The Snow that Warms

An article in Nature Sauvage, winter 2008 issue was entitled “le Froid qui Mord.”  True biting cold can even kill but, for many organisms, the disabling effects of cold are the real problem. Species that can’t control their own body temperature internally, the ‘cold-blooded’ species. don’t die because the cold overpowers their abilities to control their internal temperature. They die because the cold prevents any kind of activity. For these cold-blooded invertebrates and vertebrates, find a way to not lose heat and you may survive.

Snow helps here. Loose, fresh snow crystals trap millions of little cells of air. It insulates well. Snow crystals metamorphosed by heat or, in the extreme, ice, are much less effective as insulators.

But what is the source of heat that can be kept from escaping by a blanket of snow? The earth under the snow radiates heat as infra-red radiation. That heat comes partly from solar energy stored in the soil and rock during the previous warm season. But important amounts of earth’s heat come from radioactive decay of isotopes such as potassium-40 in the soil and rocks and, some comes from magma heat deep under the crust.

For a test of the effectiveness of snow insulation trapping earth’s heat radiation, J.C. Randolph studied the short-tailed shrew, Blarina brevicauda, in winter, near Ottawa. Winter limits the food available for these, and other, shrews. Results from an inventory of available foods and from the measurement of the metabolic requirements of the shrews showed that without the warming effects of the earth’s radiation, and the trapping of that heat by the snow, the food was not enough to let the shrews survive the winter.

In other studies of invertebrates under the snow, near Ottawa, we found that ‘cold-blooded’ representatives of 44 families of insects and spiders were moving enough under the snow that they fell into pit-traps set into the soil. Thousands of springtails, (collembolans) coated the surface of the preservative in the pit traps, their last, deadly hop. Many harvestmen also were moving under the snow as were predators such as spiders and many other invertebrates.

Temperatures above the snow fell to below -30C but temperatures between the snow and the leaf litter remained as high as +0.5C and the cold-blooded invertebrates were active in the space between the snow and the leaf litter. Under the snow, the soil was thawed but within minutes of digging a pit down through the snow to the soil, that previously unfrozen soil would become solid. The effect of the snow blanket was readily apparent to us. Especially when we found green plants growing under it. Club mosses were not only green but also had produced spores and maple seedling were sprouting their embryonic leaves, (the cotyledons). You may have noticed that when those cotyledons appear out of the spring snowmelt, they often have penetrated some of the dead leaves from the litter. That happens because the dead leaves are held down by the snowpack and the growth of the new maple leaves punches through them.

This club moss was evergreen under the snowpack. Note the yellow spore cases
In the axils along the main stem.

Wolf spiders crawl upside down using the ‘roof’ of the subnivean space as a highway to hunt under the snow when it may be -30C in the air above the snow.

That “subnivean” space under the snow develops over winter by the warmth from the earth below peeling off molecules from the snow crystals above. The points of the snow flakes are the most unstable thermodynamically, so they peel off first and the slightly warmed water moves upward in the spaces in the snow pack. As it cools, these water molecules are redeposited on snow crystals above, gradually reshaping the snow crystals into rounded, ‘popcorn’. The undersurface of the snow forming the roof of the subnivean space becomes a highway for spiders and other long-legged species. Wolf spiders and harvestmen, or ‘daddy long legs, travel commonly on this upside down highway.

Yes snow does warm many species, not just invertebrates. The Inuit know this well.

1 comment to The Snow that Warms

  • Always a favourite topic. Years ago when our daughter worked on a Grade 6 science project I helped her with an exploration of lichens under the snow. She was amazed at what else was under there. She won first place in the science fair competition.

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