I KNOW this is the right place to bring this up since the Dopers know it all!
Okay. Someone was telling that the best way to stay warm is to stand perfectly still because there is sort of this shell of heat that is around the body. The heat is eminiating from the body and there is always enough to keep the body fairly warm, as long as one doesn’t move. She knows this because there was once this TV show with some junior astronauts who were in some cockpit somewere and they had to conserve energy and they had to turn off the heat so it was like 30 degree fahrenheit and they all stuck togother and made sure not to move becauseof the heat thing.
Am I ignorant to think this sounds a bit funny to say the least? It seems to me that heat dissipates in the cooler air surrounding the body since air temperatures will naturally even out, all other factors being ignored. Granted, the body will always have warmer temperatures around it since the the body is constantaly emitting heat, as long as surrounding temperatures are cooler than the body itself. But sure that heat doesn’t make a heat shell sorta’ thing!
Ummmm… well, in the weightlessness of space, heat wouldn’t cause the air around you to “rise”, since there’d be no “up”. So I guess you would be somewhat insulated by the air immediately around you warming up. …But the practical problem (-that I see) with this plan is that there is usually some sort of air filtration/revitalization system running, that would cause air currents in the ship–else if you stayed in one spot, you’d breathe up all the oxygen there and die. So it would seem to me that your warm air is gonna get blown around anyway.
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They’re called goose pimples. When you are cold, your body hair stands on end to help create a tiny barrier of still air next to the skin. This acts like insulation to keep the body warm.
However, there are two problems with this. If the exposed skin is exposed to moving air (wind?), that moving air blows away that still air. At the same time, a still body does not generate much heat. That’s where shivering comes it. It is a body defense mechanism to generate heat.
If you are in good physical condition, have enough fuel (food and drink) to keep you running, acclimated, and performing some level of physical exertion, you can be comfortably warm at 30F with few clothes. Have you ever tried snow shoveling at 30F with a heavy jacket? In relatively little time, the physical exertion is generating sufficient heat to where you can shed that jacket. Of course, the hat, mittens and sweater should remain on to continue to trap some heat to keep you warm between shovel throws. Then again, any cool/cold wind will remove that excess body heat just that much faster (wind chill).
As for the astronauts, perhaps you are referring to the movie Apollo 13.
If any of this is incorrect, I’d rather be corrected than misinformed…
About shivering, I thought that,as well as other phycial activity, had the effect of increasing the heart rate and blood circulation. The increased blood circulation helps to maintain body temperature, hence the feeling of being warmer. (However, by bringing blood to the body’s extremities at a faster rate, where the blood cools, this would have the effect of the body losing heat faster, and would require the burning of more energy to generate replacement heat.)
While I could understand that area around the body is warmer than the wurrounding air (assuming the surrounding air is cooler than the body), since the body is constantly emitting heat, the quesiton at hand is really about what happens to the heat after the body emits it. Does it really linger around the body for any significant amount of time when left undisturbed? Or wouldn’t the heat really just dissipate through transfer to the molecules around the body?
Perhaps the reason to stand still in the example cited in the original post is in order to conserve the body’s energy. But I don’t really buy this heat shell thing as being so significant. (Sure, one’s body heat may make a particular area warmer so it wouldn’t make sense to go wandering around, but I think I’m talking about somehing different than just a general space, room or area.)
Well, in a gravity well, warm air, which is less dense than cold air rises. Starting from your feet, the warmed air being generated by body heat rises straight up around you (assuming that the body is cooler than the surrounding air, in this situation). The warmed air becomes more and more warmed as it ascends your body and then rises above you. Thus, there is very little of this heat shield around your feet, but moreso for the upper parts of the body.
Standing still allows one to be in this inverted heat cone as long as there’s no wind.
No. Wind and/or extreme cold is enough to make this heat shield ineffective for survival.
Makes sense. Without a gravity well to cause the heavier cold air to displace the warmed air surrounding the astronauts, the warmed air will stay right next to them. Also, there’s no breeze in space (assuming they shut off any air handlers that’d cause a breeze). However, if the temperature was much lower, this heat shield wouldn’t be able to withstand the loss of heat due to heat radiation (hot objects radiate heat away to less hot objects, even without a transfer medium such as air or water). The astronauts’ bodies and the warmed air surrounding them still sends their heat away to the colder environment through radiation. The lack of a convection current in microgravity helps to preserve part of it in the air blanketing them.
