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Thread: depth of frost?

  1. #1 depth of frost? 
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    I have heard it said that the frost level can become deeper due to a few sunny and warm days after a cold snap.

    Is this true? And if so, how does this work?


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  3. #2  
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    I've never heard of this but I'm going to guess that the frost line can go deeper during the warm spell, but it's in spite of the warm weather, not because of it. During the cold snap the frozen soil is warmer than the air, but colder than the soil below it. Since it is colder than the soil below it, heat transfers from the melted soil to the frozen soil, and the frost line is penetrating deeper. During the warm spell, the frozen soil is still colder than the soil below it, so the frost line continues to go deeper for some period of time. If the warm weather lasts long enough, the heat will penetrate from above.


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  4. #3  
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    It really depends more on the snow cover. Snow is a very effective insulation and a few feet of snow isolates the ground from temperature changes very well.
    Even if the ground froze hard at the start of winter the temperature of the deeper soil is usually about 42 degrees Fahrenheit or 5.5 celcius. In midwinter it is possible to dig through deep snow into unfrozen ground. At the same time you would find the soil under roads, paths or anyplace with packed snow frozen frozen hard, and likely to a respectable depth.
    Here the building code requires foundations to be 4 feet (1.2 meters) deep to avoid frost heaving.

    Permafrosts are a bit different because in their case deeply frozen soils are insulated from summer warmth by plant and soil cover so they never get a chance to thaw.
    Building on permafrost requires the builders to take measure to protect the permafrost from melting under their structures and roads.
    Parts of the Alaska Highway are built on top of layers of foam insulation.

    So if the snow cover melts in a warm spell and loses the ability to insulate the ground from the cold the ground can freeze more deeply than if the snow hadn't melted.
    Note the melting is before the cold snap, not after it in this case.
    Last edited by dan hunter; February 21st, 2014 at 05:12 PM.
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    The frost line below ground catches many people off guard when they turn off their frost protection device (heat tape) too early in the spring. Here, in the Yukon, it is inadvisable to turn off your heat tape before May 15th and most years it is wise to turn it on by October 31st, although the occasional year I have waited until November 30th if we had an early snowfall. Our local newspaper usually posts a notice to remind folks to attend to this matter, especially helpful for newcomers to the territory.
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  6. #5  
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    Quote Originally Posted by dan hunter View Post
    Snow is a very effective insulation and a few feet of snow isolates the ground from temperature changes very well. [...] In midwinter it is possible to dig through deep snow into unfrozen ground.
    In olden times, snow acquired the nickname of "the poor man's fertilizer" because farmers had noticed that the depth of snow during the winter helped predict the abundance of crops the following summer.

    Only relatively recently have scientists understood the cause and effect. Snow insulates the ground and prevents the frost from going deep or at all. The more snow, the shallower the frost layer. The shallower the frost layer, the closer to the topsoil the worms can live. And the warmer the ground, the more active the worms. The topsoil contains undigested, organic matter. The resulting worm manure is nutrient-rich organic fertilizer and soil conditioner.
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  7. #6  
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    OK guys, thanks for the input.....
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  8. #7  
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    Quote Originally Posted by redhawk44 View Post
    I have heard it said that the frost level can become deeper due to a few sunny and warm days after a cold snap.

    Is this true? And if so, how does this work?
    The deeper the frost depth longer the time lag between surface weather and its depth. If it's several feet by mid winter, such as where I was raised, a couple sunny days won't effect the depth for a few weeks.

    This is also true for temperature at depth--something I learned while studying solar homes some 30 years ago, as well as a feature climatologist have used for boreholes as a proxy for surface temperature as long as a centuries ago.



    Last edited by Lynx_Fox; February 23rd, 2014 at 01:56 PM.
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