Physics Dopers, I turn to thee. I alledge ( and, must defend myself against a friend who claims I am insane in this ) that there is a wind chill effect against other things besides skin.
It’s 0 here. It’s -9 with a wind chill. I say that if I am sitting in my car and NOT driving, that the heat will shed off of the windshield ( and, all car windows ) at a given rate. That is to say, I lose heat because glass is a heatsink.
Now, if I am going 60mph, I say that I will lose heat FASTER because 0 degrees air is flowing over the window surfaces at 60mph, removing the heat from the surface of the glass FASTER than it would were I still sitting in my driveway.
Is this true? Does my car get colder faster ( given a constant rate of heat flow from the air system in my car ) if I am driving 60 mph through icy cold air, instead of sitting still? I say that this is a Wind Chill effect- the moving air alters the heat loss rate.
Am I right, or wrong? Please, someone who gets this kind of thing, give me the Straight Dope?
Cartooniverse
p.s. It really is going below zero here tonight, this is hardly a hypothetical query.
I don’t have a definitive answer, but at a first glance it would seem to me that there would be the friction of the air moving over the car to consider. Think of the space shuttle glowing red hot as it comes in to land…
I think you’re essentially correct, but remember that, if it’s 0 degrees, and the windchill value is -9 degrees (either because of wind or because you are driving at a speed that yields that windchill value), your windshield will not get colder than 0 degrees. It should bleed heat out of the interior of the car faster, however, than it would in still air. And I don’t think the car would be moving fast enough for friction to produce any counteracting heating effect.
The wind chill is the added effect of the wind (together with the temperature) on exposed skin. If you ain’t talking about skin, it ain’t wind chill. That is not to say that other objects would get colder if they are moving, due to conduction and evaporation. So you may be correct in your conclusion, but that is not due to the wind chill.
This is one of the things that I hate about driving in winter. When it’s that cold and I’ve gotten my car all nice and warm and defrosted, I drive a few blocks to the freeway. On my way down the onramp, my windshield will first fog up and then ice over, so just as I hit freeway speed, I’ve got zero visibility and the defrosters are like, “Dude, that’s ice. Give us a couple minutes, or stop or something.”
There are three mechanisms for heat transfer: conduction, convection and radiation. Convection is the dominant mechanism at work when cold air moves across an object. When your car is standing still, you will lose heat through all conductive surfaces (but predominantly the glass since it’s less insulated than the doors, etc.) and there will be convection from bouyancy. That is, the window heats the air, the warmer air rises and is replaced by cold air, inducing a flow in the nominally still air.
When your car starts to move, convective transfer will go way up. I believe it’s proportional to velocity, but I don’t have a textbook handy. This is exactly the same effect that the windchill factor is supposed to measure: windchill is supposed to indicate that because of the increased convective heat loss due to the wind, the temperature+wind is equivalent to a colder temperature in still air. The effect of windchill is not limited to exposed skin only, but it is limited to things that are warmer than ambient temp. As Early Out points out, wind chill won’t cool a body below ambient temp (without the windchill factor) so things like rocks and trees aren’t affected. It’s only things that are warmer and can lose heat via convection.
Frictional heating at the velocities you’re driving your car are negligible.
Yes. When the car is sitting in still air, the air closest to the car heats up and provides some insulation. Moving air takes away heat by conductivity much better. That is why you have a fan to move air across the radiator of the car’s motor.
Evaporation isn’t really a heat transfer mechanism in the fundamental sense. The moisture on your skin cools because of the latent heat of vaporization and this cools your skin via conduction. Feel free to group them as convenient, but I’d tend to say evaporation isn’t really as fundamental as the others because you’d break it down into two different processes to calculate the effects. That is, there’s one set of equations for things like convection, but I’d break evaporative cooling into a temperature change in the fluid and a conductive transport from the skin. Of course, just discussing it might get us kicked to GD and drag in the crowd that likes to argue that convection isn’t fundamental either since it’s just conduction to a boundary layer.
True story: I have a friend who once asked what the big deal was about the “windshield” factor. She could not understand why anyone cared what the temperature was on a car’s windshield. When I explained that the term was actually “wind chill” factor, she could have died from embarassment. That’s why I always say, there are no such thing as stupid questions, only stupid people.
Evaporation is a form of cooling, not a form of heat transfer. One of the key differences is that there is a mass transfer in evaporation.
As to the OP: Cartooniverse, you are correct. All things, live or non-live, are affected by wind chill. Your car does lose its heat much, much faster as the wind speed (or air speed) increases. It will not be cooled below the actual still air temperature, but it will reach that temperature much faster.
Oh gosh. I really did NOT mean to use ‘wind chill’ here. I know that it only applies to living flesh, the feeling of evaporation, wind, cold, etc.
I used it imprecisely and while this ensuing conversation is HIGHLY educational, I apologize for its misuse. I meant to ask if the heat is shed FASTER if I am moving FASTER, opposed to standing still.
In this regard, we’re making some headway. :: rimshot::
Following the logic presented so far, does it then follow that if I have to use my windshield washer fluid on the highway, and it’s bitter cold out, I ought to A) Put on the defrosters on HIGH, and B) Pull over to the shoulder whilst using the fluids, so that it will work as needed, and not literally frost over on the windshield surface?
You see, I believe that the fluid - while not alive - is freezing due to it’s rapid evaporation rate, at 60mph. Would I do better to pull over and have nominally no wind on the glass when using the windshield wipers and fluids?
I know, I’m partially hijacking my own thread here but it’s a related issue.
Just to pile on, that is also why there is a fan behind the radiator. If the radiator is not shedding enough heat (on a hot day waiting at a stoplight, for example), the fan will turn on to help cool the radiator.
I’ve observed the same and believe you to be correct. This combination of heat loss to convection with the evaporation does freeze the windshield wiper stuff.
I also think that the alcohol in the fluid on the windshield evaporates faster, leaving a more water-rich mixture which freezes at a higher temp than what is advertised on the bottle.
Simple Explanation: Wind Chill is a layman explanation for what is better known as forced convection. In a sense, you could say inanimate objects experience wind chill, too. However, it is more correctly identified as (and calculated as ) forced convection.
Indeed, a warmer surface can never obtain a temp less than the cooling fluid’s temp! Wind chill is just a pseudo-relationship to say what it “feels like” to us humans. However, the truth is, exposed skin simply loses body heat faster and faster as wind speed increases…assuming the wind temp is held constant. The rate of the heat loss increases with wind speed, but the final temp of the exposed skin cannot drop below the temp of the wind itself. Of course, unexposed skin loses body heat, too…but at slower rates depending on the thickness of clothing, and such.
Yojimboguy is correct. It IS a form of heat transfer. Granted, temperature remains constant during a phase change because you are removing latent heat to cause evaporation. Still, this IS a form of heat transfer. It is a special case of heat transfer where the general governing equations need some adjustment since
dT = 0 during a phase change. (In more typical heat transfer problems, dT <> 0.) Also, if you want to get picky, the problem also requires you to account for the mass transfer… - Jinx
The context of the previous post was lost in translation. Please note it was addressing the issue about how evaporation is a form of cooling, but it was disputed by Anthracite as to whether or not it is a form of heat transfer.
(Sorry, Yojimboguy, my clarifying post was to help the OP!)
