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.
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.
Note: This article originally appeared in January.
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Some time ago an acquaintance of mine went into a long explanation of the “windshield factor”!! It was the point when weather was cold enough to ice the windshield. I was flummoxed! Same person refused to accept that he had misheard the term wind chill factor.
posted by Alice V on 1-5-2010 at 11:23 am
Critics of wind chill reports can come visit us here in the lovely upper midwest, Minnesota in particular, and see for themselves how real that wind chill feels.
posted by Anne on 1-5-2010 at 12:33 pm
That picture of the weatherman cracks me up. He’s got a look on his face like, “You caught me, wind chill is BS.”
posted by Not That Guy on 1-5-2010 at 12:35 pm
Based on the idea of wind chill, wouldn’t a high wind on a day with a temp higher than 98.6 f. (say maybe 110) make you more prone to heatstroke than just a hot day alone? This would be a different idea than the heat index that utilizes humidity and temp.
posted by tklu on 1-5-2010 at 12:53 pm
no mention of the photo from the classic Chargers-Bengals playoff game in 1981. Played in reported wind chills of -59 (using the older, less accurate formula no doubt, but still it was pretty cold.
Although you could have gotten away w/ a much more recent photo of Giants coach Tom Coughlan from the playoffs following the 2007 season (played in 2008) when NY played at Green Bay. Coughlan’s face turned so red from the wind it looked like it would turn black next (from frostbite).
link to photo
posted by Jonny on 1-5-2010 at 12:58 pm
I live outside of Fargo, and 2nd Anne’s post. ’15 F’ and ’15 F with a 25 MPH wind’ are completely different worlds.
And for what it’s worth, we hit -33 F (without wind chill) last week. +15 sounds downright balmy.
posted by EV on 1-5-2010 at 1:13 pm
People nitpick about the wind chill not being ‘exact’. Me I don’t care. The point is it’s close enough that it gives me a good indication of what the weather’s like outside.
Similarly, when converting from Celsius to Farenheit and back, I like the Bob & Doug Mackenzie method. Take the value in Celsius, double it, and add 30. (To go the other way take the value in Farenheit, subtract 30 then halve it). Is it perfect? No. But under most conditions it’s surprisingly accurate (to within a few degrees farenheit) , and it’s quick.
The point is both of these things tell you what you really need to know. How you should dress or otherwise prepare for the conditions outside today.
posted by TheBear on 1-5-2010 at 1:14 pm
Call me the 3rd on Anne’s post. I live in Winnipeg, Canada, and let me tell you – it’s called “Winterpeg” for a damn good reason. The windchill sure does suck.
@tklu – high winds on hot days serve to increase the amount of convective heat transfer, which tends to hasten the evaporation of the sweat on your skin. This evaporative effect removes heat from your body and will make you cooler.
And just as a helpful FYI from a Canadian, our national weather service is called Environment Canada.
posted by Bert on 1-5-2010 at 2:14 pm
Just to further elaborate on the first question, wind itself is not what is cooling, but rather, it is the insulating property of air that causes the wind chill effect. In stagnant cold temperatures, the surrounding air actually heats up from your body temperature, which in turn slows down the amount of heat your body releases. This is why wearing a windproof jacket works well– you are trapping heated air inside your jacket so your body doesn’t lose heat as fast. When wind is added to the equation, the heated air is quickly displaced by cool air, and so the rate of heat transfer from your body to the air increases. Also, remember that energy transfers from hot to cold, not from cold to hot.
Good post.
posted by Joel on 1-5-2010 at 2:38 pm
Okay, so if wind chill can not impact the rate at which water freezes (#4), then how did the Siple/Passel work, seeing as how it was based on observations of water freezing in different wind conditions?
posted by EMStoveken on 1-5-2010 at 3:44 pm
EM – if the actual temperature IS below freezing, then the water will freeze faster with wind than without wind because it loses its heat faster. The key to the Siple/Passel experiment would then be that the actual temperature was already below freezing.
Heat transfer between media requires there to be an actual (not perceived) temperature difference between the two media. If the water is at 40 degrees F and the air is at 35 degrees as well, wind will accelerate the drop in water temperature to 35 degrees, but since at that point the media are of equal temperature, no further heat transfer will take place. The water will not drop past 35 degrees and thus will not freeze.
posted by Bert on 1-5-2010 at 5:05 pm
I agree, great post.
@TheBear, thanks for the quick C to F conversion. Never thought of that but will now use when quickly need to estimate.
@Joel, thanks for the further explanation on heat transfer, etc.
