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Pete S
12-23-2007, 06:04 PM
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
12-23-2007, 08:32 PM
All your answers can be found here: (see #9)

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




.

Bill O
12-23-2007, 08:44 PM
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
12-23-2007, 09:16 PM
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
12-23-2007, 09:48 PM
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
12-23-2007, 09:57 PM
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
12-23-2007, 10:58 PM
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
12-24-2007, 07:49 AM
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
12-24-2007, 10:43 AM
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
12-24-2007, 10:53 AM
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.

BillPatt
12-24-2007, 12:20 PM
Bill-O,

I thought I would expand "cooling rate modifier" to describe the physics behind it.

I imagine you could get that serious cooling if it were nice and windy and bone-dry, and you are dragging yourself up out of the river. Your skin would already be shut off from bloodflow, and the wind is sucking the water off.

In Palm Springs, they use evaporative cooling a great deal - and get tens of degrees of temperature reduction. I, though, would like to see some case studies first before I would say that it would happen. All I am willing to say right now is that it is possible.

Bill O
12-24-2007, 01:24 PM
Bill-O,
In Palm Springs, they use evaporative cooling a great deal - and get tens of degrees of temperature reduction. I, though, would like to see some case studies first before I would say that it would happen. All I am willing to say right now is that it is possible.

Evaporation is very powerful in low dewpoint environments. When I lived in Salt Lake my place was cooled with a swamp cooler. Essentially just a fan drawing air over a wet mesh. Mine was old, but it worked.

I always thought they used to make ice in ancient Egypt through evaporation.

Lastly, the summit crew utilizes evaporative cooling everytime they measure the dewpoint with their wetbulb thermometer. I've seen the wetbulb turn to ice when the air temp was well above freezing.

Mike D
12-24-2007, 03:02 PM
I've seen the wetbulb turn to ice when the air temp was well above freezing.

Aha! Doesn't this show conclusively that frostbite can happen at ambient temps above freezing?

Bill O
12-24-2007, 03:42 PM
Aha! Doesn't this show conclusively that frostbite can happen at ambient temps above freezing?

If you were careless enough to leave water on your skin, and you were cold blooded.

h2oeco
12-24-2007, 06:39 PM
I do a lot of scuba diving, sailing, kayaking, etc, including in cold water/weather. Re: hypothermia, water sucks the heat out of your body about 25 times faster than air...

Steve M
12-26-2007, 10:00 PM
So, I would say that you need evaporative cooling (wet skin and high winds)to get frost bite if the temps are above freezing. High winds, dry skin, and above freezing temps will only cause frost bite like conditions and hypothermia.

Dunk a man in cold water, spin him around at high speeds and walaa, instant Popsicle. :D

Brad
12-27-2007, 09:15 AM
I asked this question of my son in law yesterday as we were hiking back into Tucks. He is a geologist, hydrologist, plant guy, DNA dude, you name it. He could understanding getting cell damage - but, not frostbite.

Mike D
12-27-2007, 10:37 AM
If you were careless enough to leave water on your skin, and you were cold blooded.

I thought wind chill was the result of evaporation of the skin's water, either on the surface or in cells near the surface. I am speaking way out of my element. Ha ha pun!

Bill O
12-27-2007, 11:35 AM
I thought wind chill was the result of evaporation of the skin's water, either on the surface or in cells near the surface. I am speaking way out of my element. Ha ha pun!

Evaporation plays a trivial role in the effects of wind chill, unless you are covered in wet clothing. Then you are dealing with something else.

Pete S
01-04-2008, 07:52 AM
Hello,
I spent the day skiing with the local emergancy room doctor ( also a ex- pro ski patrol and inventor of a step in telemark binding), and when ask the question " Can you get frostbite when the temp is above 32 F? " there was no hestitation before his answer of-- Frost bite- No, cold tissue damage-yes.
As the chair lift ride continued, he added that since the "liquids" in the body contain salt, the maximum temp for frostbite to occure may be lowered to about 30F......

Pete

Steve M
01-04-2008, 08:05 AM
Hello,
I spent the day skiing with the local emergancy room doctor ( also a ex- pro ski patrol and inventor of a step in telemark binding), and when ask the question " Can you get frostbite when the temp is above 32 F? " there was no hestitation before his answer of-- Frost bite- No, cold tissue damage-yes.
As the chair lift ride continued, he added that since the "liquids" in the body contain salt, the maximum temp for frostbite to occure may be lowered to about 30F......

Pete
Awesome, thanks for sharing. I would never have thought of factoring in the salt content of the body.

