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Camp fire melting ice in a strange way

  1. Feb 17, 2014 #1
    I was sitting by a camp fire this weekend and I noticed that the snow/ice around the fire was melting in a strange way and I couldn't think of a good reason as to why it would leave this pattern. Essentially, it looked like small icicles all aimed towards the fire, almost parallel to the ground.

    I've attached 3 images, 2 of them with my size 11 shoe in the picture to get an idea of scale.

    I imagined that the surface was rough to begin with and as a result, the IR length photons that were depositing the heat on the ice were being absorbed more in some parts than in others. They would dig a hole in a sense. But I couldn't come up with a clear reason as to why they would form this repeating pattern all around the fire.

    What exactly is causing the ice to form into hundreds of little icicles pointing towards the fire, parallel to the ground.
     

    Attached Files:

  2. jcsd
  3. Feb 17, 2014 #2
    The fire needs an influx of air. Cold air coming in just above the ground cools whatever there is under it.
     
  4. Feb 17, 2014 #3

    D H

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    Those look a bit like ice spikes (http://www.its.caltech.edu/~atomic/snowcrystals/icespikes/icespikes.htm). Hazarding a guess, was it calm that night? I suspect the spikes grew as your campfire dropped in intensity. The hot fire heated the ground to just above freezing. The ground later cooled to below freezing as you let the fire dwindle. The fire was still burning, lightly, so it was still propelling warm air upwards. That brought air in at ground level from all around toward the fire. That slight fire-ward breeze and the cooling ground created perfect conditions for making ice spikes.
     
  5. Feb 17, 2014 #4

    russ_watters

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    I disagree. You're saying these grew toward the fire, but if that were true, they would be affected by gravity and would droop -- in addition to requiring a vertical reservoir of liquid water. These are arrow-straight and so must have formed from melting. (the OP can verify)

    I'm not certain, but my guess is that this is caused by a difference in albedo. A dark spot would absorb more of the heat than a light spot, causing differences in melting.

    This is a new one though, and a very interesting/cool question.
     
  6. Feb 17, 2014 #5
    No doubt the pit for the fire was shoveled out giving a variation in density of the snow around the pit. Less dense parts of the snow facing the pit fire will change phase first, leaving denser parts to remain as a solid. The less dense parts, in addition to melting, will sublime, and re-condense onto the denser parts forming a more solid structure and eventually ice, which to re-melt has to have its whole temperature raised.

    Voko also said that cool air to fan the fire is steadily flowing over the snow, helping to re-freeze any melt onto the denser parts. The whole structure of snow can not be raised in temperature, only small parts of it.

    If one looks at a snow bank melting from the sun, a similar pattern of uneven melting forms, though not as apparent as in the photographs.
     
  7. Feb 18, 2014 #6
    I think the key point is that the melting caused by heat concucted by radiation (mostly infrared) not air.

    At first, the surface of the snow gets dark from smoke in some spots and absords heat quite effectivly at those spots. The spots start to melt through the snow but what is important to note this melting continues only on the surfaces that remain orthogonal to the direction to the fire. If the surface becomes slopy toward that direction thus making a grazing angle for the radiation it will not melt anymore : the radiation density will be too low.
     
  8. Feb 18, 2014 #7
    The snow pit was dug out a week ago and there's been constant traffic around it packing down the snow. I took the pictures on day 5/6 since it was dug out and we had a fire every evening. We'd noticed the spikes earlier but I didn't take the pictures till 5 or 6 days after we first cleared out the snow.
    The spikes were broken everyday with people walking around the pit and reformed when we got the fire going. I failed to watch them carefully to see how they were changing over the course of the night, I only noticed them the next day when it was brighter out.
    Soot and ash was deposited on the snow around the fire and the spikes formed almost everywhere around the fire. One night was a little windy pushing the flames in one direction and that area seemed to have more spikes than the rest.

    I do not believe that the spikes grew towards the fire. The boundary of the ice was always further away from the fire after each night, we were definitely melting the ice and pushing it back while forming those spikes. But to be fair, I didn't actually watch them as they were forming, I only looked at them the next day.
     
  9. Mar 9, 2015 #8
    I have had the same experience repeatedly with fresh snow. The heat energy being propagated outwards has to be sporadic and very distinctive spikes to create this pattern.
     
  10. Mar 9, 2015 #9

    DaveC426913

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    This is crazy! Just yesterday I was considering writing a post about this exact same phenomenon.

    I've noticed it around my propane heater in the backyard, and yesterday I've noticed it all over my neighborhood, all the spikes are pointing South toward the blazing sun. It does not occur in the shadows of buildings.


    I can state with certainty that the spikes are forming, not by addition of material, but by removal of material. i.e. the snow is melting away around them. The snow is melting along lines parallel with the heat radiation. It is the points where snow does not melt that forms the spikes.

    snow-melt.png


    I have not determined what is inhibiting the melting at the tips of the spike, but I suspect a combination of dirt and a positive feedback loop of melt-inhibition.


    Some things possibly related:
    1] ever notice how snow banks get very black as they shrink? The grit and dirt picked up by a plow does not melt and eventually ends up concentrated on the top of the bank.
    2] Dark objects, such as dirt and even poop heat up under the sun's rays, and you will find they melt their way down through the snow faster than light objects.
     
    Last edited: Mar 9, 2015
  11. Mar 10, 2015 #10

    A.T.

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    You mean it is created by the Sun itself, not just a heater?

    Even if the net effect is removal of material, there still could be some redistribution of material happening.
     
  12. Mar 10, 2015 #11

    A.T.

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    Assuming the melting is done by heat radiation, while the surrounding air is still below zero: Spikes have a high surface to volume ratio to give heat away to the cold air, but their surface is not perpendicular to the radiation, reducing the radiation intensity. For the concave holes it's the other way around.

    Or the air in holes heats up, and makes them bigger and bigger.
     
  13. Mar 10, 2015 #12

    DaveC426913

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    Absolutely. A walk around my neighborhood found this all over the place. I live on a lake to my south, so it is possible for spots to have dawn-to-dusk sun.
    In places where there were buildings, I saw the spikes pointing, not due south, but toward the area of open sky.

    spikes.png

    If that were true, it would be heavily influenced by gravity, and you would not get these delicate, suspended structures.

    By the way, in terms of time, I can produce these spikes very prominently with my propane heater in one sitting of a cigar - that's 45 minutes. The spikes form a perfect circle around the heater, like a throng of prostrated acolytes, in that time.

    And they point up at a shallow angle, directly towards the heater.


    Yes. This is what I think. (That's what I meant by a "positive feedback loop of melting"). I think spikes are initially precipitated by specks of dirt and the general fractal nature of a snow surface.
     
    Last edited: Mar 10, 2015
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