Love mental_floss, not just for the articles but as well as for the commentary.
posted by izzi on 1-5-2010 at 5:23 pm
Thanks izzi: It works just as well when the temperature is below zero but the furhter you stray from “normal” temperatures, the less accurate it becomes.
e.g. -40 Celsius would be -40 Farenheit instead of -50. +40 Celsius is only 104 Farenheit, not 110, but so long as you’re not calculating the temperature on the surface of the sun you should do Ok.
posted by TheBear on 1-5-2010 at 6:24 pm
Good post. I got a question for anyone out there. I recently moved to the south and over the last two days I have heard the term “Hard Freeze” thrown around quite a bit. Is a hard freeze different from a normal freeze? Is there a soft freeze? Honestly I have never lived anywhere that gets below the mid 40′s before so if these sounds like a strange questions I’m not trying to be funny I really don’t know.
BTW it was 22 degrees outside when I left for work this morning. I thought I was going to die. I don’t know how you guys do it; I saw that it was -19 in some places in the upper Midwest today, I really don’t know if I could survive that. But then again, that’s what some of the guys at work were saying when I told them that in the summer back home prolonged periods of 115 and higher were not uncommon. I guess it’s all relative.
posted by chris on 1-5-2010 at 6:44 pm
-40 windchill today. Frost on the inside of my windows, yay!
posted by Amanda on 1-5-2010 at 6:59 pm
@chris: I’m in Michigan, the way I’ve learned it, a “hard freeze” is cold enough to freeze plants solid and kill them. The equivalent of “soft freeze” is a frost that can damage plants, but probably won’t kill them. It’s handy information for gardeners. I remember a “hard freeze” froze and burst some pipes when I was kid, too.
I’m sure others out there have different definitions of “hard freeze.”
posted by Cora on 1-5-2010 at 8:20 pm
Nice post, Go Bob and Doug! Beauty,eh?
Had to laugh at the recaptcha when I got to it, “experience methanol.” I have worked for an oilfield service company for the last 20 years, and have had many days in winters in Northern Canada and Northern Russia where the temperature was easily in the -50′s, couldn’t know for sure because the thermometers didn’t usually go below -45. Methanol is used to thaw out frozen locks and parts, and we always had a large jerrycan of this with us. It pours and burns easily when diesel fuel is as thick as jell-o from the cold.
posted by John on 1-6-2010 at 5:34 am
Bert–I don’t bother with the conversion from Celsius to Fahrenheit unless I need exact numbers for some reason. I usually just go with the following rhyme:
30 is hot
20 is nice
10 is cold
0 is ice.
posted by Fruppi on 1-6-2010 at 2:43 pm
Chris, in the south you hear hard freeze more because while you may get several frosts, it is only the hard freeze that really kills plant life off. In the north, freeze is freeze!
posted by Hyacinth on 1-6-2010 at 4:25 pm
@Fruppi, it’s Bear, not Bert and where I come from you’d have to go a lot further:
10 = pleasant
0 = Better put a jacket on
-10 = cold
-20 = DAMN COLD
-30 = DAMN FREEZING COLD!
-40 = I’M NOT GOING OUT THAT DOOR UNLESS YOU DRAG ME COLD!
posted by TheBear on 1-6-2010 at 4:45 pm
@thebear
You forgot:
20 = Floridians put on hats and scarfs
-50 = Canadians put on jackets
posted by DaveP on 12-28-2010 at 12:57 am
all of you guys are crazy for living where you live. i live in southern california and if it gets lower than 68F, i’m freezing. most winters average about 75F. spring,summer, and fall average about 100F.
posted by john q on 12-28-2010 at 4:04 am
John Q, you must live in the desert, because in LA the winters average in the low 60s, and certainly 100F is way too high of an average for summer, certainly way too high for spring and fall.
posted by Yams on 12-28-2010 at 11:47 am
I once heard wind chill described as “temperature with a publicist” – but that wind makes a huge difference.
Next up: the “feels like”, or perceived temperature resulting from the combination of heat and humidity (which sounds really good right now).
posted by PartiallyDeflected on 12-28-2010 at 1:03 pm
@John Q
Heh, That reminded me of when we moved to San Diego back in 91, I’d flown back to Chicago to drive the truck down. When I got in I brought the older boys (then 3 and 5) out to help wash the truck. We were in shorts and t-shirts. The neighbor kids came out to watch – wearing down parkas.
posted by RodT on 12-28-2010 at 1:37 pm
I’m going to disagree that wind chill doesn’t affect inanimate objects. On hot days in the desert I used to soak leather bota bags in water, and then hang them from my trailer hitch in the shade. The *air* temperature was easily pushing 100°, but the water was cold enough to give me an ice cream headache.
posted by Greg Lawhorn on 1-20-2011 at 3:50 pm
That is because the evaporative action creates energy transfer from the wet leather to the open air – the same concept that was used in the old fashionded water coolers, and inside the systems in modern a/c units and heat pumps. the dry air will evaporate, “pulling” the heat energy out of the water inside – in layman’s terms, ask a a/c expert for a more complete expanation. As for the rest of the wind chill, I walked to the store yesterday where in the little village I am living in in Russia, into the wind, at -5* F. It was a much colder experience than the -20 sunshiny day we had last week.
posted by coon hat on 2-2-2012 at 1:58 am