The truth about “wind chill”: Does it even really exist?

Weather forecasters love to display those terrifyingly low numbers — but is "wind chill" all hype?

Topics: Scientific American, polar vortex, wind chill, low temperatures, Science, Weather, ,

This article was originally published by Scientific American.

Scientific AmericanThe infamous polar vortex has put the U.S. and Canada in a deep freeze several times already this winter. Alarmed weather forecasters are now routinely displaying big maps that show the extremely low wind chill values: –34 degrees Fahrenheit in Minneapolis, –36 degrees F in Chicago, –39 degrees F in Fargo, N.D., last night alone. But if the air temperature is, say, 15 degrees F, and a 20–mile per hour wind makes the wind chill –2 degrees F, would the temperature of your exposed skin drop to that temperature?

No. Your skin temperature cannot drop below the actual air temperature. The coldest your uncovered face could get would be 15 degrees F whether the wind is calm or howling at 40 mph.

So what’s the point of wind chill, then? Should we worry about it? Is it deceiving?

Wind chill is a mathematically derived number that approximates how cold your skin feels—not how cold your skin actually is. Thanks to blood in your skin and underlying tissue, your body constantly radiates heat, generating a thin boundary layer of warm air on the surface your skin that helps insulate you from the cold. If you stand still in air that is 20 degrees F and there is no wind, your skin will be warmer than 20 degrees F. Wind carries some of that heat away, however, and the faster the wind, the faster the heat loss. Once the wind surpasses 25 mph or so, it whisks away heat more quickly than your body can emit it, leaving your skin exposed to the full low temperature.

Your nerve endings and brain perceive the rapid drop in skin temperature as extreme, however. Scientists are not sure why this occurs, but they think it is a signal to close down blood vessels in the skin and extremities so more blood can flow to the body’s core, to keep your organs warm and keep you alive—even if you lose a finger or toe to frostbite in the process. Wind chill is all about perception, and the wind chill index is an attempt to gauge that perception.



Different wind chill indexes use different formulas, but in all of them the most important factors are air temperature and wind speed, says Catherine O’Brien, a research physiologist at the U.S. Army Research Institute of Environmental Medicine in Natick, Mass. The National Weather Service wind chill chart uses only those two quantities, and runs them through a model based on the tissue in a prototypical human face as well as rates of heat loss for the body. AccuWeather’s “RealFeel” index adds in effects such as cloud cover and sun angle, but because the formula is patent-protected outside scientists cannot evaluate the math.

If wind chill is not the actual temperature on your skin, why bother reporting it? This is a fair question. Some meteorologists say it would be more useful to report “minutes until frostbite” rather than wind chill values. If viewers know how quickly their exposed skin will freeze, they might be more cautious. And yet, frostbite times don’t provide very useful clues about how much clothing to wear.

The wind chill charts do show generally how long it will take skin to freeze at lower and lower values. For example, a temperature of 0 degree F and a wind of 20 mph creates wind chill of –22 degrees F and skin can freeze in 30 minutes. If the wind rises to 55 mph, the wind chill drops to –32 degrees F, and skin can freeze in 10 minutes. Remember, the only reason skin freezes faster at lower wind chill is because the body’s heat envelope is removed more quickly. Skin doesn’t freeze until its temperature is well below 32 degrees F, because its cells contain salts and other compounds that lower its freezing point below that of water. Exact times vary depending on an individual’s blood flow, fat layers and underlying tissue.

Still don’t believe wind chill temperatures aren’t real? Try an experiment: Put two thermometers outside, one in the wind and one shielded from it. When you return they will read the same. Or just ask yourself a simple question: If you are driving your car at 20 mph and you read the dashboard thermometer, then speed up to 60 mph, does the temperature drop? No. Because the air temperature has not changed. There is no wind chill for your car—even if you have given your vehicle a human name.

Realizing that coldness is all about perception raises other interesting observations. People who have a lot of body fat may actually feel colder than those who do not, despite the notion that fat acts as insulation. More fat under the skin can actually prevent heat, generated in underlying muscles, from reaching the skin, O’Brien says. Because the perception of cold comes mostly from nerves in the skin, as the air temperature drops, people with high body fat might feel colder.

Women may often feel colder than men because they tend to have less muscle mass (less heat generated) and more body fat (blocking heat to the skin), and because their generally smaller size gives them a larger ratio of surface area to mass they tend to lose heat faster. Every person is different, of course.

Whether or not wind chill is a useful number, meteorologists on television and radio are running wild with it. After all, reporting wind chills of –36 and –39 sounds more dramatic than temperatures of 0 and a paltry –2. “It’s getting a little ridiculous now,” O’Brien says. “Sometimes it’s hard to find the actual air temperature.”

