Millions of people in the central and eastern United States are experiencing frigid weather, with wind chill temperatures that are in the single digits in many places. And a storm is expected to bring strong winds to much of the East Coast over the weekend.
Wind chill is a measure of how quickly bodies lose heat when you combine low temperatures with high winds. And wind chill conditions can be dangerous.
"The stronger the winds [and] the colder it is, the more likely you are to develop frostbite in a short amount of time or hypothermia," says Jessica Lee of the National Weather Service's Weather Prediction Center.
Here's how wind chill works and how to protect yourself and those around you.
What is wind chill?
Wind chill is basically how cold the air feels. For example, if the thermometer shows an air temperature of 25 degrees and the wind is blowing at 30 miles per hour, it will feel like it's 8 degrees outside.
That's because wind removes heat from your skin as the air blows over you. The faster the wind is blowing, the faster it removes heat from your body.
"Think of your body as being like a little heater, and you have a thin layer of warm air right against your skin," Lee explains. "When it's windy, the wind is whisking away the insulating layer of warm air and replacing it with colder air, which then makes your body cool down faster."
The National Weather Service publishes a chart that shows how much wind chill to expect when the air temperature is between 40 degrees and minus 45 degrees Fahrenheit, and the wind speeds are between 5 and 60 miles per hour.
How do I know if the wind chill conditions are dangerous?
The chart above includes thresholds for frostbite, which is when the skin freezes.
For example, if the air temperature is 30 degrees Fahrenheit, but there are 50 mile-per-hour winds, the wind chill will be 12 degrees. In those conditions, exposed skin will be frostbitten within 30 minutes.
And when the air temperature is lower, it takes less wind to push conditions into extreme territory. For example, if the air temperature is minus 5 degrees Fahrenheit, it takes 35 mile-per-hour winds to freeze exposed skin in just 10 minutes.
Frostbite can cause permanent damage, and the stages leading up to full-blown frostbite can cause temporary damage to tissues, as well as significant pain and discomfort, according to the Centers for Disease Control and Prevention.
If the air temperature isn't below freezing, you can't get frostbite, but you could still be at risk for hypothermia, which is when the body gets dangerously cold. Hypothermia can be deadly.
Who is most at risk from wind chill?
Wind chill impacts are calculated for adults. Children, who have less body mass and often get cold more easily, are more at risk for hypothermia in very cold conditions.
Elderly people and people with underlying medical conditions may also be at greater risk, as well as people who work or play outside for long periods.
Pets and other animals that venture outside in the cold, such as dogs, horses and livestock, also experience wind chill, although fur and hair help retain heat better than naked skin.
Wind chill also affects objects, although not to the same degree as it affects people and animals. Wind cools off objects more quickly, if they are warmer than the air temperature. For example, if you turn off your car, the engine will cool more quickly if there are stiff winds.
However, wind chill does not cause inanimate objects to get colder than the air around them.
How can I protect myself from wind chill?
"The important things are you want to dress warmly and you want to minimize your time outdoors," Lee says.
Those who do not have reliable shelter should seek a safe indoor space when it is windy and cold.
Wearing warm layers can help keep the body's core temperature in a safe zone, even when the wind is removing heat from the skin. Wearing windproof coats, pants, shoes and head coverings can help prevent hypothermia in wind chill conditions.
And to prevent frostbite, cover all your exposed skin. "This includes your ears, your nose, your fingers, your toes, even your cheeks and your chin," Lee says. "Grab a scarf, grab a hat."
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