A garden full of snow looks like a solved water problem. It is not, quite. Snow is mostly air, and turning it into water costs a surprising amount of fuel at exactly the point in a winter emergency when fuel is the thing you are rationing.
Worth doing, worth doing properly.
How much water is actually in it
A full 10 litre bucket of ordinary snow gives you about a litre of water. That is the number worth internalising, because it changes how many trips you plan and how much fuel you budget.
Ice is dramatically better on every count: it yields nearly its own volume, and because it conducts heat far better than snow it melts faster for the same fuel. If there is ice available, from a frozen butt, a pond edge, or icicles, take the ice first and leave the snow.
Never eat it
The starter-water trick
Putting dry snow into a dry pot over heat is the classic mistake. Snow is a poor conductor, so the snow touching the metal melts, the rest bridges over the gap, and the pot scorches with nothing in contact with it. On a lightweight pot you will burn through it. Everything then tastes of scorched metal.
Always start with a few centimetres of liquid water in the pot. Keep back a cupful from the last batch specifically for this. Add snow to the water a handful at a time, letting each addition melt before adding the next, and keep the pot at least a third full of liquid throughout.
If you have no water at all to start with, add snow very slowly, keep the heat low, and stir constantly until you have a liquid base to work from.
Two more things worth knowing:
- Compact the snow before it goes in. Squeezing it into a snowball first gets far more mass into the same pot volume and cuts the number of cycles.
- Keep a lid on it. Melting snow with the lid off wastes a large share of the heat and loses water as steam.
What it costs in fuel
The page opened by saying melting snow costs a surprising amount of fuel. Here is the number, because it changes the planning.
Melting ice into water absorbs roughly 334 kJ per kilogram, and it absorbs all of it at zero degrees without the temperature rising at all. That is the latent heat of fusion, and it is why a pot of snow sits there apparently doing nothing for a long time.
Raising that same litre from zero to boiling takes about 419 kJ.
So nearly half the fuel goes into producing water you cannot see being produced. In practice it is worse than that, because a pot of loose snow has poor contact with the base and much of the heat goes past it into the air.
What follows from it:
- Budget roughly twice the fuel you would allow for boiling the same water from a tap.
- Take ice over snow every time. Ice is nearly ten times the yield per pot load and it conducts heat far better, so it melts faster for the same fuel.
- Melt in bulk, not by the cupful. Bringing a pot up to temperature repeatedly wastes the heat in the pot itself.
- Insulate the pot if you can, and always keep the lid on.
- Do not boil what does not need boiling. If the meltwater is going through a filter, or is being chemically dosed, the melting is all the fuel you need to spend. See judging raw water before you treat it.
Fuel is the constraint in a winter emergency, not snow, and this is the page where those two meet. See split, stack and season firewood and what to burn.
Melting without fuel
Worth knowing, because the fuel arithmetic above is the whole problem and these methods cost none of it.
- Body heat. A bottle of snow inside your coat, against your torso but not against skin, melts slowly and reliably while you do something else. Slow, and it costs nothing and it is always available.
- Sun and a dark surface. Snow spread thinly on a dark tarpaulin, a black bin liner or a dark tray, angled towards the sun, melts on days well below freezing. Angle it so the meltwater runs off into a container rather than soaking back into the snow.
- A dark container of packed snow in a south-facing window is the indoor version and works on any bright day.
- Residual heat. A pot of snow on a stove that has gone out, on a warm hearth, or beside a fire rather than on it, uses heat that would otherwise be lost.
- Indoors, in a bucket. Simply bringing snow into a heated room melts it eventually, and it is the cheapest method there is if you have any heat at all. Cover it, and expect it to cool the room slightly.
- Layer it in a bag hung above a container, so the meltwater drips out as it goes, which is the classic and needs no attention.
Combine them. Start a batch melting passively in the morning and finish it on the stove, and the fuel cost drops considerably.
Choosing what to melt
Snow is not sterile. It is condensed atmospheric moisture that has fallen through whatever is in the air and then sat on whatever it landed on.
Take it from a clean, undisturbed surface well away from:
- Roads and pavements, which means road salt and vehicle residue.
- Roofs, unless you already collect from them and know what they are made of.
- Anywhere animals have been. Discoloured snow of any shade goes unused, no exceptions.
- The top surface after a still, dry spell, which collects deposited dust and soot.
Dig down and take clean snow from below the surface crust, not the top layer.
Then treat it as you would any surface water. Melting is not disinfecting: it happens well below the temperature that kills anything. Filter it if it is visibly dirty, then boil it or dose it with bleach at the ratios on the water page. If you have already built a biosand filter, meltwater is exactly the sort of low-sediment input it handles well.
The one genuine bonus: meltwater from clean snow is very low in dissolved minerals, so it is kind to a hollow-fibre filter and will not scale a kettle.