Surface water can be contaminated with pathogens both directly and via runoff. However, waterborne pathogens don't go with water when it evaporates. Water fresh from the sky should in theory not need to be purified from biological agents. The only contamination in fresh snow should be the airborne particulates trapped in the snow when it forms in the atmosphere. These are potentially harmful to human health, but being non-living will not be affected by boiling. If you were seriously concerned about contaminated snow, you would want a filter that would catch both PM10 and PM2.5 (particulate matter between 2.5-10 microns and less than 2.5 microns). But filters are liable to freezing and I don't think many people carry them in winter. Now if you're melting water, it's likely you're going to be using at least some of it right then for a drink or meal, so it makes sense to just boil it. But as long as you are not using old or dirty snow, boiling should not be necessary. It should also be possible to melt a bottle's worth of water and just continue adding snow to your bottle to keep it topped up, assuming you take care to keep the bottle warm enough for the snow to be melted. This would save both time and fuel.

I'm expecting someone to link me to an old thread where this has already been discussed, b/c few of my "awesome" ideas have turned out to be original. But I thought I'd throw this one out there anyway since the videos I have seen testing stoves' melting capabilities take the water all the way to boiling and that seems unnecessary unless you actually need boiling water right then.

Edited by spelt on 10/14/2011 14:26:00 MDT.

check these out

Edited by meld on 10/14/2011 15:00:16 MDT.

"The only contamination in fresh snow should be the airborne particulates trapped in the snow when it forms in the atmosphere."

Haven't you ever seen muddy little animal tracks in fresh snow?

You don't know where they've been, and animals don't wash after using the toilet.\

--B.G.--

That's been my take on snow water as well. If it's fresh and clean, it's clean enough. Thing is, in the snow I am usually looking for something hot to keep me warm, so I end up getting it close to boil anyways.

Bob. I assumed we are talking about obviously clean, untouched, white snow.
Yes, if it has brown stuff on it, it's another story....

Yes, I was assuming a person would avoid snow with visible disturbance or dirt visible to the naked eye.

@ Spelt:
I could not imagine otherwise.

Kat, yup!

Interesting about the pasteurization. Makes sense but I had only ever thought about it in terms of commercial food safety. Of course without a thermometer or a tuned sense for the right temp, it's easier just to boil. And the time required at pasteurization temp might well put you at a boil anyway as your stove runs while you wait.

While cross-country skiing, I have often been out during a snow storm, and I have followed fresh animal tracks through the fresh snow. When I return 30 minutes later, the tracks are invisible, but they are still there.

Normally when we collect fresh snow to melt, we scrape off the top one inch of snow, and then collect snow underneath that. However, that still isn't perfect.

--B.G.--

Bob, fair enough. Good cautionary measure to only scoop shallowly. I still think it would be less risky not to treat snow from where an animal may have walked than from a stream where you know many animals have been. Just looking at the surface area available for snow collection vs concentrated flows of liquid water, it has to be less likely that an animal has walked through your source point if your source is snow.

It takes a lot of energy to melt snow. So, you'll always be better off if you can find any liquid water. Just don't fall into a stream at -40. If it's cold enough that there is no liquid water you'll probably want hot water anyway and if you want it to stay liquid you'll get it pretty hot. Heavy hot tent campers in the north carry a crow bar to chop through lake ice to get liquid water.