wind-chill factor (WCF) 1

[25362]

I was used to the definition of WCF as the cooling effect of a combination of temperature and wind velocity, expressed as the heat lost by the skin surface exposed to higher convection heat transfer coefficients, depending on the body size, shape and metabolic rate. Re-reading an old thread I saw a possible relation of WCF with the cooling effect by sweat evaporation, which I belief applies to the body's reaction to warm weather. Would the experts clarify this point ?

 

[pmover]

a google search revealed many hits:

see:

http://www.answers.com/topic/wind-chill

and
Definitions of Wind Chill Factor on the Web:

The apparent temperature which describes the cooling effect on exposed skin by the combination of temperature and wind, expressed as the loss of body heat. Increased wind speed will accelerate the loss of body heat. A wind chill factor of 30 degrees or lower on exposed skin will result in frostbite in a short period of time.

http://www.weca.org/nws-terms.html

The amount of cooling one "feels" due to the combination of wind and temperature. The formula to calculate wind chill is: WC=.0817(3.71 V^.5 + 5.81 - .25 v)(T-91.4)+91.4 where V=wind speed in MPH and T=temperature F.
ggweather.com/glossary.htm

A calculation that takes into account the effect of the wind to provide a reading of the apparent temperature (as opposed to the actual temperature as registered on a thermometer).

http://www.hometravelagency.com/dictionary/ltrw.html

Effect of wind blowing away the warmed air near the body.

http://www.carlwozniak.com/clouds/glossary.html

a factor applied to temperature that attempts to better represent the feel of low temperature, wind and humidity on people

http://www.capgo.com/Resources/Sensors/SensorGlossary.html

Wind chill is the apparent temperature felt on the exposed human (or animal) body due to the combination of air temperature and wind speed. ...
en.wikipedia.org/wiki/Wind_chill_factor

good luck!
-pmover

 

[JStephen]

"A wind chill factor of 30 degrees or lower on exposed skin will result in frostbite in a short period of time"

I think that statement is just plain wrong- the actual temperature needs to be below freezing. If it's 50 degrees, you may die from hypothermia, but you won't get any frostbite, no matter what the wind is.

 

[pmover]

JStephen,

although it seems unlikely or difficult to believe, be assured that it is possible for individuals to suffer from frostbite at 30°F or lower - exposed flesh. just ask any snow machiner whom has ridden in those temps. it may take awhile, depending upon heat removal rate from skin and other individual human biological factors, but it has and can occur.

-pmover

 

[ivymike]

p - that's not what he was saying - if the thermometer reads 50deg and the wind chill makes it feel like 30 (J's scenario), that's different from the thermometer reading <30 (your scenario).

 

[pmover]

ivymike,

thanks for the clarification! apology's to JStephen.

-pmover

 

[rmw]

Wind chill factor is just something that the weather persons on the evening news can use to seem more self important and melodramatic.

Really important if someone robs you of your clothes while skiing, though. Otherwise, just a weather persons term.

rmw

 

[zdas04]

Why is this so difficult for engineers? WCF is simply a rate of heat transfer stated in odd units. If the WCF is 20F, then an entity will lose heat at the same rate as if the temperature were 20F with no wind. That is far more useful than just a "weather persons term".

I guess we would be more comfortable with it in terms of BTU/(ft^2*R*sec).

David

 

[tkordyban]

The Wind Chill Factor is not the equivalent of an air temperature and convective heat transfer coefficient. It can be deceptive, because it implies that objects can be cooled to a temperature below the actual air temperature. If the air temperature is 40F and the wind speed is high, the Wind Chill might be 28F. That makes you think water might freeze, because it is below the freezing point of water (32F). But it doesn't matter how fast the wind is blowing, water (and any other object exposed to the air) cannot be cooled below 40F.

I don't know why the weather people can't just say, "It feels like 40F and 20 miles/hour out there." Why do they have to combine it into a single Wind Chill number that nobody understands and is deceptive?

