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The related question Is it safe to drink snow? tells us that in addtion to then normal considerations for any wild water source, you also need to consider how the snow may tip your metabolism into hypothermia

If it is cold and you are traveling and/or without a heat source, what is the best way to melt snow with your body heat to have the least chance of causing hypothermia? I think there are really two choices, put in your mouth and eat/drink it; or put in a container and hold it near your body to melt, then drink it.

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If you are on the edge of hypothermia then any body cooling on any part of your body is a bad idea. I suppose you need to weigh up the risks cold vs drink! – Liam Jan 7 at 17:14
    
I'd think the best answer to this is "don't." Melt it with sunlight or some similar energy source instead. Better yet, heat it up with said energy source and then drinking it hot will help prevent hypothermia. :) Moral of this story: if you're going someplace cold with lots of snow and a significant risk of hypothermia, take a reflective dish of some sort with you or at least some foil that could be used to make one. – reirab Jan 8 at 21:41
    
If I HAD to do this in your scenario I'd say - place it in a container in an area where you had enough insulation and add more insulation over it. If you do not feel the cold from it then it is having minimal impact but the heat which previously leaked to air is now leaking into the ice.) – Russell McMahon Jan 9 at 7:05
    
@RussellMcMahon it sounds like you want to vote for the answer by Joel – James Jenkins Jan 10 at 10:40
up vote 12 down vote accepted

The best way to melt snow is to put it in a bottle inside your jacket under your mid layers while you're on the move and let your body heat melt it. Do not place it against the skin, leave a layer or two between you and the bottle. It's advisable to always leave your bottle in your jacket in subzero temperatures, it can freeze if left in your bag.

Melting snow by body heat is not a fast process, so some foresight is required on your part, if your bottle is getting empty, and you don't know when you're going to find running water next, then put some snow in your bottle to replace what you're drinking. Dropping snow in liquid water will help melt it faster than trying to melt a bottle of dry snow.

Putting snow in your mouth is fine as long as you're warm, I do it all the time, but you are right that putting snow in your mouth will cool you down, so don't do it if you're already cold.

If you are borderline hypothermic, then it would be unwise to try and expend additional body heat in order to melt snow, you need to get up, get moving, and get warm before you can even try it. This will be more difficult when you are dehydrated, as hydration has a huge role in hypothermia.

One other option for melting snow is using solar radiation. Again, it's not a quick process, and you can only do it on a bright sunny day when the temperature is right. This is exactly how the survivors of the 1972 Andes flight disaster got water while they were stranded at the top of the snowy Andes mountains. They used reflective pieces of wreckage from the plane to catch the suns rays, and put small amounts of ice and snow on them then collected the drips in bottles to drink. They also ate each other... but that's another story.

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"They ate each other"... I'll give you my arm, if I can gnaw on your leg? – ab2 Jan 7 at 21:55
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@ab2 good point; the living ate the dead. Only the living ate, and no one ate the living. – James Jenkins Jan 7 at 21:57

Pee on it.

To keep the water drinkable, you'd want to have the liquids separated but still have good thermal transfer between them. A well equipped traveller will pick his/her thermos bottle and a condom, pee in the condom (ladies would probably do it the other way around), tie the condom and put in the bottle, fill the rest with snow, cap, wait and drink.

You'll curse your choice of [condom] flavour but might live another moment. If getting hypothermic, you might want to hold the filled condom inside gloves/jacket for some time before sticking in the bottle, doesn't need to be steaming hot to melt some water.

Too bad you'd probably have nothing to pee left at this situation, could try anyway.

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Nice idea although it's a bit awkward. Now that's a reason to always carry condoms while being out ;) – Wills Jan 7 at 22:47
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Actually maybe improve on this by peeing into a container of snow. Is drinking urine safe? > diluting the urine with snow would lower the density of solids in the urine while putting all the heat in the urine directly towards melting the snow. – James Jenkins Jan 7 at 23:52
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Just to add might be more pleasant as well to have a spermicide free condom. – Aravona Jan 8 at 9:07
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Gross. Wouldn't work anyways. Your pee is only body temperature, so it's not going to work any faster than putting your bottle in your shirt, and you aren't going to have any pee in the first place if you're dehydrated. – ShemSeger Jan 8 at 14:02
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@ShemSeger With all the snow around, how would you possibly be dehydrated? And of course it isn't faster (although it is probably more thermally efficient, but only marginally so), but it doesn't lower your body temperature because you're expelling mass that you don't have to worry about getting cold anymore, which is great. – corsiKa Jan 8 at 18:58

Physically there are two ways for you to heat up the snow: by heat conduction and by heat radiation. Conduction means you place it somewhere close to your body. It does not matter whether this is directly in your mouth or on your belly, you will lose the same amount of energy. The only option to heat it up without losing additional energy is by radiation, i.e. placing the snow next to you outside your insulation layers. Unfortunately that will never be enough energy to melt a substantial (if even noticeable) amount of snow.

