Wind Chill and Frostbite: Reading the Cold

You check the temperature before a lift ride, you see a number near zero, and you decide whether to add a layer. That number tells you only part of the story. The wind chill is the figure that describes how cold the air actually feels on exposed skin, and it is the one worth reading before a long chairlift ride in the Northeast.

The details cited in this entry were checked on National Weather Service wind chill chart.

Wind chill is based on the rate of heat loss from exposed skin caused by wind and cold. As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature. The air does not have to be extreme for the effect to matter. At 0°F with a 15 mph wind, the wind chill is -19°F, and at that wind chill exposed skin can freeze in 30 minutes. Thirty minutes is a single lift line and a long ride on an exposed chair.

What the wind chill number actually measures

The National Weather Service index does not measure the air. It describes what the combination of cold and wind does to a human face. The formula calculates wind speed at an average height of 5 feet, the typical height of an adult human face, based on readings from the national standard height of 33 feet, the typical height of an anemometer. That adjustment matters on a mountain, where the summit anemometer reads a wind that never reaches the face of the person sitting below it.

The index is built on a human face model. It incorporates heat transfer theory based on heat loss from the body to its surroundings during cold and breezy days, lowers the calm wind threshold to 3 mph, uses a consistent standard for skin tissue resistance, and assumes no impact from the sun, meaning a clear night sky. Read that last assumption carefully. On a bright blue January morning, the chart is describing something more severe than what you feel, because bright sunshine may increase the wind chill temperature by 10°F to 18°F.

Why the chart stops at two limits

Wind chill temperature is defined only for temperatures at or below 50°F and wind speeds above 3 mph. Below that wind speed the air is not moving enough to drive the calculation, and above that temperature the effect is not the concern. If you are standing in the base lodge at 55°F, the chart has nothing to tell you. If the ridge is reporting a 2 mph breeze, the number you see is essentially the air temperature.

A club trip leader reading the morning forecast is looking at two numbers, not one. The air temperature tells you what your equipment is dealing with. The wind chill tells you what your skin is dealing with. On a long ride, the second number governs.

You can read the full explanation and the formula on the National Weather Service wind chill chart, which lays out the effect across the range the index covers.

What the wind chill means for frostbite

The question that comes up most on a cold morning is whether frostbite is possible when the thermometer is above freezing but the wind chill is below it. The answer is no. The air temperature has to be below freezing for frostbite to develop on exposed skin. Wind chill cannot bring the temperature below freezing for humans and animals when the thermometer says it is above freezing, so you will not get frostbite at 34°F no matter what the wind does. You can still get hypothermia from exposure to cold, which is the quieter risk in wet, windy, above-freezing conditions.

In short, you can only get frostbite if the actual air temperature, not the wind chill temperature, near your skin is below freezing. That distinction is the one worth carrying up the lift. The wind chill does not lower the temperature of your skin below the air; it speeds up how fast you lose heat toward it.

Does wind chill apply to your skis, your car, your pipes?

No. Wind chill applies only to people and animals. The only effect it has on inanimate objects, such as car radiators and water pipes, is to cool the object more quickly to the current air temperature. The inanimate object will not cool below the actual air temperature. If the temperature outside is -5°F and the wind chill is -31°F, the car radiator in the lot will not drop lower than -5°F.

This has a practical consequence in the parking lot at the end of the day. Your skis and boots will be cold, but not colder than the air. The windshield will frost at the air temperature, not at the wind chill figure. What the wind chill governs is you, standing in the lot in a base layer and a shell, loading a roof box.

Does humidity or a lake change the number?

When the National Weather Service tested the index, researchers applied it to 12 test subjects. The results showed that relative humidity was an insignificant weather parameter, affecting the outcome by less than 1 degree, so it was left out of the calculation to keep it simple. That is worth knowing in the Northeast, where a damp cold is often described as feeling worse than a dry one. The index does not carry humidity.

Being near a large body of water does not enter the formula either. The movement of air across exposed skin, not the moisture in it, is what the chart describes.

What about children and older skiers?

The tests that were done on wind chill were conducted on adult subjects. For legal and safety reasons, the National Weather Service did not ask for child volunteers. The recommendation that follows is to use the existing chart as a starting point and be even more cautious with children, the elderly, and persons with compromised health. A race league parent watching a junior group on a windy afternoon is working from an index that was never validated on that group. The chart is a floor, not a ceiling, for caution.

When warnings and advisories are issued

Criteria for issuing official National Weather Service Extreme Cold Warnings and Cold Weather Advisories are set locally. In the Rochester, New York area, Extreme Cold Warnings are issued when the apparent temperature, with or without wind, is expected to fall at or below -25°F. Cold Weather Advisories are issued when the air temperature or wind chill temperature is expected to fall between -15°F and -24°F.

Because the thresholds are local, a club running trips to several ranges should know which forecast office covers each destination. A -20°F morning that triggers an advisory in one part of the Northeast may sit under a different headline elsewhere. Cold is dangerous with or without wind, and the advisory language is the operation's signal that the day needs a plan.

How to read the numbers before you ride

Start with the air temperature, because that is what determines frostbite and what your wax, your layers and your car are dealing with. Then layer the wind chill on top of it. A 10°F day with a 25 mph summit wind is a different day from a 10°F day with a 5 mph breeze, even though the base lodge reads the same.

Two habits follow from the chart. First, cover the face when the wind is running, because the index is a face model and the face is where it bites. Second, shorten the ride rather than the layers: a lift that takes ten minutes is ten minutes of exposed skin, and the chart's 30-minute frostbite figure at -19°F is measured on skin that is exposed and still. A club that talks about the wind on the ride up is doing the simple version of what the index does with numbers.

Weather Ready Nation information from the National Weather Service, including the full chart and the calculator the agency offers alongside it, sits behind the page linked above. The Centers for Disease Control and Prevention publishes additional cold-related health safety guidance for anyone planning a day on the hill.