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Is "Wind Chill" Real?


Ken H.

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I’ve always struggled to understand any sound science behind the concepts of wind chill factor and heat index. I understand that moving air will dissipate heat away from something that is generating heat (a human) faster that still air, but, “feels like” ??? How can you quantify a feeling? To me it’s all weather reporting hype for the sake of melodramatic numbers.

 

Once again I got into a go round and round about the need to freeze protect a vehicle down to the wind chill temperature. I maintain that while the wind will cause a warm vehicle, or any inanimate object, to cool faster it can’t cause it to cool further. I.e. - sink below the actual temp. Of course the other guy is predicting dire results if my antifreeze isn’t good to -60 or more, when the actual temp never sinks below -40 or whatever.

 

So who’s full of ice?

 

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Wind chill is not going to cool an object any lower then the temperature of the medium (not including evaporative cooling).

However as you know, it will cool it faster.

 

Here is an explanation of wind chill.

 

Edit: Wind chill factor and heat index only apply to animate objects.

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I’ve always struggled to understand any sound science behind the concepts of wind chill factor and heat index. I understand that moving air will dissipate heat away from something that is generating heat (a human) faster that still air, but, “feels like” ??? How can you quantify a feeling? To me it’s all weather reporting hype for the sake of melodramatic numbers.

 

The way it made sense to me is this: The temperature is T degrees, the wind is blowing at W mph (or kph if you prefer). Now, if the local news reports "T", they'll get people calling in saying that it feels colder than that. And they'll be right; it does feel colder because of the wind. How much colder? Well, in still air, it would have to be X degrees to feel as cold as it actually does with the wind chill. That difference between T and X is the wind chill factor. It's not completely made up and it gives the weatherman a little more cover.

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Edit: Wind chill factor and heat index only apply to animate objects.

 

Wind chill applies to exposed human flesh. Specifically, the human face. And apparently, we can blame the Canadians for it.

 

Human skin freezes faster when (a) it's colder or (b) wind is blowing on it. Thus, more wind has the equivalent effect of more cold. That's wind chill. It is real, but it's probably not as precise as the wind chill tables make it out to be.

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As usual, you are correct. It can not get colder than the actual air temperature.

 

Now for the "feels like" business. In a couple more months, you will be telling us you can "feel" the temperature w/o out a thermometer. :grin: And, like all Canadians, will be darn proud that it was only -38 F, but it felt like -60 F.

 

A curse on your cold Canadian air. :wave::rofl:

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Joe Frickin' Friday
I understand that moving air will dissipate heat away from something that is generating heat (a human) faster that still air, but, feels like ??? How can you quantify a feeling? To me its all weather reporting hype for the sake of melodramatic numbers.

 

"Feels like" refers, I believe, to how quickly you can develop frostbite on exposed skin, and is the same as the wind chill temperature calculation, e.g. whatever the actual air temp is, if the wind chill temp is -10F, the weather guessers will report "feels like -10F," and your skin will develop frostbike as quickly as if you were standing out in breezeless conditions with an ambient (non-windchill) temp of -10F.

 

The frustrating thing for me is that they (the weather guessers) obsess over wind chill temps to the point that it's sometimes hard to find the ACTUAL temp - which is what I need to know so I can dress appropriately for sitting in my car, inside which the air ain't moving real quick.

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I maintain that while the wind will cause a warm vehicle, or any inanimate object, to cool faster it can’t cause it to cool further. I.e. - sink below the actual temp. Of course the other guy is predicting dire results if my antifreeze isn’t good to -60 or more, when the actual temp never sinks below -40 or whatever.

 

 

I can't help with the scientific part,but I have seen a lot of car radiators freeze to slush when moving,then overheat,when the antifreeze didn't have the 20 degree buffer.Almost always on a cold engine.

 

It sure feels colder when you put your bare hand in front of the radiator while the engines running.

 

 

 

 

 

The rule of thumb(for the last 50 years)is to have the radiator protected to -20 degrees below the lowest temp expected.

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I maintain that while the wind will cause a warm vehicle, or any inanimate object, to cool faster it can’t cause it to cool further. I.e. - sink below the actual temp.

No one ever said that it did. The concept of wind chill only applies to living things in that moving air will draw heat from your body much more rapidly, increasing negative physiological effects such as hypothermia and frostbite with increasing wind speed. Those effects is very real and probably worth understanding if you live in Canada... :grin:

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....Of course the other guy is predicting dire results if my antifreeze isn’t good to -60 or more, when the actual temp never sinks below -40 or whatever.

 

Well, it looks like you've got your answer(s), so I won't add what would be my redundant 2 cents worth...

 

...but this thread inspires my to wonder if a thread reviewing the most flagrant misunderstands of science, and physics in particular, that we've seen others demonstrate might not be fun and enlightening?

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Tee's heat transfer 101

 

The temperature of an object is a function of the heat it exchanges through one or more of the three modes of heat transfer:

 

1. Conductive - heat loss and gain through direct contact with an object. Dependent on temperature differential, mass and material thermal properties. This is how your grip and seat heaters work.

