Just make sure that they are pumped dry and if really worried, store them with all your "intermediate warmth" stuff at night. In my case, that's the stuff between my sleeping bag and Mountain Serape overbag. Purification is less a problem than procurement in the winter. Melting snow is a long, fuel intensive, process. I will carry a larger container in the winter suitable for that task. It's best to break through ice to get to running water underneath if you can. I've never had a cracked filter in sub freezing temps by being careful to pump all of the water out.

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