TLDR:
You do not want to waste body heat (energy) to melt snow when almost in hypothermia in any way.

If you are short of dying of dehydration and almost in hypothermia, choose the option that needs least movement and removed insulation, e.g. once getting some snow into a bottle. Get this bottle into the sleeping bag and occasionally put some of it into your mouth. Anyway, in this situation, if you are still functioning, you will probably not function rationally but on instincts...

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For those who miss convection as heat transfer: I simply disregarded it. Though the only somewhat substantial heat transfer by convection, breathing, might even have more of an effect than radiation. Still not nearly enough to melt snow.... – imsodin Jan 7 at 17:33
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In actuality, snow melted in a container near your body heat will not necessarily cause your body to loose as much temperature as swallowing snow. The heat stored in insulation material is incidental to the heat your body produces, and doesn't directly consume your body heat. You body is keeping insulation material warm anyway, and a divot of heat in the insulation isn't going to consume more heat, it is more simply redistributed through the insulation. – Escoce Jan 7 at 18:08
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That is sadly not true. Any heat "stored in insulation material" that goes into the snow will be replenished. Heat is continuously "dissipating" and the amount of heat lost is not given a priori only by the body. The amount of heat dissipated per time depends on the temperature between the inside and outside of the insulation and the heat isolation properties of the insulation. If you place a body of snow in between layers of the insulation you essentially exclude the insulation on the outside for this local area. – imsodin Jan 7 at 18:35
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but it is dissipating anyway, therefore a divot in the insulated heat is more of a distribution issue. The body is renewing the stored heat continuously anyway. Whereas putting a heat sink "the snow" directly into the heat source (mouth/stomach) has more heat dissipation affect then placing the snow within the folds of insulation. – Escoce Jan 7 at 18:39
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@Escoce heat is dissipating because the temperature of near environment is cooler. If you put a container (sealed non-insuliated) of snow in your sleeping bag the snow will lower the air temp in bag and your body will work to warm the space. If you put a container (sealed non-insuliated) of boiling water in the bag, the temp in the bag will warm and your body will use less energy to keep it's self warm. – James Jenkins Jan 7 at 19:09

Very simply: if you eat snow, all the energy that would be melt the snow is energy currently in your body. Lots of heat loss direct from your body.

If you put snow in an outer layer of your clothes, then much of the energy that would melt the snow is energy that was in the process of escaping to the outside air. Effectively the snow is capturing some of the heat that left your body already.


To help understand this more clearly, imagine we were to wrap a person in one more very thin layer of clothing which doesn't actually trap heat, but rather measures how much thermal energy is crossing through it at every point. If the person puts the snow in a layer of clothing close to the outer layer, then it will absorb some of the energy in those outer layers. We'll see that the clothing in that area gets colder. This means that our special layer is measuring less heat crossing it near where the ice has been put because the heat flux across is proportional to the temperature difference, which is now smaller. What that means is that less heat is escaping into the air.


Here's an extreme example to consider: The person is wearing just the right number of jackets - one more and he'd overheat. Then he puts on yet another thick jacket and waits. Eventually that outer layer is above freezing and he's overheating. He wants to stop overheating, so he puts lots of snow just inside his outer jacket. That's approximately like taking his outer jacket off, so it gets him back to normal (at least until the snow melts). If he takes that same amount of snow and puts it directly inside his inner layer, he'll end up very cold.

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@imsodin I see you do some physics - Consider the total energy in the system, and focus on the flux of energy leaving the body/snow/clothes system. Placing the snow in an outer layer does increase the amount of energy leaving the body - I'm not arguing that. But it cools the clothing locally. This means there is a reduced thermal gradient in the outer layer. So less energy is going into the surrounding air. So some of the energy ultimately comes from the body, but some of it is coming from "stealing" energy that would have left the system. – Joel Jan 8 at 1:03
    
So you could come up with something like: Stuff snow into a bottle. Place the bottle on top of your ruscksack resting against your outer layers. Tie a spare item with some decent insulation value over the top to keep it in place. Hike and wait some time. Or put a bottle of snow in an outer pocket with extra insulation on the outside. Of course if it's cold enough this might not deliver enough heat to melt any/appreciable quantities of snow. – Chris H Jan 8 at 9:44
    
@Joel Your somewhat strange "death-comment" made me not take you seriously - I see I am mistaken now (I still do not get the comment). Your argument is clearly correct and I was pondering it or more precisely the significance of this effect, but neglected it on a hunch. Doing some "more structured model thinking" after your explanations makes me reconsider. It is certainly more efficient this way, but how much more efficient is kind of hard to estimate. I will add some of my thoughts to my answer when I have time. – imsodin Jan 8 at 11:37
    
Most efficient: Add more insulation, then eat snow to maintain desired temperature. Getting too hot? Eat more snow. Had enough to drink? Reduce insulation. – Russell McMahon Jan 10 at 22:57

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