into the wind, with the 10-metre forecast wind scaled to face height. It is not defined above 10 °C or in near-calm conditions, and it says nothing about sun, humidity or wet clothing. The frostbite bands in the tool are Environment Canada’s published hazard table; the NWS chart shades the same formula into 30-, 10- and 5-minute frostbite zones, which the band labels name. The heat index is the NWS regression (Rothfusz 1990) of Steadman’s apparent temperature for a 1.7 m, 67 kg person in shade with a light breeze; its error is about ±1.3 °F against Steadman’s table, and the NWS applies the humidity adjustments and the low-range formula exactly as implemented here. The tool uses the metric wind chill form directly and converts for °F display; the two published forms agree within 0.2 °C.

Sources: NWS – Wind chill chart and formula, Environment and Climate Change Canada – Wind chill index: hazards and formula, NWS – The Heat Index equation (Rothfusz 1990, SR 90-23), NWS – Heat index chart and categories, Osczevski & Bluestein 2005, Bull Amer Meteor Soc – The new wind chill equivalent temperature chart.

Feels-like values are estimates for an average adult and are not medical advice. Children, older people and anyone with heart or circulation problems are at risk sooner; follow local weather warnings.

Wind chill versus heat index versus humidex

Wind chill and heat index are the two “feels like” measures used by the US and Canadian weather services, one for cold and one for heat. Canada reports humidex in summer instead of the heat index; it uses dew point rather than relative humidity and runs a few degrees higher for the same conditions. Many forecast apps show a single “RealFeel” or “apparent temperature” that blends wind, humidity and sun with proprietary weights, so small differences from this calculator are expected.

Questions

Why does the calculator say "not applicable"?
The wind chill formula was fitted only for temperatures at or below 10 °C (50 °F) and winds of at least 4.8 km/h (3 mph). Outside that range the published tables stop, so the tool reports the air temperature as the feels-like value instead of extrapolating.
At what wind chill does frostbite occur?
Environment Canada’s table puts exposed skin at risk of freezing in 10–30 minutes from a wind chill of −28 °C (−18 °F), in 5–10 minutes from −40 °C, 2–5 minutes from −48 °C and under 2 minutes from −55 °C. The NWS chart shades the corresponding 30-, 10- and 5-minute zones.
Can the heat index be used below 27 °C (80 °F)?
The regression is only meant for 80 °F and above. Below that the NWS uses a simpler formula averaged with the air temperature, which the tool applies and labels; the result is usually within a degree of the air temperature.