Wind chill calculator

Why wind makes cold dangerous

Your skin warms the air immediately touching it, creating a boundary layer a few millimeters thick that acts as insulation. Still air is a genuinely poor conductor of heat, so that layer does real work — it is the same principle behind double-glazed windows and down jackets, both of which trap air rather than block cold.

Wind destroys the boundary layer. Moving air carries the warmed molecules away and replaces them with cold ones, continuously, so your skin has to reheat a fresh supply of air every second. The rate of heat loss can multiply several times over, which is why 20 °F with a 30 mph wind is genuinely more dangerous than 5 °F on a calm day.

One thing wind chill does not do is change the temperature of objects. Your car's engine block, a water pipe, or a puddle will not freeze any faster at a wind chill of -20 °F than at an air temperature of 20 °F — they cannot get colder than the air around them. Wind chill applies to bodies that generate their own heat, which means people and animals. Pipes freeze because of prolonged cold air, not because of the wind chill value, although wind does speed up how quickly they reach air temperature.

The 2001 formula and why the old one was wrong

The original wind chill index came from Antarctic research done by Paul Siple and Charles Passel in 1945, who measured how fast water froze in a plastic cylinder at various wind speeds. It was a reasonable first attempt and it stayed in use for over fifty years, but it overstated the danger considerably — a plastic cylinder full of water is not much like a human face.

In 2001 the US and Canadian weather services replaced it with a formula built from human trials. Volunteers sat in a refrigerated wind tunnel with thermal transducers taped to their faces while researchers measured actual heat loss from cheeks, forehead, nose and chin. The new model assumes a calm walking pace, an adult face at about 5 feet above ground, and no sunlight.

The change made a real difference to the numbers. Under the old index, 0 °F with a 20 mph wind gave a wind chill near -39 °F. The current formula puts the same conditions at about -22 °F. Cold weather did not become safer in 2001; the measurement simply became honest, which also meant warning thresholds had to be rewritten.

Frostbite, hypothermia and what to actually do

Frostbite is tissue freezing, and it starts at the extremities furthest from your core: fingers, toes, ears, nose and cheeks. The early stage, frostnip, produces numbness and pale or waxy skin and recovers fully with rewarming. Once ice crystals form inside the cells, damage becomes permanent, and the affected area may feel deceptively fine because the nerves have stopped reporting.

Rewarm frostbitten skin in water held between 99 and 108 °F, never with direct heat from a fire, a heating pad or a car heater — numb skin cannot feel itself burning. Do not rub the area, since ice crystals tear tissue when moved. And do not rewarm at all if there is any chance the part will refreeze before you reach shelter; thawing and refreezing does far more damage than staying frozen during the trip out.

Hypothermia is the greater killer, and it is quieter. It begins when core temperature drops below 95 °F, and the early signs are shivering, slurred speech, clumsiness and confusion. The dangerous part is that confusion means the person often insists they are fine, or stops shivering and feels suddenly warm. Most hypothermia deaths in the United States happen at air temperatures between 30 and 50 °F, not in extreme cold — usually involving wet clothing, wind, alcohol, or all three. Wet clothing conducts heat away roughly 25 times faster than dry, which is why staying dry outranks staying bundled.

The formula

National Weather Service wind chill, in use since November 2001
T = air temperature (°F), V = wind speed (mph)

WC = 35.74
   + 0.6215·T
   - 35.75·V^0.16
   + 0.4275·T·V^0.16

Valid for T ≤ 50°F and V > 3 mph.
At or below 3 mph the formula returns the air temperature itself.

The V^0.16 exponent is the important part. Heat loss does not grow linearly with wind speed — it climbs steeply at first and then flattens out. Going from calm to 10 mph makes a large difference; going from 40 to 50 mph barely moves the number, because the boundary layer has already been stripped away as completely as wind can strip it.

Practical tips

• Cover every bit of skin. A face is where frostbite usually starts, and a balaclava or scarf is worth more than an extra sweater.
• Dress in layers you can open. Sweating into your clothing in the cold is how people get hypothermic on the walk home.
• Mittens beat gloves in serious cold, because your fingers warm each other.
• Alcohol makes you feel warmer while dilating skin blood vessels, which dumps core heat faster. It is a factor in a large share of hypothermia deaths.
• Keep a blanket, hat and gloves in your car through winter. Wind chill matters most when a breakdown turns a 20-minute drive into two hours of waiting.

Frequently asked questions