Why Is There Less Snow on the Ground than What Has Fallen?

Many places in Upper Michigan experience plenty of snowfall during the winter months, with some locations averaging over 200”. However, there is never that much snow on the ground at any time. In fact, there is rarely 30% of that much on the ground. For example, during the winter of 2001-2002, 319.8” of snow fell at the NWS office in Negaunee Township. Yet, the deepest snow pack measured during the season was 41”. So far this season, over 120" has fallen, with a maximum snow depth of 33". So, what happened to all of that snow? There are several processes which allow for a lower snowpack than the actual snowfall. Here is a summary of those processes.

 

Compression

This is one of the main reasons snow packs seem to shrink over time or remain low even as snow continues to fall. As snowflakes accumulate, the “fluffiness” of the snow results in air pockets between the flakes. The combination of gravity pulling the flakes downward and snowflakes breaking apart under the weight of new snow removes these air pockets.  This is most evident with lake effect snow, where the new snowfall can compress to half as much on the ground over just one day.  Placing some numbers to this concept, falling snow has a density on the order of 10 to 20 inches of snow per one inch of water while old snow on the ground will have a density of about 3 to 5 inches of snow per one inch of water.

 

Wind

Wind has a similar effect to compression. Snow blown by the wind will fill in the empty air pockets within the snow pack. On a windy day after a fresh snowfall, the snow pack can lower a significant amount because of this effect.

 

Melting

Melting will decrease the depth of snow on the ground as the snow turns into liquid water and seeps through the snowpack below. The melt water will fill in crevasses remaining within the snowpack and potentially refreeze. Otherwise, if the ground is generally unfrozen, the melt water will filter into the soil below the snow. Melting will have the greatest effect with fall and spring snowfalls when temperatures are most likely to surpass the freezing mark on subsequent days. Also, by March, the influence of the sun's energy will be strong enough to melt snow on sunny days.

 

Sublimation

Sublimation is the process by which a solid turns into a gas. In this case, snow (solid) turns into water vapor (gas). This is a relatively minor process and does not remove significant amounts of snow from the snow pack. However, on dry days with light winds and temperatures above freezing, this process can actually deter melting because sublimation cools the air just above the snow pack (just as misters cool air by evaporation).



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