I saw in a newspaper the other day a weather report that had the "Real Feel" temperature index. It purported to included temperature, wind speed, humidity, altitude, barometric pressure, and the metabolism of the reader. I wondered how they knew my metabolism before I bought the paper. Perhaps it should also include the gender of the reader. My wife always thinks the room temperature is too hot and I always think it is too cold, and we are sitting in the same room. Shouldn't that be included in how cold a temperature "feels"?

 

[JStephen]

zdas, one reason it's difficult for engineers is that the concept is flawed, and engineers realize it. You just can't make a blanket rule that "heat loss at x temperature and no wind is the same as heat loss at y temperature in 40 mph wind". To do that, you'd have to know the size and geometry of the item losing heat, the surroundings, humidity, factor in radiation heat transfer, etc. If that idea worked, it would sure enough simply the world of heat transfer.

The concept is no less or more valid at high temps than at low. But you will NEVER hear a weatherman say "It is 90, but the wind chill is 78"- doesn't sound impressive, so no point.

 

[zdas04]

I'm not getting the problem. I can calculate an intantenous rate of heat transfer from a body that is 98.6F into a 20F heat sink. If the body cools to 97F then the rate is no longer valid, but so what. The WCF contains all of the information to calculate a valid initial rate of convection. That doesn't mean that on a 40F day, a body will cool to 20F, and it doesn't imply that. To me the only reason for this conversation is our collective refusal to think of a concept stated in temperature units as a "rate".

David

 

[rmw]

WCF is not only a function of temperature, it is a function of relative humidity. On a cold dry winter day, human skin exposed to air at a given velocity will lose moisture, causing evaporation on the skin, hence the "feel" of the colder temperature.

Keep the wind the same, the ambient temperature the same, and vary only the relative humidity, and the WCF changes.

That is why the WCF is much more pronounced on the bone dry high altitude ski slopes (where you better not have too much skin exposed for too long) and the same temperature on the Texas Gulf coast.

Same with heat index which the weather people fall in love with in the summer time. Opposite effect here. On humid days, moisture does not readily evaporate from the skin, making it feel hotter than it really is. Same temperature in Utah, and it feels a lot different than the Texas Gulf Coast.

rmw

 

[JStephen]

Here's a pretty good discussion of the issue. It still contains an error or two but is worthwhile:

http://en.wikipedia.org/wiki/Wind_chill

According to this, current "wind chill temperature" is reported based on wind velocity and temperature only. Humidity, density of air, and radiation heat transfer have nothing to do with the reported number, although they obviously do affect the actual and perceived coldness.

As pointed out in this link, the wind chill factor was originally worked out for human faces (not human bodies). The wind chill factor varies depending on the surface it's applied to- so faces, wet rubber, and monkey heads all have different wind chill factors. The orientation of a surface in the wind would affect wind chill, so your face will have a different wind chill factor on the upwind side than on the downwind side (wonder which they based it on?) If you have a beard, the wind chill factor would be different. If you're fatter or skinnier, have higher or lower metabolism, etc, the factor would vary. It varies with height above the ground. Not surprisingly, different people disagree about how it should be defined- because it's not just a simple rate.

The equation listed in the link is:
Twc = 35.74 + 0.6215Ta - 35.75V^1.5 + 0.3275TaV^0.16
where Twc = wind chill temperature, Ta = actual temperature, both in F, V = wind speed in mph.

 

[rmw]

Since wikipedia is user contributed, and anyone can say anything, I'll stand by my remarks until a more definitive source is presented.

And, I'll add one.

A 40 degree F wind, whether blowing hard enough to make human exposed skin think it is 28 or minus 28 will only cool a dry object to 40 degrees F, (or whatever approach temperature is relavent). If the object is wetted, then the wet bulb temperature will be reached (approached). And that is dependent upon the relative humidity.

rmw

 

[JStephen]

rmw, I think if you'll read on down in that article, you'll find it discusses this. Note that the equation given is stated to be the "reported" wind chill factor as typically given on news, not that it's any more or less correct than anything else.

 

[JStephen]

Perhaps this link will be more definitive:

http://www.nws.noaa.gov/om/windchill/windchillglossary.shtml

 

[rmw]

OK, that is more definitive, and I stand corrected.

rmw