I have heard many people say that eating snow actually can increase dehydration since the energy required for the body to heat up and melt the snow is greater than the benefits received from the moisture in the snow.
Truth or fiction? And please back it with solid physiological evidence.
The calories to melt even frozen water are pretty small, and the water gained is certainly greater than that used to aid the use of those calories.
http://en.wikipedia.org/wiki/Calorie
So 1 calorie = 1 degree celcius (roughly with minor variation). Okay easy. Except it takes 80 times that to melt it initially. Screw snow, let's figure on ice cubes.
1 liter of water frozen to ice at 0 °C
1 liter water = 1 kg water, (1000g * 80) 80,000 calories to melt the ice and then 37500 for the next 37.5° (to achieve 37.5 °C, temperature of the human body)
117500 little c calories
That's 117.5 dietary calories, because a dietary calorie is actually a kilocalorie.
How much water does it take to process 117.5 dietary calories? No where near a liter or we'd all be dead the next time we ate a christmas supper.
But if you're still in doubt... well average daily intake is 2000 calories. So if we assume ice is net negative then it must take more than a liter of water per 117.5 calories... which means that the human daily intake of water would need to be 17 liters.
Based on this article, but researched and did all the math myself to fact check it.
All the advice I've seen emphasises melting snow before consuming to avoid lowering your core body temperature (rather than, specifically, risking dehydration). If possible, melt the snow using a stove, or alternatively, pack the snow into a waterproof container and keep it in a pocket or your sleeping bag until it melts.
News stories such as this one, suggest that eating snow aided survival, however, I would guess there is a balance to be struck between succumbing to the effects of dehydration versus hypothermia which will depend on the environmental conditions.
As with any relatively unscientific field, there is a lot of lore out there that may have originally had a good scientific foundation, but the restrictions or specific conditions have long been forgotten and the answer takes on a life of its own out of context. The myth about eating snow seems to me to be one of these things.
The main point is that it takes a lot of energy, which when coming from your body means warmth and/or calories, to melt water to drinkable form. This is true, but the details are rarely considered. Russell has already give an good answer going over the numbers to show that the energy required is relatively minimal.
However, another point I never see addressed is that the energy is often free. If you're hiking up hill, even in winter usually, your body has to dump energy as heat. If you're cold and facing hypothermia, eating snow is probably not a good idea. Otherwise though if you're capable of dressing yourself so as to be too warm for whatever activity you are doing, you have excess heat that has to be gotten rid of somehow. Therefore except in unusual cases, melting snow with your body warmth is free since you just end up cooling yourself off less in other ways.
I have experienced this personally. Many years ago I was hiking up Mt Lassen in northern California in the summer. I had taken insufficient water with me and was getting thirsty. The weather was nice and I was hiking comfortably in shorts and a T shirt with a windbreaker in reserve in my pack. When I got high enough so there were patches of snow, I kept some in my mouth and got water from that. It definitely felt nice and made me less thirsty eventually.
The biggest problem you realize if you ever tried eating snow is that it takes a lot to get a reasonable amount of water. You simply can't put that much snow in your mouth at one time, and there is a lot of air in it, so the amount of water you get from a mouthful of snow is very limited. Still, it's better than nothing and makes you feel better. It does slow you down noticably since you have to pause to pick up fresh snow for another mouthful regularly. I suspect in warm conditions that you're barely replacing the water you are losing regularly anyway, but that's still better than losing it without replacing it.
So to me it seems that if you're thirsty and there is clean enough snow around, go ahead and "eat" it unless you are cold and hypothermia is a concern. Dehydration, after all, is a serious problem too.
If you can, melt it beforehand - not because of dehydration but because of the obvious; it'll cause you to get rather cold rather quickly!
If you can't and it really is an emergency (the only time I'd suggest it might be worth considering) then I haven't found a general consensus on whether it's a good idea, probably because it comes very much down to judging the situation - hydration vs. core body temperature. Do bear in mind that you won't get anywhere near the amount of fresh water from snow though so it may not hydrate you as effectively as you might be imagining.
One final point, never drink / eat coloured snow. I don't just mean the obvious cause of coloured snow - it can have all sorts of colours to it, and aside from the obvious this is most commonly caused by bacteria which are often rather nasty if ingested. This will almost always make the situation worse, not better!
Yes, but it will really make you cold. It takes about 30 times more heat to heat water (melt ice) from 31 to 33 degrees (F) than it does to heat it from 33 to 35 degrees. That heat comes from your body if you eat snow.
There are reported deaths from eating snow during WWII (Eastern Front). I presume due to hypothermia and/or the general poor health of the soldiers concerned.
Another site points out that snow is excellent at catching polution. Their reasoning is a bit fuzzy, but as a scientist I agree with their conclusion.
One point I would like to add to all the good answer before:
Snow is good at catching pollution. But it isn´t good in containg minerals. Apart from short-term hydration, you should NOT rely on snow as water source for a longer period of time because you simply won´t get the electrolytes you need - at least if you don´t supply them for example through food. This is also true for rain water and to some extent for ice.
Also I would like to point out, that Russels calculation may be right, but it doesn´t take into account where the heat is consumed. So it can be true that it doesn´t need to much heat, it can also be true that it lowers your core body temperature. I would try to melt it outside my body (e. g. in a bottle close to my body) or in the mouth, if necessary.
The most authoritative source I know is the Wilderness Medicine textbook, and it has some very useful information on this problem.
Snow is mostly air.
Let's say you stuff yourself full of snow. Unfortunately, the snow you eat is mostly air and not water. So it is extremely hard to get fully hydrated because though you get full, when the snow melts into water you only get a fourth or a fifth of the volume of the snow in water.
The solution to this is sometimes quite easy. Dig. If you have no choice but to eat unmelted snow for water, dig down. The deeper you dig in the snow, the more dense the snow is. And when denser snow melts it yields more water.
Melt the snow first.
Obviously, the ideal solution is to melt the snow first, so you can drink liquid water. The trick here is melting the snow. Wilderness Medicine includes a lot of case reports and has one definite conclusion: PLEASE do not use your body heat to melt the snow. You will increase your risk of hypothermia in a survival situation. Any other method to melt the snow would be extremely helpful to your survival.
