❄️ Wind chill
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How cold the wind makes it feel on exposed skin — the combination of low temperature and wind speed that drives heat loss.
Wind chill describes how much colder the air feels on exposed skin when wind is taken into account. Moving air carries heat away from your body far faster than still air, so a modest sub-zero temperature in a strong wind can feel bitterly, even dangerously, cold. Wind chill is expressed as an equivalent temperature — the still-air temperature that would feel the same.
Your skin warms a thin boundary layer of air right against it. In still conditions that warm layer insulates you. Wind continually blows it away and replaces it with cold air, so your body must keep reheating fresh air and loses heat much faster. The stronger the wind, the faster this heat loss — though the effect levels off at very high wind speeds.
Modern wind chill is calculated from the air temperature and the wind speed measured at the standard height, using a formula adopted by North American weather services in 2001 based on how fast a human face loses heat. It applies only at or below about 10°C and with appreciable wind. Below freezing, the numbers become important for frostbite risk.
Frostbite occurs when skin and the tissue beneath it freeze. The colder the wind chill, the faster it sets in: around -27°C wind chill, exposed skin can freeze in about 30 minutes; near -40°C it can happen in 10 minutes or less; in extreme conditions, in just a couple of minutes. Fingers, toes, ears, nose and cheeks are most at risk.
Wind chill describes heat loss from a warm object. A person or animal generates heat, so wind accelerates their cooling and they feel colder. An inanimate object like a car or a thermometer will not drop below the actual air temperature no matter how hard the wind blows — it simply reaches that temperature faster.
No. Wind chill only describes how fast warm objects lose heat. A car, pipe or thermometer cannot get colder than the actual air temperature, though wind makes it cool to that temperature faster.
Below about -27°C, exposed skin can freeze within 30 minutes. Near -40°C it can take 10 minutes or less, and even faster in extreme conditions.
Wind chill is only meaningful when it is cold, generally at or below 10°C. In warm weather, wind is described instead through the heat index and how it affects sweating.
No. It is an equivalent 'feels-like' temperature for exposed skin. The true air temperature is higher; wind chill expresses how cold it feels because of heat loss.