Wind Chill Index

You can read about wind chill in wikipedia but, in short, wind chill or chill index, or wind chill factor, is the felt air temperature on exposed skin due to wind. The wind chill temperature is never higher than the air temperature, and the windchill is undefined at higher temperatures (above 10 °C).

Paul Allman Siple and Charles Passel developed the first wind chill formulae, working in the Antarctic before the Second World War. Then formulae are evolved, and this calculator uses the one from U.S. National Weather Service

where Twc is wind chill index, in Celsius, Ta is air temperature, V is wind speed in km per hour

Windchill Temperature is only defined for temperatures at or below 10 °C (50 °F) and wind speeds above 4.8 kilometers per hour (about 1.3 meters per second)
This calculator outputs the wind chill index for given air temperature and wind speed and plots the chart for a given temperature where wind speed changes from 2 to 35 meters per second. Thus you can see how windspeed affects wind chill. For those curious, 35 meters per second is Beaufort number 12 - Hurricane with other values as follows.

Wind speed: 1-2 knots (0.3-1.5 m/s) - Beaufort number 1 - Light air
Wind speed: 3-6 knots (1.5-3.3 m/s) - Beaufort number 2 - Light breeze
Wind speed: 7-10 knots (3.3-5.5 m/s) - Beaufort number 3 - Gentle breeze
Wind speed: 11-15 knots (5.5-8.0 m/s) - Beaufort number 4 - Moderate breeze
Wind speed: 16-20 knots (8.0-10.8 m/s) - Beaufort number 5 - Fresh breeze
Wind speed: 21-26 knots (10.8-13.9 m/s) - Beaufort number 6 - Strong breeze
Wind speed: 27-33 knots (13.9-17.2 m/s) - Beaufort number 7 - Near gale
Wind speed: 34-40 knots (17.2-20.7 m/s) - Beaufort number 8 - Fresh gale
Wind speed: 41-47 knots (20.7-24.5 m/s) - Beaufort number 9 - Strong gale
Wind speed: 48-55 knots (24.5-28.4 m/s) - Beaufort number 10 - Storm
Wind speed: 56-63 knots (28.4-32.6 m/s) - Beaufort number 11 - Violent storm
Wind speed: ≥64 knots (≥32.6 m/s) - Beaufort number 12 - Hurricane

Wind Chill Index

Air temperature, Celsius

Wind speed, meters per second

Calculation precision

Digits after the decimal point: 1

Calculate

Wind Chill Index, Celsius

 

Wind chill dependency on wind speed

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So, why do we need to know the wind chill? According to Environment Canada these are the meanings of wind chill index

Wind Chill 0 to -9
Low risk, a slight increase in discomfort
Dress warmly, stay dry

Wind Chill -10 to -27
Uncomfortable, risk of hypothermia if outside for long periods without adequate protection.
Dress in layers of warm clothing with a wind-resistant outer layer. Wear a hat, mittens or insulated gloves, a scarf, and insulated, waterproof footwear. Stay dry. Keep active

Wind Chill -28 to -39
Exposed skin can freeze in 10 to 30 minutes; risk of frostnip or frostbite: Check face and extremities for numbness or whiteness. Risk of hypothermia if outside for long periods without adequate clothing or shelter from wind and cold.
Dress in layers of warm clothing with a wind-resistant outer layer. Cover exposed skin. Wear a hat, mittens or insulated gloves, a scarf, neck tube or face mask, and insulated, waterproof footwear. Stay dry. Keep active

Wind Chill -40 to -47
Exposed skin can freeze in 5 to 10 minutes. High Risk of frostbite: Check face and extremities for numbness or whiteness. Risk of hypothermia if outside for long periods without adequate clothing or shelter from wind and cold.
Dress in layers of warm clothing with a wind-resistant outer layer. Cover all exposed skin. Wear a hat, mittens or insulated gloves, a scarf, neck tube or face mask, and insulated, waterproof footwear. Stay dry. Keep active.

Wind Chill -48 to -54
Exposed skin can freeze in 2 to 5 minutes. Very High Risk of frostbite: Check face and extremities frequently for numbness or whiteness. Serious risk of hypothermia if outside for long periods without adequate clothing or shelter from wind and cold.
Be careful. Dress very warmly in layers of clothing, with a wind-resistant outer layer. Cover all exposed skin. Wear a hat, mittens or insulated gloves, a scarf, neck tube or face mask, and insulated, waterproof footwear. Be ready to cut short or cancel outdoor activities. Stay dry. Keep active.

Wind Chill -55 and colder
Extremely High risk: exposed skin can freeze in less than 2 minutes. DANGER! Outdoor conditions are hazardous. Stay indoors.