Wind chill

Discussion in 'General Science & Technology' started by origin, Jan 7, 2014.

1. originTrump is the best argument against a democracy.Valued Senior Member

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This is a nice day to talk about wind chill. It was about -40F wind chill this morning here.

Most of the members here know what the wind chill is but there is sometimes confusion on what exactly wind chill is telling us. Some people will have the mistaken belief that a wind chill of say -25F means that your automobile needs to have antifreeze protection to less than -25F.
Wind chill is actually a heat transfer effect. The simplest way to look at this is to use a simplified heat equation.

$\dot{Q} = \dot{m} C_p (T_{hot} – T_{cold})$

$\dot{Q}$ = heat flow
$\dot{m}$ = mass flow rate (air)
$C_p$ = heat capacity (air)
$T_{hot}$= Person, automobile, etc.
$T_{cold}$ = air temp.

So if you increase $(T_{hot} – T_{cold})$ you increase heat flow
If you increase the $\dot{m}$ you increase the heat flow
If you were to have a fluid with a higher $C_p$ you would increase the heat flow.

One of the limitation of this simplified equation is if the $\dot{m}$ is zero then $\dot{Q}$ is zero (in practice this is never actually true because there will always be some movement of the air at the skin surface due to convection, unless the skin temp is equal to the air temp).

Anyway if there air temp is less than the skin temp you will have heat flow out of the skin. If the air temp is held constant and the mass flow is increased then the heat flow out of your skin will increase just as if you decreased $T_{cold}$. What is actually happening is that the air molecules that have a low Translational Kinetic Energy (TKE) will come in contact with the skin or warm boundary layer of air near the skin and the high TKE of the skin will transfer its energy to the air molecule increasing the TKE of the air and decreasing the TKE of the skin. With a higher mass flow the more air molecules with low TKE will come in contact with the skin removing more energy from the skin.

So what affect does wind chill have on an automobile? If the car engine is warmed up and then turned off on a cold day the engine will cool following the above equation. The higher the wind speed the faster the engine will cool – so a higher wind chill will actually have an effect on an automobile. But the engine will only cool to the point that $T_{hot}$ = $T_{cold}$ . No heat transfer can possible occur if there is no delta T, so it does not matter if the wind is blowing at 60 mph and the wind chill is – 48F the car will never be colder than the air temperature of -5F.

3. Buddha12Valued Senior Member

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None. Wind chill only affects many living things not inanimate objects.

5. andy1033Truth SeekerValued Senior Member

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In the real world its more complex than your equation, no matter what your applying it to.

7. originTrump is the best argument against a democracy.Valued Senior Member

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If you looked at the equation you would know that it does affect things other than just living things. Why do you blow on hot soup? The point is that the wind chill number does not mean that the actual temperature of an object can decrease below the air temperature regardless of the wind speed.

8. originTrump is the best argument against a democracy.Valued Senior Member

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Yes I know, that is why I stated that it was a simplified equation. That does not mean that it is not useful in understanding wind chill.

9. brucepValued Senior Member

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His model is perfectly reasonable for finding the result he's looking for. I'm not to sure you're much of an expert in real world physics. This is real world. When T_hot = T_cold no further heat transfer will occur. During the process of reaching thermal equilibrium the > the wind speed the > the wind chill the faster path to thermal equilibrium. A little more real world 'insulate yourself from these effects when you're going to be exposed'. Wear some more clothes like mom said.

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Cool....

I've just returned from an early morning surf at Maroubra beach, with a temperature of 22C, and water temp of 20C

11. brucepValued Senior Member

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So I found a link for Maroubra beach outside Sydney. The picture they have for the right hander at the beach break looks like backdoor Pipeline. Looks like that could be fun if you don't mind sand packed up your buttcrack.

12. originTrump is the best argument against a democracy.Valued Senior Member

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Well the afternoon sun has raised the temperature here to a whopping -16C and with the wind chill it is about -30C. So as you might guess I hate you.

13. brucepValued Senior Member

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Sounds like you're near the frozen tundra at Lambeau Field in Green Bay Wisconsin?

Last edited: Jan 7, 2014
14. AsguardKiss my dark sideValued Senior Member

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ah to be in the tropics, chilling out escaping the 40C heat in Australia in the topics where it is always around 32C, even the rain feels wonderful here

15. originTrump is the best argument against a democracy.Valued Senior Member

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Nope, upstate NY. We see these temperatures pretty much every year at least once or twice but having the high winds at the same time is just brutal. I'm getting to old for this crap.

16. exchemistValued Senior Member

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…not to mention the risk you run of being choked to death by your thymus gland at such temperatures…..

17. originTrump is the best argument against a democracy.Valued Senior Member

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I hate when that happens...

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19. scorpiusa realistValued Senior Member

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your car will be just as cold as the air temps,assuming the thermo gage on my truck transmision is acurate enough,
doesnt really matter too much anyways,I was down to -32 C when working in Edmonton Alberta and even without pluging in the block heater my Silverado started every time easy...today high energy ignition systems are just totaly awesome unlike way back then..