Do You Purify Water from Melted Snow

If you melt snow for drinking water in the winter, do you need to boil it for purification? ~ Photo by Paul David Gibson

As kids we used to eat snow and icicles all the time during the winter, with no ill effects.  If we follow some common sense guidelines we can use snow while backpacking as a plentiful supply of water.  The question we often debate is: “If you melt snow for drinking water in the winter, do you need to boil it for purification?”

Water from melting clean white snow is generally considered safe to drink.  We are probably at greater risk for getting giardia from hiking partners who do not practice good hygiene than getting sick from snow water.

Nevertheless, we often have to boil water anyway for cooking and warm drinks, so if you do end up heating the water to that point you’ll get some peace of mind.  By the way, don’t melt ice.  It may seem like a good idea, but harmful bacteria can live for months in ice.

Choose your snow wisely

Clearly, you want to choose the cleanest snow possible for your water needs.  A good start is to avoid collecting snow that has any color, except white! Fresh fallen snow is safest. The longer snow sits on the ground the more nasty stuff falls on it and is absorbed from the layers of snow beneath.

Choose snow far from game trails, water sources, and other locations where animals may have done their business. Find clean snow away from trees to avoid snow with bird droppings and other stuff falling from trees. Dig beneath the surface of the new snow to find the cleaner snow.

Avoid all “colored” snow:

Did you know that snow sometimes comes in colors?  You’re no doubt familiar with yellow snow, but how about red, green, or brown?  If the snow isn’t nice and white and fresh–avoid it.

  • “Yellow” snow – no explanation needed, right?
  • “Red” or “Watermelon” snow – contains an algae that thrives in freezing water. The algae have a laxative property.
  • “Green” and “Brown” snow produced by algae are also seen.

When you find the perfect snow, gather it in a small tarp, stuff sack, or a pulk and haul it back to camp.  This conserves your energy by making one trip to the “good” snow field.

How much snow would we need to melt for our water needs?

In my unscientific tests, I melted 1100 ml of packed snow, which yielded approximately 470 ml of water.  This is about 2 cups of water,  not counting the cup of water I added to the pot at the start.  The water was heated just enough to melt the snow.  If you boil the water, more will evaporate through steam, of course.  The yield was approximately 43% of the packed snow volume.  Your mileage may vary.

Ice is denser than snow and would produce more water through melting.  However ice may contain harmful bacteria that could be released upon melting.  So, unless you plan to boil the water produced from ice, just use fresh snow instead.

How to Melt Snow the Right Way

The snow-melting process seems like a no-brainer, but there is a right way to do it.

Contrary to what you might think, you can burn water.  At least I can.  If you don’t pay attention while melting the snow, you can burn water, too.  What actually happens is that the water in the pot evaporates and leaves the snow just sitting there, thus scorching the bottom of the pot.  The pot smells nasty and the water tastes like burning tires. (If I knew what that tasted like.)

The Process:

  • Incorrect: Pack snow into a pot and put it on the fire. Ignore it.
  • Correct: Put about an inch of water in the cook pot and heat it.  Start adding a little snow and stir as it melts. This takes time.

If when you add the snow to the pot it absorbs all of the heated water, then you didn’t start with enough water in the pot.  Add more water now or you will end up scorching the water. You’ll also want to use a lid on the pot to conserve heat, but open the lid to stir the water as the snow melts. If you’re using a stove, a wind screen will definitely help to use fuel efficiently.  (You knew that though!)

Does the Water from Snow Need to Be Purified?

Not necessarily. If you’ve chosen a snow source wisely, purification isn’t as critical.  Melting the snow without boiling should be all that is needed. The water is likely safe to drink, but it will have “stuff” floating in it.

Stuff is a technical word.  Even fresh snow has stuff in it that was picked up in the sky when it was forming.  It’s the same stuff that ends up in bodies of water when it rains.  Stuff — dust particles, pollen grains, seeds and plant and insect fragments are carried by winds and deposited with snow on the surface.  However the presence of some of this stuff in new snow doesn’t mean there’s enough of it to make you sick.   This floaty “stuff” in melted snow bothers some people.  So strain it out and drink up.

What If I Want to Purify It?

If you want to purify it, go for it.  You’ll have to either boil it or use Chlorine and iodine based water disinfectants.  If you prefer the latter, you should heat the water to at least 60 degrees Fahrenheit though. Chlorine and iodine based disinfectants will not work in ice cold water.  When the water is warmed though, you can use AquaMira, Potable Aqua Tablets, Katadyn Micropur Tablets, or your water filter.

Go with your gut.  If you have a sensitive system, disinfect your water.  Otherwise, if you are drinking smart, melting freshly fallen snow for water, you are probably good to go.

Go Ahead and Boil Water if

Here are some reasons why you might want to boil the water anyway:

  • You worry a lot about what’s in your water
  • You want a hot drink
  • Cooking your meal requires hot water
  • You’ll be going to bed shortly.  (see “Keep Water from Freezing” below)
  • You are not rationing fuel for a fire or stove

Keep Water from Freezing

Fill up water bottles with hot (not boiling) water and put them in your sleeping bag.  They’ll help warm you up, and you’ll keep them from freezing.

Extra water from melting snow can be stored overnight by burying the cook pot with its cover under about one foot of snow.  The air trapped in snow makes it a good insulator.  Even when the air temperature is below freezing, water stored in this way should be prevented from freezing.

I hope this post will help answer some questions you might have had about purifying water made from snow.  If you have other insights you can share on this topic, please join the conversation by leaving a comment below!

Water Purifying Resources:

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