Peace.
OK, to nitpick my own thread: warmer objects do not emit heat radiation to cooler objects exclusively. All object above absolute zero radiate some sort of heat. It’s just that warmer objects radiate a lot more heat to the cooler object than the other way, resulting in a net loss of heat for the warmer object and a net gain for the cooler, making it look like that only the warmer object did anything.
Why do you put a fan on a wood stove? To move air and increase performance and heat distribution. So without moving air the warm air stays at the stove and requires the movement of the air to heat the surrounding area quicker. I picked up this little piece of information recently. Supposedly it’s the same effect that causes something to achieve ambient temperature quicker as in wind chill. I’ve also seen the heat shell used to explain wind chill and the heat transfer layer surrounding the body. When you go into a cold enviroment from a warm enviroment if the wind is blowing you cool quicker than if it’s not blowing.
This heat shell is kind of the opposite effect of wind chill. The so-called wind chill factor occurs because when you have a warm object in a cold airstream, it loses heat faster by convection than it would by conduction alone in still air. Convection comes in two forms: forced convection where a fluid (air) is blowing across a warm body (you), and natural convection where the warm body heats a thin layer of air which then rises because of decreased density, which induces a flow in otherwise still ambient air. Natural convection is the effect moriah mentioned. Natural convection still cools the body faster than conduction alone, but not nearly as fast as forced convection.
If you’re an astronaut in zero gravity, there is no natural convection because the lower density hot air doesn’t rise. If you’ve also turned off your ventilation fans so there’s no forced convection, you will sit inside a bubble of air warmed by your body, and this heat will only be lost by conduction (and some diffusion in the air). Of course, you’ll also sit inside a growing bubble of exhaled CO2, so if you’re more worried about suffocating than freezing, you’ll want to swim around a little.
As moriah points out, all these heat transfer mechanisms (conduction, natural convection, forced convection, and radiation) are almost always present. It’s just that some tend to dominate and make other effects negligible. If you’re in a strong wind, the effect of natural convection exists but is completely dominated by the forced flow.
It sounds like the OP’s friend was referring to events that happened during the Apollo 13 mission. On this mission in April 1970, an oxygen tank ruptured and the damage caused the loss of not only the stored oxygen, but other gases and the ability to use the fuel cells to generate power. Without going into great detail about it, the 3 astronauts (Jim Lovell, Fred Haise, & Jack Swigert) were forced to shut down power to most of the systems in their craft to conserve power.
So this was a pretty specific scenario that included zero-gravity conditions as well as minimal air movement. In that case, it was true that the astronuats found they could conserve some small amount of warmth by huddling very still. Any movement would cause the warmer layer of air to dissipate. Regardless, it was a pretty small effect and certainly wouldn’t save you from hypothermia in the long run. In almost any circumstances an ordinary person could conceivably encounter, there would be gravity (and thus convection currents) and some air movement (due to wind or an HVAC system).
By the way, I don’t think it would be accurate to describe these men as ‘junior astronauts’. Although it was the first time in space for at least Swigert (maybe Haise too, can’t remember), by the time an astronaut is being sent into space, they wouldn’t be considered junior anymore. And anyway, Lovell was very experienced at that time.
If the air/water around you isn’t moving pretty much at all, there will be a small layer of heated air/water around you - you can try it yourself:
Lay in a tub of water that’s a little bit too cool for your liking, and stay perfectly still for say 2 minutes. Then start swishing around - you’ll feel colder water wash around you when the slightly warmer water heated by your body starts mixing with it.
When I was a kid learning to swim I remember being in the deep pool with about 6 others all wearing life jackets. We were told to form a ring with our feet tucked up in the center. After a few minutes they told us to break apart and swim away. The temperature drop was very noticable.
Also try sitting outside when it’s very cold. While stationary (not exercising or building up heat otherwise) you’ll stay warmer if you huddle up and don’t move. Start turning your head around, moving your arms, turning and exposing more of your body to “fresh” cold air and you’ll get cold quicker.