Needle IceWhen the air cools to below the freezing point on a winter evening, and the sun sets, the

Needle IceWhen the air cools to below the freezing point on a winter evening, and the sun sets, the ground surface loses heat via radiation to the sky. The soil water at the surface then turns into ice crystals that grow slowly downward. If the circumstances are right (the air in and out of the thousands of tiny soil pockets is moist, freezes slowly, and thaws frequently) the result is needle ice, tufts of long, bristle-like ice crystals.The needles lengthen by expansion (water expands 9% on freezing) and by growth (crystallization of more and more water, added to the lower end of the needle). The lengthening needles exert pressure both upward and downward; many tufts are forced up through the surface, carrying little caps of soil on top. Needle size ranges from the typical 5 cm to the rare 30 cm – once the dawn turns into day and the soil warms, the tufts soften and bow over. In addition to soil, ice needles can also grow on fallen logs and other dead wood.Riverbanks are common places to find needles, especially in regions too mild for rivers to freeze. Needles grow best here because there is a lot of soil moisture, and the river acts as a heat sink. This keeps night temperatures at a steady downward decline, instead of plummeting. Soil displaced by needle ice rolls down the slope into the water or are washed away when the river rises. In areas with what I call “Goldilocks” winters (cold but not too cold, this is not a technical term), this is the primary cause of riverbank erosion.Photo: Ruth Hartnuphttps://flic.kr/p/q5u7gRReference: Dr. James R. Carter, Illinois State Universityhttp://bit.ly/1QwIET7 -- source link

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