It's cold! What difference does snow make?
I've been noticing, for the past week or two, an interesting phenomenon about the soils I've been treading upon between home, work and several field sites. Our daily maximum temperatures have been in the low 20s F (-5 oC) with nightly minimums in the low 10s (-11 oC) for about a week. Yet, the 2 to 4" of snow we've had on the ground has completely prevented the soil from freezing. The soil remains soft and very 'diggable.' In fact, I planted a some small bare root tree saplings at home the other day for lack of any better alternative. I've also been out soil sampling a bit. Digging holes to a 20" depth was no problem at all and I rarely encountered even a small chunk of frozen surface. However, the soil froze immediately to my tools once exposed to the cold air.
I think we've all heard the old adage that snow is a good insulator, but it's hard to fully appreciate snow insulation directly in such cold winter temperatures. I remember attempting to apply this often-repeated principle a few times as a kid. We'd dig holes in the snow drifts and lie inside them expecting, waiting to warm up. I was always disappointed. It didn't seem to work. Or did it? Perhaps my impatient nose and toes were not the proper instruments to measure this effect.
The graph to the right depicts average daily air temperature and average daily soil temperature (at 4" depth) during the winter of 2004-2005 near Escanaba (45.8551 Lat, -87.1844 Long), a very northern Michigan town along the northern shore of Lake Michigan. Though the average air temperatures frequently dip below freezing, and often below 20o, the soil temperature remains above freezing until early January. I can't be sure, but I strongly suspect the soil was covered with an insulating layer of snow during this time. A sudden drop in soil temperature occurs in mid-January - typical of a snowmelt or a big wind event, leaving the soil surface bare, which allowed the subsequent big decrease in air temperature to below 0 oF (-18 oC) for a few days to penetrate the soil and reduce its temperature to below 20 oF (-7 oC). I suspect the soil was re-covered with snow again in late January resulting in a return to soil temperatures right back up around the freezing mark, even though air temperatures remained in the 10-20o range.
Often, winter near-surface soil temperatures are not tightly correlated with air temperatures because of the insulating effect of snow at the surface. Snow cover insulates the soil below in two directions. It prevents cold ambient air temperature from cooling, and it also prevents warming via solar radiation. Snow reflects more solar radiation than soil, so less heat is absorbed into the surface. Snow cover also reduces the temporal variability of soil temperature, reducing number of freeze-thaw changes while the soil is covered. An deeper snow cover insulates the soil more than shallow or compacted snow.
All these effects of snow are extremely important in determining the outcomes of living systems beneath it. Soil and air temperatures under the snow layer, and the number of freeze-thaw cycles the soil endures, affects small animals, microorganisms, insects and invertebrate life and plants in addition to the physical characteristics of the soil itself.
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