 

2. Convective - heat loss and gain through contact with a fluid (liquid or gas.) Dependent on factors too numerous to list but fluid velocity is one of the primary ones. This is your wind chill, and why your radiator has a thermostat to slow the circulation rate in cold weather.

 

3. Radiative - heat loss or gain directly through the difference in object temperatures. Varies exponentially to the fourth degree of the temperature differential of two objects. This is why it is so cold on a clear night (the night sky is near zero degrees kelvin) and not so cold on a cloudy day during Winter. This is also how your heat lamp works.

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Joe Frickin' Friday
...but this thread inspires my to wonder if a thread reviewing the most flagrant misunderstands of science, and physics in particular, that we've seen others demonstrate might not be fun and enlightening?

 

That describes a large part of my participation on this site. It's a never-ending battle...

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That describes a large part of my participation on this site. It's a never-ending battle...

 

Yep, some of your and Ed's contri's (nitrogen in tires always seems to be a favorite) in particular came to mind when I was thinking about this.

 

I always learn something from those :thumbsup:

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I maintain that while the wind will cause a warm vehicle, or any inanimate object, to cool faster it can’t ause it to cool further. I.e. - sink below the actual temp.

 

 

I think you are partly right,a bit of poking around on a automotive theory form indicates as long as the radiator surface area is dry,the temp will not sink any lower than the actual air temp,however if there is any moisture on the metal,the evaporative cooling from air passing over it can remove additional heat from the metal,causing it to be lower than the air temp.

 

 

might be a good idea to stick with the -20 margin.Probably won't hard freeze at -40 but usually the antifreeze turns to slush that blocks the coolant flow.

 

 

 

 

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might be a good idea to stick with the -20 margin.Probably won't hard freeze at -40 but usually the antifreeze turns to slush that blocks the coolant flow.

Makes sense from a margin of error perspective. But isn’t that just compensating for an error in the actual protection of the mixture? E.g. – The mixture is suppose to/thought to not freeze until -30 but actually does at -20. And has nothing to do specifically with wind chill?

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I’ve always struggled to understand any sound science behind the concepts of wind chill factor and heat index. I understand that moving air will dissipate heat away from something that is generating heat (a human) faster that still air, but, “feels like” ??? How can you quantify a feeling? To me it’s all weather reporting hype for the sake of melodramatic numbers.

That's an odd position for Mr. Perception-is-Everything to take! ;)

 

When I was on the pro patrol at Bridger Bowl, we treated a lot more frostbite when the wind was whippin'. It made work a lot more miserable for us too, especially when doing avalanche control on exposed ridges before sunrise. So yeah, it's real.

 

As far as quantifying it is concerned, I believe the formula is based on how much more quickly water will freeze, and since we're basically water bags, this affects us in a measurable AND perceptible way.

 

I had a less scientific, but nonetheless predictive model for determining when wind chill became critical, without knowing the exact temps or wind speed: If the snow didn't make much noise when you walked on it with ski boots, you could tolerate freezing temps, even under a steady breeze, for a long time without worrying about frostbite. If the snow made a crunching sound, you could tolerate it for periods up to an hour or so. If the snow made a squeaking noise, frostbite could occur within minutes.

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Great article! Goes along with what I was thinking, but at a more detailed level by revealing the flaws in the formulas used; that both are pseudoscience. At best.

 

Actually, I thought the linked article was somewhat lunatic-fringe anti-scientific:

 

I do not dispute the fact that the feeling of cold is sharper when there is wind or that we have the feeling that it is hotter during the summer when the moisture is high. My point of contention is in the invention of a quantified formula to describe this reality. Science does not have an answer for everything and creating a formula that quantifies a "feeling" illustrates the dressing-up of science for a goal which is unsuitable for it and which it will never belong to. A feeling is specific to each individual. Science studies REPRODUCIBLE phenomena, this can not be applied, by definition, to a feeling. . . .

 

It is based on a model of the human face, and incorporates modern heat transfer theory, that is, the theory of how much heat is lost by the body to its surroundings during cold and windy days. The model of the human face, how, when and on who was it modeled? Is it possible to base a measurement on something as variable as the morphology of a human face? What happens for those who proudly wear a beard?

 

It's a line of argument that's used by anti-scientific types as well as Intelligent Design advocates: Science can't be exact about X, therefore it's useless.

 

At some level of abstraction, all science is an approximate model of reality. Newtonian physics is an model that fails at quantum dimensions and near-light speeds, but it works real well on your local pool table.

