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Suppose a man falls into very cold water and gets their foot stuck under a heavy rock. Fortunately, his head is above water and someone is able to call for help. The paramedics want to keep him warm while they work on freeing his foot. They put a hat on his head. Should they also wrap him in a blanket?

Moby Disk
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    Well, isn't it just better to try it out? – user3459110 Jul 03 '14 at 15:58
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    @AwalGarg - Sure, let's all go get our feet stuck under heavy rocks & call some paramedics to help us. Should only take a couple hours for us to complete the experiment. We won't lose any blood or fall into a coma from the cold. – trysis Jul 03 '14 at 22:14
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    @trysis Well, I will be the observer then, OP will get his feet stuck, you become the paramedics and answerers will help me observe... what say? – user3459110 Jul 04 '14 at 02:57
  • Well, it depends a lot on what the blanket is made of. But even in the worst case, it will probably still help, just not a lot. Using a well isolating blanket that doesn't soak up on water, though, could be very effective. It's hard to tell how effective a cotton blanket would be, since it goes limp and soaks up a lot of water, reducing the insulating properties when wet. – Luaan Jul 04 '14 at 08:17
  • @AwalGarg - If the OP is up for that, sure. Won't be any skin off our backs. As it will be his life & limb we will be risking, though, he shouldn't be forced to. Even "For science!!!" only goes so far. – trysis Jul 04 '14 at 22:07
  • This of course assumes the blanket doesn't inhibit the work on freeing the foot. If it means only two people lifting the rock instead of three, I'd say it's probably not a great idea! – corsiKa Jul 04 '14 at 22:28
  • @AwalGarg why not make it an electric blanket for good measure? – jkeuhlen Jul 05 '14 at 02:38
  • @jkeuhlen Because Santa is not coming with his underwater dynamo to make electricity there! – user3459110 Jul 05 '14 at 16:31
  • Sure, this is why a wetsuit keeps you warm(er). – copper.hat Jul 07 '14 at 01:51
  • Funny, the question was asked by Moby Di(ck)sk. Hehehe – Torsten Hĕrculĕ Cärlemän Jul 09 '14 at 03:24
  • To be a complete prat and miss the entire point... unless it's jkeuhlen's electric blanket, the blanket's not actually warming you, it's just reducing heat loss, right? – Smithers Dec 26 '14 at 22:01

8 Answers8

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Yes.

There are three mechanisms of heat loss (this applies generally, not just to the man in this example) radiation, conduction and convection. In most everyday cases radiation can be neglected so we just have conduction and convection.

Conduction is just the transfer of heat along a static object. For example if you hold the end of a metal bar in a flame pretty soon the heat will conduct along the bar and the end you're holding will get hot too. In this case if our man is surrounded by a region of still water he will lose heat by conduction into the water.

Convection is the transport of heat by a moving fluid. For example if you stand in still air on a winter day it may not feel too cold, but if a gale is blowing you'll start feeling cold very quickly. This happens because your body heats the air immediately around it then the wind whisks that warmer air away and replaces it with cold air.

Now back to your question. We can't do anything about conduction. The man is in the water and we can't change the thermal conductivity of the water. But we can do something about convection. Our man's body heat will soon heat the water immediately around him, and we don't want water currents carrying away that warm water and replacing it with cold water. Wrapping a blanket tightly around the man will trap a layer of water near his skin and prevent convection from cooling him.

John Rennie
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    Thanks! Don't we also get some benefit to conduction as well, because the conduction through wet cotton should be slower than the conduction through pure water? – Moby Disk Jul 03 '14 at 14:56
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    @MobyDisk Yes it will have an effect. However, cotton is able to soak up a large amount of water, due to its build-up. Therefore, heat is able to conduct through the water soaked by the cotton blanket. However, as there are channels blocked by the cotton strands, it does slow down the rate of transfer of heat, however, it is not really that significant. – Gummy bears Jul 03 '14 at 14:59
  • I thought wet suits worked primarily by blocking conductive heat transfer (because the suit is neoprene with air bubbles in it), which ends up blocking convective transfer as well. Would cotton really have a noticeable effect on convection? It will still transfer a significant amount of heat by conduction into the surrounding water which moves the heat away; just like your skin. How can you have a significant effect on convective heat transfer but an insignificant effect on conductive transfer at the same time? Cotton seems like a negligible difference wrt. your skin anyways? – Jason C Jul 03 '14 at 18:24
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    @JasonC No; a wet suit is well.. wet. The neoprene bubbles are not closed cell, and become filled immediately with ice-cold water, which absorbs heat, and stays right where it is. The suit is solely to stop convection. Think of the compression and expansion changes and buoyancy changes for a scuba diver, if the bubbles were air filled... – DJohnM Jul 04 '14 at 02:49
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    @User58220 All wet suits are made from closed-cell neoprene, e.g. Patagonia, Yamamoto. Ones advertised as "open-cell" are referring to the lining and the neoprene is still closed-cell with gas bubbles (example). There's also wikipedia, if you count it as a legitimate source. – Jason C Jul 04 '14 at 05:57
  • @JasonC: it's been several decades since I last wore a wetsuit, but my recollection of it was that after removing it you could squeeze it and water would come out as from a sponge. However since my memory may well be at fault (or of course wet suit design may have changed in the last thirty years) I've removed the reference to wet suits from my answer. – John Rennie Jul 04 '14 at 06:00
  • @JohnRennie Still; the effectiveness of cotton would really depend on the thickness. Imagine, say, wrapping a window screen around yourself on a windy day to stay warm. If the cotton is wrapped tightly around you, its inability to stop conduction will cause any heat trapped in the thin cotton/water layer to be quickly conducted out and carried away by convection with minimal effect. But if it were a very thick cotton blanket then it would make more of a difference, the goal being to have as much relatively still water trapped as possible, so that that water/cotton mix can act as a buffer. – Jason C Jul 04 '14 at 06:08
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    @JohnRennie (By the way the "sponge" effect you're remembering with the wet suit is water coming out of the lycra / nylon / wool / etc. lining, and also water trapped inside the suit which enters via neck / wrists / ankles -- but not water from inside the neoprene itself, which is waterproof. Wet suits work by trapping water inside tightly and insulating it from the water outside, which cotton cannot do.) – Jason C Jul 04 '14 at 06:10
  • Everything has an effect in physics, the point is: is it measurable? – DarioP Jul 04 '14 at 10:18
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    I am not sure if all the terms in your answer are used precisely. Convection is a heat transfer in fluids, yes, but it is happening because of both conduction and advection combined, it's not a separate complimentary process. Due to conduction heat is transferred into the fluid, and due to advection, that is bulk movement of fluid particles, the heat is transferred away, with the wind or down the river. When you use blanked, you directly encumber advection, not convection. The convection will still be taking place because of diffusive and conductive heat transfer. – luk32 Jul 04 '14 at 16:17
  • @DarioP If it isn't measurable, it you can't claim it has an effect. You can only hypothesise based on other measurements and theories. – corsiKa Jul 04 '14 at 22:26
  • @luk32 Well said; I had tried to say the same thing but failed. I don't think this answer is very accurate. – Jason C Jul 06 '14 at 03:56
  • Remember that there will be convection in the man-blanket-water system, just as there is in the man-water system. In a computer, a high quality heat sink completely seals in a processor and prevents air movement directly around it, but is no real barrier to convective cooling of the processor - we wouldn't install them if it was. The effectiveness of a blanket will still largely depend on the barrier it presents to conduction. – DeveloperInDevelopment Jul 06 '14 at 11:55
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The blanket would help a bit in the same way it keeps the body warmer in cold air, by inhibiting convection and allowing an interface of warmer air between the blancet and the body, but water is a better heat conductor than air and it will not be very efficient.

