How Wind Chill Is Calculated and How Fast Frostbite Sets In

Wind chill is the temperature that bare skin “feels” when wind strips heat away faster than still air would. The U.S. National Weather Service calculates it from the air temperature and the wind speed, and at a wind chill around -18°F (-28°C), exposed skin can develop frostbite in roughly 30 minutes. Colder wind chills shorten that time to 10 minutes or less.

What does wind chill actually measure?

Wind chill measures how fast exposed skin loses heat, expressed as the still-air temperature that would cause the same loss. Your body warms a thin layer of air right next to the skin. In calm air that layer stays put and acts as a little insulation. Wind blows it away and replaces it with colder air, so the skin keeps giving up heat.

The number only describes people and animals, which produce their own heat. A car, a pipe or a bottle of water cannot cool below the actual air temperature, however hard the wind blows. Wind only makes them reach that temperature sooner.

How is wind chill calculated?

The current formula was adopted by the National Weather Service and Environment Canada in 2001. It is based on a model of heat loss from a human face walking into the wind, with wind measured at about 5 feet (1.5 m), roughly face height. In Fahrenheit and miles per hour it reads:

Wind chill = 35.74 + 0.6215T – 35.75(V^0.16) + 0.4275T(V^0.16)

Here T is the air temperature in °F and V is the wind speed in mph. The formula is used only when the temperature is 50°F (10°C) or lower and the wind is above 3 mph. Some worked examples:

30°F with a 10 mph wind feels like about 21°F.
20°F with a 20 mph wind feels like about 4°F.
0°F with a 15 mph wind feels like about -19°F.
-10°F with a 30 mph wind feels like about -39°F.
-20°F with a 25 mph wind feels like about -51°F.

The older index, built from water-freezing experiments in Antarctica in the 1940s, gave much colder numbers and exaggerated the danger at moderate temperatures. The 2001 version replaced it. The National Weather Service wind chill chart lists the values for common combinations, so most people never need to run the math.

How fast can frostbite set in?

Frostbite time depends mostly on the wind chill. The National Weather Service chart shades three bands for exposed skin on an average adult:

Around -18°F (-28°C) wind chill: frostbite possible in about 30 minutes.
Around -35°F (-37°C): about 10 minutes.
Around -50°F (-46°C) and colder: 5 minutes or less.

Those times are averages, and real conditions change them. Wet skin, tight boots, poor circulation, diabetes, smoking, alcohol and exhaustion all speed up freezing. Children and older adults lose heat faster. Touching cold metal or spilling gasoline on skin can cause frostbite in seconds, because both pull heat away much faster than air.

Which body parts freeze first?

The nose, ears, cheeks, chin, fingers and toes freeze first. They stick out from the body, have little muscle to make heat, and the body cuts blood flow to them in the cold to protect the core. Covering the face and hands does more against wind chill than an extra layer on the chest.

What are the signs of frostbite?

Frostbite develops in stages:

Frostnip: skin turns red, then pale, and stings or tingles. It goes away with rewarming and leaves no damage.
Superficial frostbite: skin looks white or grayish, feels waxy and may go numb. Blisters can form a day or so after rewarming.
Deep frostbite: skin turns hard and cold, and the tissue underneath freezes. Blisters fill with dark fluid and the skin can turn black. This needs hospital care.

Numbness is the dangerous part, because people stop feeling the cold that would warn them. Check each other’s faces outdoors.

What should you do if someone has frostbite?

Get the person indoors and warm the affected area in warm water, about 99 to 104°F (37 to 40°C), or with body heat, such as fingers tucked into an armpit. Do not rub the skin or use snow, and keep away from heating pads, stoves or fireplaces, because numb skin burns easily. Do not rewarm a frozen area if it might freeze again before you reach shelter. Deep frostbite and any blistering, hard or blackened skin need medical care.

If the person is shivering hard, confused, slurring speech or very drowsy, treat it as hypothermia, which means the core body temperature has dropped below 95°F (35°C). Call 911. The National Weather Service winter safety pages list the warning signs of both.

How do you dress for a low wind chill?

Wear a windproof outer layer, since wind is what drives the heat loss.
Layer underneath with wool or synthetics, which stay warmer than cotton when damp.
Cover the face, ears and neck with a hat, scarf or face mask.
Choose mittens over gloves; fingers kept together stay warmer.
Keep socks and gloves dry and carry spares on long outings.

The weather service issues cold weather advisories and extreme cold warnings when wind chills reach dangerous levels for the area, and the thresholds differ by region.

Quick answers

Can wind chill freeze water pipes faster?

Wind can cool a pipe faster, but the pipe cannot get colder than the air. If the air is above 32°F (0°C), the pipe will not freeze, whatever the wind chill says.

Do pets feel wind chill?

Yes. Dogs and cats lose heat through their ears, paws and tails, and short-haired and small animals are most at risk. Shorten walks when the wind chill falls well below freezing.

Is there a “heat” version of wind chill?

The heat index plays a similar role in summer. It combines temperature and humidity to estimate how hot it feels.