How Do You Calculate the Wind Chill?

iStock/HelenL100
iStock/HelenL100

What does it really mean when my weatherman says that it feels like minus-20 in Chicago? Is there a wind chill thermometer somewhere, or is he just using a mathematical formula? Let's answer these and some of the other pressing questions about the ubiquitous winter statistic.

Why does wind make us feel cold?

When the wind blows across the exposed surface of our skin, it draws heat away from our bodies. When the wind picks up speed, it draws more heat away, so if your skin is exposed to the wind, your body will cool more quickly than it would have on a still day.

Who came up with the idea of calculating wind chills?

American explorer and geographer Paul Siple and his fellow explorer Charles Passel made the first breakthroughs in wind chill research while on an expedition in the Antarctic in 1940. Siple and Passel suspended bottles of water outside a hut at their base station and measured how long it took the water to freeze under various wind conditions. After taking hundreds of these readings, the pair had a good idea of how rapidly heat was lost at different wind speeds.

What exactly is a wind chill temperature?

When Siple and Passel did their research, they weren't really trying to develop a temperature equivalent that alarmist weathermen could trot out. In fact, their original measure expressed the heat loss in a more esoteric unit: watts per square meter.

The idea of expressing wind chills in terms of an equivalent temperature—the "feels like" language we hear on the news—didn't start until the 1970s. Before the switch, weathermen would report the wind chill in three- or four-digit numbers which were a bit difficult for viewers to wrap their heads around. American weathermen started translating wind chills into temperature equivalents in order to give viewers a more familiar term.

If the air temperature is 40 degrees but the wind chill is 28 degrees, will water freeze?

Nope. Although high winds can make those of us with a pulse feel more miserable than normal, they don't have the same effect on inanimate objects. Lower wind chills mean that inanimate objects cool to the air temperature more quickly, but even high winds can't force the object's temperature below the air temperature. That means that in the above example even though your skin might think it's 28 degrees, your water pipes will still be a balmy 40 degrees.

So is there an actual formula for wind chill?

You bet. Just in case you ever find yourself with a calculator, thermometer, and anemometer but without access to The Weather Channel, the Fahrenheit version of the equation looks like this:

Wind Chill = 35.74 + 0.6215T – 35.75(V^0.16) + 0.4275T(V^0.16)

T is the air temperature in degrees Fahrenheit, and V is the wind speed in miles per hour.

Wait, shouldn't how cold the wind makes you feel depend on all sorts of variables like your body type, and whether you're walking into the wind?

Those things certainly affect how quickly the wind cools a person's skin. The above formula makes some simplifying assumptions to get its numbers. Basically, the wind chill factor you hear reported assumes that your exposed face is roughly five feet off the ground, it's night, and you're walking directly into the wind in an open field at a clip of about 3 mph.

Are those conditions really all that realistic?

If you're in a profession that involves a lot of night-field-walking, sure. Otherwise, maybe not. Critics of wind chill reports note that lots of factors can mitigate the reported wind chill. Your weatherman may tell you that it feels like 50 below outside, but if you're dressed warmly, standing in the sun, or in an area with cover like buildings and trees that block the wind, you will feel significantly warmer.

So if the formula is arguably somewhat dubious, can we just disregard wind chill reports as frivolous statistics?

Not so fast. While the methodology concerning wind chill calculations is still being debated in some quarters, that doesn't mean that the measurements are altogether useless. Remember, the basic concept behind wind chill is that stronger winds will cause exposed skin to cool more quickly. The faster skin cools, the faster frostbite can set it. As wind chills drop south of -50 or so, the onset of frostbite can take as little as five minutes, so it's worth keeping an eye on the wind chill even if the notion of your skin "feeling like" a certain temperature may be a bit misleading.

Has the formula always been the same?