Divers who stay long in the water have water suits whose material is designed to keep the body temperature comfortable even in quite cold water. Different types for different waters.

In the emergency you are talking about cutting up a wet suit will offer better protection.

p.s. when I was young we had woolen bathing suits, and it is true that they are warmer than the synthetics we get now.

anna v
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Blankets do not "warm you." Blankets slow down heat transfer by convection, by simply restricting the movement of a medium which may carry heat. Air absorbs heat from solid objects by conduction. Air does not conduct that heat to other air molecules very well. So, if you restrict the movement of those air molecules, you will limit the ability of that air to carry that heat away from the solid object from which it originally absorbed the heat. As a secondary factor: The closer the temperature of the air is to the temperature of said solid object, the less heat absorbed by the air. So, as that trapped air absorbs more heat and rises in temperature, the less heat it can then absorb.

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The Blanket will help but the amount depends on the type of blanket cotton or other fabric blankets will be of fairly small help because the water can still flow through them just not as quickly as it would without the blanket in the way where as non porous material blankets would be of significant more help as they stop the flow of the water carrying away more heat

Chichi90504
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I suspect that the only significant benefit that you can achieve from a thin layer of cotton is obtained wrapping it tightly especially around the limbs so that the blood is forced to stay deeper dissipating less heat.

Although it is for sure true that a thin layer of cotton will somehow limit the convection, to effectively contain the heat losses you need to create a thick layer of static water and looking at the thickness of diving gears, I believe that the effect of a blanket is almost negligible if just loosely wrapped around the unlucky man.

DarioP
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It definitely will and I can attest to this. During my swim career while waiting on the wall for your set there was always some discomfort when kicking of the wall after having been on it for a moment or two. The discomfort is because a "shell" of water around your body, about an inch, had been warmed up and remained there assuming no one had swam by you and the water was calm. You could feel this on other people as well. So a blanket around you would buffer against natural currents in the water and if it's thick enough, doesn't convect heat well, and the temperature difference isn't to large it will keep you warm. This is how a blanket works when dry by the way. It simply keeps the air inside warm. The specific heat of air is about 50% greater than the specific heat of cotton according to engineeringtoolbox.com. It is the still air inside that does the insulating, not the fabric.

aPhysicist
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Simple answer is no!! Blankets only work in air. Since air is a bad conductor of heat and blacnkets use this air to provide as a cushion for your heat and the external environment. Since in water there wont be much air and water too is a bad conductor of heat. Hence you should not use blanket in water

jaskaran
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Short answer, no, it will not warm the body underwater.

Consider, what is the temperature of cold water? If cold water is that of the arctic area or thereabouts, which is usually about 5 to 10 Celsius, then those paramedics better work fast, blanket or not. Be that as it may, remember to calm this man down first by reassurance/s as panic just increases the stress on the body, and that water, whether trapped or not in neoprene or woolen blanket, will rob the body heat.

Water below 21 C (~70 F) is generally considered cold, and that of Pacific Ocean even in San Diego/CA area will be a chilly 10 to 15 C (~ 50 to 60 F). If I was the rescue team [paramedic], I would highly recommend putting the face into that very cold water as this would invoke the mammalian diving reflex pathway. It may seem inhumane, but will increase the chances of recovery post-rescue. Then work on unsticking of the foot. Have a read from here, http://bit.ly/TWf3f8 for cold water survival/rescue.

Solutions
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    Your answer does not explain why the blanket would have no effect. While maybe relevant to actually rescuing a person from cold water, I think the question asks for the physical principles involved, otherwise it would be off-topic, anyway. – Jonas Greitemann Jul 06 '14 at 07:58