Featured Slide Shows

  • Share on Twitter
  • Share on Facebook
  • 1 of 7
  • Close
  • Fullscreen
  • Thumbnails
    AP/Jae C. Hong

    Your summer in extreme weather

    California drought

    Since May, California has faced a historic drought, resulting in the loss of 63 trillion gallons of water. 95.4 percent of the state is now experiencing "severe" drought conditions, which is only a marginal improvement from 97.5 percent last week.

    A recent study published in the journal Science found that the Earth has actually risen about 0.16 inches in the past 18 months because of the extreme loss of groundwater. The drought is particularly devastating for California's enormous agriculture industry and will cost the state $2.2 billion this year, cutting over 17,000 jobs in the process.

       

    Meteorologists blame the drought on a large zone (almost 4 miles high and 2,000 miles long) of high pressure in the atmosphere off the West Coast which blocks Pacific winter storms from reaching land. High pressure zones come and go, but this one has been stationary since December 2012.

    Darin Epperly

    Your summer in extreme weather

    Great Plains tornadoes

    From June 16-18 this year, the Midwest was slammed by a series of four tornadoes, all ranking as category EF4--meaning the winds reached up to 200 miles per hour. An unlucky town called Pilger in Nebraska was hit especially hard, suffering through twin tornadoes, an extreme event that may only occur every few decades. The two that swept through the town killed two people, injured 16 and demolished as many as 50 homes.   

    "It was terribly wide," local resident Marianne Pesotta said to CNN affiliate KETV-TV. "I drove east [to escape]. I could see how bad it was. I had to get out of there."   

    But atmospheric scientist Jeff Weber cautions against connecting these events with climate change. "This is not a climate signal," he said in an interview with NBC News. "This is a meteorological signal."

    AP/Detroit News, David Coates

    Your summer in extreme weather

    Michigan flooding

    On Aug. 11, Detroit's wettest day in 89 years -- with rainfall at 4.57 inches -- resulted in the flooding of at least five major freeways, leading to three deaths, more than 1,000 cars being abandoned on the road and thousands of ruined basements. Gov. Rick Snyder declared it a disaster. It took officials two full days to clear the roads. Weeks later, FEMA is finally set to begin assessing damage.   

    Heavy rainfall events are becoming more and more common, and some scientists have attributed the trend to climate change, since the atmosphere can hold more moisture at higher temperatures. Mashable's Andrew Freedman wrote on the increasing incidence of this type of weather: "This means that storms, from localized thunderstorms to massive hurricanes, have more energy to work with, and are able to wring out greater amounts of rain or snow in heavy bursts. In general, more precipitation is now coming in shorter, heavier bursts compared to a few decades ago, and this is putting strain on urban infrastructure such as sewer systems that are unable to handle such sudden influxes of water."

    AP/The Fresno Bee, Eric Paul Zamora

    Your summer in extreme weather

    Yosemite wildfires

    An extreme wildfire burning near Yosemite National Park forced authorities to evacuate 13,000 nearby residents, while the Madera County sheriff declared a local emergency. The summer has been marked by several wildfires due to California's extreme drought, which causes vegetation to become perfect kindling.   

    Surprisingly, however, firefighters have done an admirable job containing the blazes. According to the L.A. Times, firefighters with the state's Department of Forestry and Fire Protection have fought over 4,000 fires so far in 2014 -- an increase of over 500 fires from the same time in 2013.

    Reuters/Eugene Tanner

    Your summer in extreme weather

    Hawaii hurricanes

    Hurricane Iselle was set to be the first hurricane to make landfall in Hawaii in 22 years. It was downgraded to a tropical storm and didn't end up being nearly as disastrous as it could have been, but it still managed to essentially shut down the entire state for a day, as businesses and residents hunkered down in preparation, with many boarding up their windows to guard against strong gusts. The storm resulted in downed trees, 21,000 people out of power and a number of damaged homes.

    Debbie Arita, a local from the Big Island described her experience: "We could hear the wind howling through the doors. The light poles in the parking lot were bobbing up and down with all the wind and rain."

    Reuters/NASA

    Your summer in extreme weather

    Florida red tide

    A major red tide bloom can reach more than 100 miles along the coast and around 30 miles offshore. Although you can't really see it in the above photo, the effects are devastating for wildlife. This summer, Florida was hit by an enormous, lingering red tide, also known as a harmful algae bloom (HAB), which occurs when algae grow out of control. HABs are toxic to fish, crabs, octopuses and other sea creatures, and this one resulted in the death of thousands of fish. When the HAB gets close enough to shore, it can also have an effect on air quality, making it harder for people to breathe.   

    The HAB is currently closest to land near Pinellas County in the Gulf of Mexico, where it is 5-10 miles offshore.

  • Recent Slide Shows

Comments

Loading Comments...