 

There is a physically verifiable and reproducible effect: wind causes a "chilling" effect on human skin. There's a scientific explanation for it:

 

There is a thermal boundary layer surrounding the skin which may be several millimetres thick. This boundary layer acts as an insulator. When it is cold and the wind is blowing, the air feels colder than it does when it is calm because the wind blows away the boundary layer. In a perfect calm, if free convection could be suppressed (as it is in microgravity), the boundary layer would be infinitely thick. Add a wind, and the only still air that remains would be the air in the immediate vicinity of some surface, like the skin. The stronger the wind, the thinner the layer. Because the outer layers of still air are blown off more easily than the ones closer to the skin, when it is nearly calm, a small increase in wind speed causes a much greater thinning of the boundary layer thickness than the same increase in wind speed when the wind is already strong.

 

Convective heat loss is really conduction through an insulating boundary layer. The insulation of the boundary layer depends on its thickness. When there is wind, the thermal resistance of the boundary layer is smaller, the heat loss is higher, and the temperature of the skin is closer to the air temperature. Humans do not sense the temperature of the air but the temperature of the skin. Because skin temperature is closer to the air temperature when it is windy, the wind causes it to feel colder.

 

Now, we could report wind chill in kilocalories per square meter per hour like the Canadians, but no one would understand that except Mitch. So we create a model of "equivalent temperature" to relate skin heat loss to ambient temperature and wind velocity. It is, of course, an inexact model, but so what? Does the variation in heat loss if you're fat or skinny or have a beard really matter? Would it really be more logical to base wind chill on "whole body cooling . . . while naked"? The fact that the wind chill index has built-in inaccuracies and ambiguities doesn't mean it's pseudo-scientific or reduce its usefulness. If we wanted a super-accurate wind chill index, we could come up with a formula that incorporates relative humidity, body mass index, core temperature, metabolic rate, skin moisture content, instantaneous wind speed at a specific location, and a whole bunch of other variables. And we would spend hours every morning taking measurements and doing calculations so we'd know whether to wear a scarf or not. Instead, we have a more approximate, but more useful, function based on two variables.

 

Both of which are approximations anyway. Why does the NWS tell me the temperature and wind speed at the airport at the top of the hour when I don't live at the airport and it's half-past the hour? What good is the wind chill number when it's not even based on the weather where I am? :eek:

 

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At some level of abstraction, all science is an approximate model of reality. Newtonian physics is an model that fails at quantum dimensions and near-light speeds,

Ah! You can't explain everything! So you admit that there must be a god... :grin:

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Joe Frickin' Friday
Now, we could report wind chill in kilocalories per square meter per hour like the Canadians, but no one would understand that except Mitch.

 

Thanks for getting the correct dimensions for heat flux. :Cool:

 

The fact that the wind chill index has built-in inaccuracies and ambiguities doesn't mean it's pseudo-scientific or reduce its usefulness.

 

I won't claim it's entirely useless - but I sure wish the weather folks wouldn't gloss over the actual air temperature. As I've mentioned earlier, sometimes that information is more useful than the windchill.

 

Tonight on the news they were citing super cold temps in the northern midwest states. The voiceover mentioned an actual temp of -30F somewhere in ND, and a windchill of -33F somewhere further south; at the same time the onscreen map showed simply -30 and -33 at those locations, no mention of windchill or actual.

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This made me chuckle. I am notorious at my house for my comments that "I don't believe in windchill" which my daughters think is a nonsense statement. I say it because:

 

1 I got tired of saying how when I was their age I had to walk to and from school in -40 weather uphill both ways against the wind and that they are softies

 

2 I don't go outside in winter in Alberta in flipflops and t shirts so the wind doesn't really get through my parka

 

3 Most times the radio reported windchill is way too low - often it is not even windy at all in my neighborhood when they quote a windchill 10C below the actual temperature

 

As a scientific aside - when I was a lad in the 60's the weather reports would talk about a temperature wind chill index 10-20F below the actual temperature. Then about 15 years ago in a metric craze the Canadian weather reports started to say "... the wind chill is 278 Watts/sq meter which will freeze exposed skin in less than 3.8 minutes.". This of course was a meaningless statistic that no one understood. At least now they have gone back to quoting a temperature based wind chill.

 

I just wish they would give it up entirely as people now assume it tells them everything they need to know. I'd rather take a walk outside to find out how cold it is - the red wine and the fireplace are always more appealing afterward.

 

 

 

 

 

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Well I just got back last night and the temps were in the 20's.

At 140 mph it was colder than sitting in a heated room.

Must be sumptin to it.

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I maintain that while the wind will cause a warm vehicle, or any inanimate object, to cool faster it can’t ause it to cool further. I.e. - sink below the actual temp.

 

 

I think you are partly right,a bit of poking around on a automotive theory form indicates as long as the radiator surface area is dry,the temp will not sink any lower than the actual air temp,however if there is any moisture on the metal,the evaporative cooling from air passing over it can remove additional heat from the metal,causing it to be lower than the air temp.

 

 

might be a good idea to stick with the -20 margin.Probably won't hard freeze at -40 but usually the antifreeze turns to slush that blocks the coolant flow.

 

 

 

 

Actually, the ice formed by the -40 temps would prohibit evaporate :grin: cooling.

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