With oppressive temps currently baking Southern California, I thought I'd escape for a moment by listing a few tips on melting snow for water that I've learned these past few seasons snow camping. What's attractive about melting snow instead of carrying water is you can save a lot of weight. One can of fuel can easily give you four liters of water or more, which would be prohibitively heavy to carry up the mountain—especially when you factor in the cruel load of winter camping gear and climbing hardware.
Regardless, carrying water becomes moot once the temperature drops past a certain magic number and your bag or bottle of water freezes into a solid brick. So, if you're going to camp in winter, you're going to want to know how to melt snow to get water. Sounds simple enough, right?
Well, I've found it's a little tricker than you'd expect.
Use a Lid
Melting snow takes a great deal of energy—ie, fuel. Efficiency is therefore going to be one of your primary goals. If your melting methods are poor, your water yield per can of fuel will plummet. Here are a few tips that should make a big difference:
Ah, beware the wind. Any wind—and especially a gale—will greatly increase the amount of fuel that it takes to burn a liter of water. You can try cooking in your tent, but the associated hazards are considerable—and potentially even fatal. The safer solution is to find other ways to shelter your stove.
Presumably if it's winter, you're going to have a lot of snow to work with. Dig out a kitchen area, cut snow blocks to make a wind break, and in the most wind-protected end, dig out a little alcove (ie, a cave - see photo 1) in which to put your stove. This won't be perfect, but if you align your kitchen so that the open end faces downwind, it will provide considerably more protection than nothing.
Not enough snow to do this? Use any natural shelter you can find, including piled rocks, bushes, etc. Complement with your own body for extra wind protection. Also, consider using a Jetboil or MSR Reactor stove, which are specially designed to offer superior performance in windy conditions.
Choose your snow wisely
Obviously, you want to choose the cleanest snow possible for your water needs. Avoid discolored snow, and avoid snow that is beneath overhanging tree branches. Staying away from vegetation as a general rule is probably a good idea.
Dig beneath the surface snow, which usually has the most dirt and/or pollen on it, to find cleaner snow below. Whether or not you choose to purify the water you get from the snow is a personal matter. I am generally comfortable drinking unpurified meltwater, though I do filter it through an MSR coffee filter (photo 3) to get the worst of the grit and pine bark out.
It's often convenient to bring a stuff sack to fill with snow for melting at your stove site, as you may otherwise get tired walking back and forth carrying handfuls of snow. If camping in a group, you'll also want to make clear where you're getting your snow for water, so no one pees on it in the night by mistake.
If you do decide you want to purify your melted water, your best choice is probably iodine pills or Aquamira. In winter, purification pumps/filters like the Katadyn Hiker or MSR Hyperflow will tend to freeze and become useless. Also, boiling all your water will not be a viable choice as a purification method. It is too fuel-intensive.
Never Heat a Dry Pot
This is important! Always keep a little water in the bottom of your pot to efficiently transfer energy to the snow. Put an inch or so of water in your pot, then pile in snow. As the snow melts and the water level rises, pour out the water, keeping that same inch at the bottom.
Don't Heat the Water
Another efficiency tactic: don't waste energy heating water. Your goal is to melt snow, not warm up water. Once you've got liquid water, you've got what you need. Toward this end, keep adding snow to the water as room becomes available in your cook pot. In particular, add snow and swirl/stir just before you pour out any water. This helps recover any lost energy spent heating water.
Use a Lid
This is a big one. I've heard that putting a lid on a pot greatly reduces the amount of energy needed to boil water. In practice, I haven't noticed much difference in boiling speed, but I notice a huge difference when it comes to melting snow. I believe that by putting a lid on your pot, you trap hot air and steam inside the pot, which speeds up the melting process, and greatly improves melting efficiency. This also allows you to regularly swirl your pot, mixing up snow and water, without risk of spilling anything. Again, this boosts efficiency.
Find Passive Ways to Melt Snow
You don't need a stove to melt snow. Fill your pot with snow, put on the lid (to keep the bugs out), and put it in the sun while you're out climbing and skiing. You'll have a little more water to drink when you get back to camp.
While you're climbing, you can add small amounts of snow to your liter bottle if temperatures are above freezing. Using this little trick, you can climb with only a liter of water, but drink quite a bit more than a liter all day. Just find a patch of clean snow, stuff some into your bottle, and keep climbing. Remember: there has to be at least an inch or so of water in your bottle for this to work. Don't overfill it with snow: the snow will absorb all the water, and you may end up with an undrinkable slushy. Finally, this method only works in relatively warm, sunny weather.
Another popular passive-melt method is to fill a black plastic bag at camp with snow for a higher-capacity snowmelt engine. The dark plastic absorbs energy from the sun, melting large batches of water for you while you're out climbing.
Utilizing passive methods is an important tactic when winter camping. Anything you can do to use less fuel will be of great benefit. Nothing is worse than hearing your stove run out when you're short on water.