Wind Chill Calculator & Frostbite Time

About the Wind Chill Calculator & Frostbite Time

Wind strips away the thin layer of warm air around your skin, so the same air feels colder and exposed skin freezes sooner. The wind chill temperature puts a number on that. It is the calm-air temperature that would cool exposed facial skin at the same rate. This calculator uses the Wind Chill Temperature index of the US National Weather Service and Environment Canada, from the air temperature and the wind speed in km/h, mph, m/s or knots.

It also shows how fast frostbite can happen, from two official sources:

• the NWS frostbite time (30, 10 or 5 minutes), read from the shading of the official NWS chart;
• Environment and Climate Change Canada’s risk level, from low to extreme, with its guidance.

A full wind chill chart, NWS in °F or a metric chart in °C, highlights your conditions. The index applies at or below 10 °C (50 °F) with wind of 3 mph (4.8 km/h) or more; for lighter wind it uses Environment Canada’s light-wind formula.

Common uses

• Deciding how to dress for a winter walk, run, ski trip or bus stop wait, and how long exposed skin is safe.
• Reading winter forecasts and cold weather advisories.
• Planning outdoor work, events or school recess in the cold.
• Science lessons on heat transfer and how wind cools the body.

The formula

The Wind Chill Temperature index, in US units, is WC = 35.74 + 0.6215·T − 35.75·V^0.16 + 0.4275·T·V^0.16 (T in °F, V in mph). In metric form it is WC = 13.12 + 0.6215·T − 11.37·V^0.16 + 0.3965·T·V^0.16 (T in °C, V in km/h). The calculator uses the form for your unit; the two agree within 0.1 °C. V is the wind speed at the standard anemometer height of 10 m (33 ft). The index converts it to the wind at the height of an adult’s face (about 5 ft) and is based on a model of the human face.

The NWS defines the index only at or below 50 °F (10 °C) and for winds above 3 mph. Environment Canada reports it when the temperature is 0 °C or below and the wind 5 km/h or more. For winds over 0 but under 5 km/h it uses W = T + [(−1.59 + 0.1345·T) ÷ 5]·V, which meets the main formula at 5 km/h. The NWS index also assumes no sun, as on a clear night. The NWS says bright sunshine may raise the wind chill by 10–18 °F (about 6–10 °C).

Frostbite times

The NWS chart shades each cell by the time for exposed skin to freeze: 30, 10 or 5 minutes. The calculator reads the cell at or beyond your conditions — the temperature rounded down to the next 5 °F and the wind rounded up to the next 5 mph — so the time is never longer than the chart shows. Beyond the chart (colder than −45 °F or windier than 60 mph) frostbite can only come faster.

Environment Canada groups wind chill values instead:

• 0 to −9: low risk;
• −10 to −27: moderate risk;
• −28 to −39: high risk, exposed skin can freeze in 10–30 minutes;
• −40 to −47: very high, 5–10 minutes;
• −48 to −54: severe, 2–5 minutes;
• −55 or colder: extreme, under 2 minutes.

It adds that in sustained winds over 50 km/h frostbite can happen faster than these times. The two agencies’ times can differ for the same conditions, so the calculator shows both.

What wind chill does not mean

Wind chill describes people and animals only. A car radiator or water pipe cools faster in the wind, but never below the air temperature. For example, in −5 °F air with a −31 °F wind chill, the radiator will not drop below −5 °F. Frostbite needs the air itself to be below freezing. Above 32 °F (0 °C) a cold wind cannot freeze skin, though long exposure — especially when wet — can still cause hypothermia. The NWS tested the index on adults, and advises even more caution with children, older people and anyone in poor health.

Sources

• NOAA National Weather Service, Understanding Wind Chill (formula, limits, sunshine, examples) and the NWS Wind Chill Chart, whose frostbite shading the calculator uses.
• Environment and Climate Change Canada, glossary: wind chill (metric and light-wind formulas) and Wind chill and the Wind Chill Index (risk levels, chart and examples).

Further reading: R. J. Osczevski & M. Bluestein, “The new wind chill equivalent temperature chart”, Bull. Amer. Meteor. Soc. 86, 1453–1458.

Limitations

• The index is for exposed facial skin, from a face model tested on adults, with no sun; it does not include clothing, wetness or exertion.
• Frostbite times are general guidance for exposed skin, and people differ. The NWS chart covers 40 to −45 °F and 5–60 mph.
• Gusts are not included — use the sustained wind speed from a forecast or observation.
• Not medical advice: if you suspect frostbite or hypothermia, get medical help.

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