What is wind chill and how does it work?

http://www.washingtonpost.com/weather/2022/02/22/wind-chill-explainer/

What is wind chill? Here’s how it makes the air feel colder.

Wind chill doesn’t change the temperature — it alters how we feel it

By Matthew Cappucci

February 22, 2022 at 12:27 p.m. EST

If you’re from the Northern Tier, Rockies, Midwest or New England, odds are you’re no stranger to “wind chill.” You probably feel its effects all the time during the winter, when even the lightest stirring or breeze can turn a seasonably cold day downright bone-chilling.

Stronger winds and below-freezing temperatures can bring the risk of frostbite and hypothermia as well, making wind chill an important figure to understand and plan for anytime outdoor recreation is to be considered.

Frigid temperatures and wintry, severe weather to cover much of the U.S. this week

Most television weathercasters and mobile apps alike display values associated with the wind chill, but few take the time to define what it means.

What is wind chill?

The premise of wind chill is simple: When the ambient air is cooler than your body temperature (roughly 98.6 degrees), any stitch of wind will blow away the insulating layer of mildness that forms around you.

What is the polar vortex, and how does it change the weather?

The human body emits heat, which generates a cushion of warmth surrounding a person. The faster the wind, the quicker a person sheds their warm layer, leaving them exposed to the cold once again. The body has to work at a faster pace to combat that constant loss of heat.

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When someone says “it’s 24 degrees outside, but the wind chill is 17,” they’re really saying that, at a temperature of 24 degrees with some wind, the rate of heat loss from an individual to the environment (and what the individual perceives) is equal to what 17 degrees and no wind would feel like.

How is wind chill calculated?

Atmospheric scientists and physicists have crunched the numbers and found a way to quantify this wind-induced dissipation of heat — by treating humans like a power source. We can calculate how much heat a human radiates in units of watts per meter squared.

Watts are units of power, and they can be converted into calories per second. You know the phrase “burning calories”? Yep. Think about expending the energy contained in your food, but as heat!

The “per meter squared” term comes from the surface area of an average human. That makes sense, since we radiate heat out of our skin.

The formula above is used by the National Weather Service and by computer algorithms to calculate wind speed. The constants, or numeric values, come from assumptions mathematicians made about the average dimensions of a human, etc. It’s also presumed that sunlight isn’t playing more than a negligible role in heating a person. Wind chill is defined only for temperatures below 50 degrees and winds over 3 mph.

Wind chill in practice

Let’s assume the temperature is sitting right at the freezing mark. With a sustained 10 mph wind, the wind chill, sometimes advertised as a “feels like temperature,” would be 24 degrees. At 20 mph, that wind chill would drop to 20 degrees. How about 32 degrees amid a full-fledged 50 mph New England blizzard? The wind chill would be a frigid 14 degrees.

According to Weather Underground, the coldest wind chill ever recorded in the United States (utilizing the current wind chill formula first employed in 2001) occurred on Jan. 16, 2004, atop the 6,288-foot summit of Mount Washington in New Hampshire. A temperature of minus-41.8 combined with winds gusting over 87 mph brought a wind chill of minus-102.6 degrees.

How to dress to stay warm when it’s super cold

The Weather Service will issue wind chill advisories and warnings when conditions are dangerous, and there are a few tips that you can follow to be safe: stay dry, stay covered, dress in layers and stay informed with local news and alerts.19Comments

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By Matthew CappucciMatthew Cappucci is a meteorologist for Capital Weather Gang. He earned a B.A. in atmospheric sciences from Harvard University in 2019, and has contributed to The Washington Post since he was 18. He is an avid storm chaser and adventurer, and covers all types of weather, climate science, and astronomy.  Twitter

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