Snow is free water until you try to drink it

Quick answer: Melt snow on a stove, with a little water already in the pot, and plan on about an ounce of canister fuel per quart. Eat snow only when you're warm and moving, never as your main water when you're already cold.

Fill a one-quart pot to the brim with fresh powder and set it on the flame. When it's done melting, you'll have a thin layer of water in the bottom. Snow is mostly air. That's the first surprise, and the fuel bill is the second.

Why snow is the most expensive water you'll make

Melting takes far more energy than most people expect. It takes about as much heat to turn a quart of snow into a quart of freezing water as it does to warm that quart of water from freezing to around 176°F. And it isn't even liquid yet when you've spent it.

So think of snow as water you pay for twice. Once to melt it, and again to warm it to something you can drink or cook with. Ice is better than snow. It's denser, so each pot holds more water and you refill less often. If you have a choice between a fresh drift and a hard, clean block of old snow or ice, take the dense stuff.

Why eating snow chills you

Your body can do the melting, but it doesn't do it for free. Every quart of snow you swallow pulls heat out of you: the energy to melt it, plus the energy to bring it up to body temperature. Run the arithmetic and a quart costs you somewhere around 100 to 120 calories of heat. You'd also need a lot of snow to get that quart, because of all the air in it.

Do that when you're cold and underfed and you're burning the very thing keeping you steady. Your lips and mouth cool down too, and cold hands holding wet snow lose heat fast.

Be fair about it, though. A handful of snow on a sunny ski day, when you're sweating and warm, isn't going to hurt you. The problem is relying on it as your water when you're already chilled, tired or stuck.

The same logic applies to the old trick of melting snow in a bottle inside your jacket. It works, slowly, and it spends your body heat to do it. Use it as a last resort, and only with a bottle that's mostly water, not packed with snow.

Melting it right

1. Start with a splash of water. Put an inch of water in the pot first, from a bottle or a stream if you have one. Dry snow in a dry pot can scorch the bottom and leave a burnt taste in every cup. It can also damage thin cookware.
2. Add snow in small handfuls. Let each one collapse before you add the next. Pack it down with a spoon. Don't dump a full load on top and let it sit as an insulating lid over the burner.
3. Keep the lid on. It saves more fuel than anything else you can do for free.
4. Use a windscreen. Wind steals heat from a canister stove faster than cold does. Bend foil around the pot (not around the canister, which can overheat) or use the one that came with your stove.
5. Set the stove on something flat and solid. A square of foam pad or a small board works. Snow softens under a hot stove, and a tipped pot of boiling water is a burn you don't want.
6. Pick clean snow. Skip anything pink, yellow or gray. Stay away from trail edges, roads, your camp's bathroom area and spots where animals have been. Take it from a deeper, untouched layer.

Do you need to boil it?

Melted snow isn't automatically safe. If you're deep in clean backcountry, many people judge the risk low, but you can't see what's in it. A rolling boil is the sure fix: one minute at most elevations, three above roughly 6,500 feet, according to the CDC.

Boiling costs fuel, which brings us to the next problem. And don't count on a filter in freezing weather. If a pump or hollow-fiber filter freezes with water in it, it can crack inside where you can't see it, and then it won't work. Keep it in a sleeping bag or an inside pocket at night.

The fuel math

These are rough planning numbers from the physics, not the result of a stove test. Your stove, the wind and your altitude will move them.

• Melt only: plan on about an ounce of canister fuel per quart.
• Melt and boil: plan on about two ounces per quart.

Take two people who need around six quarts of water on a winter day. Just melting it is roughly six ounces of fuel. Boil all of it and you're closer to twelve. Add breakfast, dinner and a hot drink at the end of a cold day, and a standard eight-ounce canister doesn't last as long as you'd think.

So do the sum before you leave, and then add a canister. Fuel is cheap, and running out of it in the snow is not.

Gear that makes this easy

Roughly what you'll spend:

• Canister stove with a pot: An MSR PocketRocket-style stove runs about $50. An integrated system like a Jetboil is usually $100 to $150. It's more efficient with a lid and heat exchanger, which matters when every ounce of fuel is water.
• Liquid-fuel stove: The MSR WhisperLite is around $100 or a little more. It burns white gas and does better in deep cold than canisters, which lose pressure as it drops. If you stick with canisters, keep the next one warm in a jacket pocket.
• Fuel: An eight-ounce canister is typically $8 to $10.
• Wide-mouth bottle: A Nalgene-type bottle, about $15 to $20. Check that it's rated for hot water before you pour boiling water in. Store it upside down in your pack so ice forms at the bottom, not in the threads.
• Insulated sleeve: $10 to $15. It buys you hours before the bottle freezes.
• Pot: Aluminum is cheap and heats evenly. Titanium is lighter but runs $30 to $40 and can scorch more easily. Whichever you pick, get one with a lid that fits.

A bonus: pour hot water into a bottle and put it in your sleeping bag at night. It's a heater, and you'll have liquid water in the morning.

Two safety rules that matter more than the stove

Never run a stove inside a closed tent. It can fill the space with carbon monoxide, and you won't smell it. Cook in the vestibule with the door open, or outside, and keep fabric well back from the flame.

And if you're lost or stranded, water comes after shelter and warmth. Getting out of the wind and getting dry matter first, as laid out in what to do in the first hour you're lost in the woods. Cold also dulls thirst, so drink on a schedule, and check the color when you pee. Dark means drink more.

Take a class before you need it

Reading gets you the idea. Hands get you the habit. Pack your own stove, practice in your backyard on a cold day, and see how long a quart really takes with your gear. Then book a winter skills course from a qualified instructor, and an avalanche-awareness course if you'll be anywhere near mountain slopes. They'll correct the mistakes you can't see yourself making.

The first time you melt snow shouldn't be the night it matters. Do it in the driveway with the porch light on and a hot drink waiting.