The above formula is actually a fairly new development that the National Weather Service introduced in late 2001. During the year 2000, the National Weather Service and its Canadian counterpart had independently started looking for ways to improve the wind chill formula, partially because they had a sneaking suspicion the old formula overstated just how cold it was. This overstatement may sound innocuous, but the weather services worried that it would lull citizens into a false sense of security if it led people to believe they could withstand colder temperatures than they realistically could.

Since the collaborating weather services knew that the old wind chill formula was broken—"feels like -50" actually felt much warmer than standing around on a windless 50-below day—they recruited a group of volunteers to walk on treadmills in a refrigerated wind tunnel. Using sensors on the subjects' skin, scientists calculated a more accurate formula. You can read one of the test subjects' thoughts on the experiments here.

What's the lowest wind chill ever recorded?

With all of the tweaks in the formula over the years, it's tough to say definitively, but how's this for chilly: on July 4, 2003, a remote weather station in east Antarctica picked up a minus-94 degree day. That would be plenty frigid on its own, but the wind was also blowing at 75 miles per hour, which would be good for a wind chill of about minus-150.

This post originally appeared in 2010.

Denver's Temperature Dropped a Record 64 Degrees In 24 Hours

Leonid Ikan/iStock via Getty Images
Leonid Ikan/iStock via Getty Images

One sure sign summer is over: On Wednesday, residents of Denver, Colorado were experiencing a comfortable 82-degree day. Just before midnight, the temperature dropped to 29 degrees. Between Wednesday and Thursday afternoon, the Denver airport recorded a differential of 79 degrees down to 24 degrees. At one point on Wednesday, a staggering 45-degree drop was seen in the span of just three hours.

All told, a one-day span saw a 64-degree change in temperature, from a high of 83 to a low of 19, a record for the state in the month of October and just two degrees shy of matching Denver’s all-time record drop of 66 degrees on January 25, 1872. On that date, the temperature plummeted from 46 degrees to -20 degrees.

Back to 2019: Citizens tried their best to cope with the jarring transition in their environment, to mixed success. On Wednesday, the city’s Washington Park was full of joggers and shorts-wearing outdoor enthusiasts. Thursday, only the most devoted runners were out, bundled up against the frigid weather.

The cold snap also brought with it some freezing drizzle which prompted several vehicular accidents, including 200 reported during Thursday's morning commute. It’s expected to warm up some in the coming days, but residents shouldn't get too comfortable: Melting ice could lead to potholes.

[h/t KRDO]

Fall Foliage Is Running Late This Year

Free art director/iStock via Getty Images
Free art director/iStock via Getty Images

The August arrival of the pumpkin spice latte might have you feeling like fall is in full swing already, but plants aren’t quite so impressionable. According to Travel + Leisure, the best fall foliage could be coming a little later than usual this year.

Historically, the vibrant transformation starts to sweep through northern regions of the Rocky Mountains, Minnesota, and New England in mid-September, and reaches its peak by the end of the month. Other areas, including the Appalachians and Midwest states, don’t see the brightest autumn leaves until early or mid-October. The Weather Channel reports that this year, however, the forecast from the National Oceanic and Atmospheric Administration predicts unseasonably warm temperatures for the next two weeks, which could impede the color-changing process.

Warm temperatures aren’t necessarily bad for fall foliage, as long as they occur during the day and are offset by cool nights. Since meteorologists don’t expect the overnight temperatures to drop off yet, plants will likely continue producing enough chlorophyll to keep their leaves green in the coming days.

The good news is that this year’s fall foliage should only be about a week late, and meteorologist David Epstein thinks that when leaves do start to change color, we’re in for an especially beautiful treat. If the current weather forecast holds, he told Boston.com, we'll "see a longer season than last year, we’d see a more vibrant season than last year, and it would come on a little earlier than last year, which was so late.”

Though poor weather conditions like early snow, heavy rain, drought, or strong winds can cause leaves to fall prematurely, most trees right now are in a good position to deliver a brilliant display of color after a healthy, rain-filled summer.

Find out when you’ll experience peak fall foliage in your area with this interactive map.

[h/t Travel + Leisure]

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