Featured News | General | Science

The Sound Beneath Your Boots

Needle ice formations on a recent cold, wet December morning in the woods of Vermont.

By: Shelby Perry, Stewardship Director
December 19, 2017

There is a specific kind of late fall/early winter day, when the air is cold and crisp but before the ground is covered in snow, that I just love. You know these days, with a clear blue sky above and the glow of the rising sun painting the horizon, when you walk and you just feel grateful to be outside in this moment, listening to the sound of foraging squirrels in dry leaves and the soil crunching under your feet.

I relish these moments; for a long time I got so lost in them that I forgot that the soil around here doesn’t normally crunch. I just carried on, crunching down trails, reveling in the magic of the woods. Of course once I picked up on it I couldn’t help but investigate.

As it turns out that crunching soil is really ice, or more specifically an ice formation known as needle ice. Needle ice is formed when the air is below freezing but the soil still contains liquid water. As the water at the soil surface freezes, capillary action within the soil draws up more water from the soil, which in turn freezes at the surface. The result looks a bit like ice that has been squeezed through a very tiny spaghetti maker: densely packed tiny needles of ice. Often these needles are capped with a thin layer of soil that has been pressed up by the growing columns of ice below.

For needle ice to occur the soil must be dense enough to hold water, but porous enough to contain tiny spaces suitable for the adhesion required for capillary action. The basic science of what draws the needle ice out of the soil is exactly the same as what causes a paper towel to draw in spilled liquid, except with needle ice the liquid is freezing as soon as it is drawn up.

So next time you are hiking along on a crisp cold day, and you hear that telltale crunch beneath your boot, be sure to stop for a moment and check out the intricate and beautiful needles of ice that you just pulverized beneath your boot.

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