Bill O
01-04-2008, 08:49 AM
Hello,
Frost bite- No, cold tissue damage-yes.
As the chair lift ride continued, he added that since the "liquids" in the body contain salt, the maximum temp for frostbite to occure may be lowered to about 30F......

Pete

I'd also imagine that if we are talking about hands, ears, nose you'd have to be a very sick person for those to freeze even at 30F since our body has a great heat pumping and distribution system.

Somewhere back in the confusion of this thread I was trying to get at the frozen tissue-no, tissue damage-yes answer. I think it was lost in the complexity of my response.

Steve M
01-04-2008, 09:38 AM
I'd also imagine that if we are talking about hands, ears, nose you'd have to be a very sick person for those to freeze even at 30F since our body has a great heat pumping and distribution system.

Somewhere back in the confusion of this thread I was trying to get at the frozen tissue-no, tissue damage-yes answer. I think it was lost in the complexity of my response.
I didn't find it confusing. I knew that's what you were saying.

mary
01-04-2008, 03:13 PM
not to barge into your post....but i will add what i have learned and come to know about frostbite and frostnip....

your body works to stay alive first and to stay functioning second....

in conditions of prolonged cold exposure....your body sends signals to the blood vessels in your arms and legs telling them to constrict (narrow)....by slowing blood flow to the skin your body is able to send more blood to the vital organs....supplying them with critical nutrients while also preventing a further decrease in internal body temperature by exposing less blood to the outside cold....as this process continues and your extremities (the parts farthest from your heart) become colder and colder....a condition called the hunter’s response is initiated....your blood vessels are dilated (widened) for a period of time and then constricted again....periods of dilatation are cycled with times of constriction in order to preserve as much function in your extremities as possible....but when your brain senses that you are in danger of hypothermia (when your body temperature drops significantly below 98.6°F) it permanently constricts these blood vessels in order to prevent them from returning cold blood to the internal organs....when this happens frostbite has begun.

frostbite is caused by 2 different means: cell death at the time of exposure and further cell deterioration and death because of a lack of oxygen....

in the first....ice crystals form in the space outside of the cells....water is lost from the cells interior and dehydration promotes the destruction of the cell....in the second....the damaged lining of the blood vessels is the main culprit....as blood flow returns to the extremities upon rewarming....it finds that the blood vessels themselves are injured by the cold as well....holes appear in vessel walls and blood leaks out into the tissues....flow is impeded and turbulent and small clots form in the smallest vessels of the extremities....it is because of these blood flow problems thatcomplicated interactions occur and inflammation causes further tissue damage....this injury is the primary determining factorof the amount of tissue damage you will have in the end....it is rare for the inside of the cells themselves to be frozen....that phenomenon is only seen in very rapid freezing injuries like those that are produced by frozen metals.....


it really comes down to pathophysiology....cold exposure leads to ice crystal formation....cellular dehydration....protein denaturation....inhibition of DNA synthesis....abnormal cell wall permeability with resultant osmotic changes.... damage to capillaries and pH changes....rewarming causes cell swelling.... erythrocyte and platelet aggregation....endothelial cell damage....thrombosis....tissue edema....increased compartment space pressure....bleb formation.....localized ischemia....and tissue death.... underlying responses to these injuries include generation of oxygen free radicals....production of prostaglandins and thromboxane a2....release of proteolytic enzymes and generalized inflammation.....tissue injury is greatest when cooling is slow....cold exposure is prolonged....rate of rewarming is slow and especially when tissue is partially thawed and refreezes.....


frostnip is a nonfreezing injury of the skin tissues....usually of the fingers.... toes....ears....cheeks and chin....numbness and tingling are present but no tissue injury occurs....symptoms develop when blood vessels supplying the affected tissues narrow because of the cold temperature....frostnip occurs at temperatures of about 15°C (59°F)....

a more significant nonfreezing injury from exposure to cold temperatures is chilblains....as tissue temperature drops below 15°C (59°F) tissue injury progresses....the walls of small blood vessels break and the tissues swell....



so....if anyone made it to the end of this post....i hope it helped....and i hope all the dots didnt drive you crazy....its how i tend to type when not working on an official document....

Brad
01-04-2008, 03:17 PM
Mary, we thank you! Hopefully your typing fingers are toasty and warm.

Mike D
01-04-2008, 09:56 PM
This whole discussion reminds me of a book I got for Xmas a while back called Surviving the Extremes. It was a fascinating read about the enormous variation humans worldwide expose themselves to, both to live and to explore. It contained stories of people on death's door who made inexplicable recoveries, physiological accounts of the body's efforts to save itself, and the types of adaptations made over generations of living in drastic climates.

http://findarticles.com/p/articles/mi_m1200/is_13_165/ai_n6100275/pg_1