Is it possible to get frostbite......

[Pete S]

Hello,
We all know that it is possible to to become hypothermic at any air temperature.

When the windchill values are cold, cold, cold, but the air temperature is above 32 F, is it possible to get frostbite?

I'm looking for some science behind the answers.....

Thanks for thinking about this question, Pete

[EricJ]

All your answers can be found here: (see #9)

http://www.weather.gov/os/windchill/...glossary.shtml




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[Bill O]

I guess there's your answer. I always thought that your skin did have to freeze to get frostbite. That is what causes the damage. The water in your cells freezes, expands and destroys the cells.

Not much science behind that answer though.

[Mike D]

The air temperature has to be BELOW freezing in order for frostbite to develop on exposed skin. Wind chill can bring the temperature to below freezing for humans and animals.

The first sentence seems to give a clear answer, but the second sentence just muddies the waters. My question is, when it says "temperature" does it mean "skin temperature" or "effective air temperature"? Either way, if a wind chill value below freezing is not capable of turning liquid water into ice, what significance does the number have?

Sincerely with more questions than answers,
Mike

[Brad]

Read #2 - for humans and animals the heat is taken out of the skin resulting in a lower temperature than the air temp. Read about the car radiator - this is not true for objects.

My conclusion is that if the air temperature is above freezing and the wind chill is below freezing, our skin temp will be drawn down to the wind chill temperature and frostbite will bite you.

[Steve M]

Read #2 - for humans and animals the heat is taken out of the skin resulting in a lower temperature than the air temp. Read about the car radiator - this is not true for objects.

My conclusion is that if the air temperature is above freezing and the wind chill is below freezing, our skin temp will be drawn down to the wind chill temperature and frostbite will bite you.

I always understood the wind chill to be just a feeling. i.e...it is what your body perceives the temp to be, not that it is that temp. If the wind is stripping away heat, does that mean that your skins temp is actually going below the temp of the air or just feels that way?

[Bill O]

My conclusion is that if the air temperature is above freezing and the wind chill is below freezing, our skin temp will be drawn down to the wind chill temperature and frostbite will bite you.

That's not true. Wind chill temperature is just a feeling and nothing more. If the temp is 40 and the windchill is 10 your skin will not freeze.

I always though that to get frostbite your cells must actually freeze. But I just read that damage actually starts to occur when blood vessels narrow and proper blood flow does not reach your skin. Your body does this to conserve heat, but it sacrifices your skin in the process.

Now this is just a guess, but above freezing this process probably just leads to numbness and some recoverable skin damage.

Below freezing this process exacerbates the problem. Blood flow reduction not only damages cells, but it lowers their temperature allowing them to freeze more easily.

[Patrad Fischroy]

That's not true. Wind chill temperature is just a feeling and nothing more. If the temp is 40 and the windchill is 10 your skin will not freeze.

I always though that to get frostbite your cells must actually freeze. But I just read that damage actually starts to occur when blood vessels narrow and proper blood flow does not reach your skin. Your body does this to conserve heat, but it sacrifices your skin in the process.

Now this is just a guess, but above freezing this process probably just leads to numbness and some recoverable skin damage.

Below freezing this process exacerbates the problem. Blood flow reduction not only damages cells, but it lowers their temperature allowing them to freeze more easily.

Agree Bill, wind chill is a cooling rate modifier, it does not change the actual temperature.

[BillPatt]

That's not true. Wind chill temperature is just a feeling and nothing more. If the temp is 40 and the windchill is 10 your skin will not freeze.

I have always understood wind chill to be more than a feeling. Otherwise, why do we blow on hot coffee to cool it down?

Wind Chill operates with two mechanisms: evaporative cooling, and modification of the boundary layer. Let's take the second one first.

When any object is at a different temperature than the surrounding air, a boundary layer of air forms right next to that object. If left undisturbed, that boundary layer can actually act as an insulator, reducing the amount of heat transport to/from the object. This is why there is a mirage above a hot road - the air, even though it is a fluid, just does not do large-scale convection well.

If anything strips that boundary layer away, like a breeze, then there is 'fresh air' to be heated by the underlying object. This is why the hot road mirage is best on a calm day, and almost invisible on a breezy day. That is also why blowing on coffee works - you strip away the superheated air lying directly atop the liquid, replacing it with air cooled from expansion through your pursed lips.

All of this will only get the object down to the ambient air temperature, however. That is why we have to look at the second effect: evaporative cooling.

Your skin is always moist. Some nervous folks are moister than others. But everyone has at least a little moisture on their skin. When that moisture cnanges from liquid to vapor, it removes some heat from the skin. The boundary layer is again modified to include this vapor, which acts to supress further evaporation. This is why a 'dry heat' feels cooler than the murderous mugginess of NYC in the summer, even though the ambient temperatures are the same. When the boundary layer is in the 90s, RH-wise, you just aren't going to cool anymore, and need a fan.

In the winter, then, you are hit with the double whammy - not only is the wind stripping away the insulating layer of air surrounding your skin, but the moisture on your skin is evaporating, cooling you further.

A long answer, but that is the science behind it. Now, I don't know if frostbite has been clinically observed in windy conditions with the ambient above freezing, but clearly, it could happen. Especially if the skin is wet from snow/rain/meltwater.

Hope this helps.

Bill

[Bill O]

I have always understood wind chill to be more than a feeling. Otherwise, why do we blow on hot coffee to cool it down?.........

You're right. Patrad described it better than I could, its a cooling rate modifier. I took some liberties in simplifying everything. I chose to leave out the evaporative effect. For the most part I think it makes little difference in the cold. For one, if your skin is cold its not going to sweat. Second, once you stop sweating and your skin is dry there is no more evaporation. Yes, you can work up a sweat hiking up then stop hiking and freeze with all that sweat.

Back to the original question. No, you can't freeze your skin when its above freezing unless you have some serious evaporative cooling going on. Yes, you can get frostbite like damage to your skin when its above freezing.