Snow hanging from my car
Snow is an important part of our Niagara Frontier landscape, an aspect about which we have decidedly mixed feelings. We missed it this year when we had none through all of November, we then tired of it after a few days of shoveling, but finally our attitude will change again with the arrival of spring when we review those lovely snow scenes we photographed.
A fascinating article by Ellen Rathbone, who writes regularly for the blog, Adirondack Almanack, led me to rethink my own understanding of the physics of snowpack. Some of what follows is drawn from her column.
As I write in mid-January, our local weather forecast office informs me that we have seven inches of snow on the ground, but that near Jamestown the snow is more than 36 inches deep. The meteorologists also indicate that we have had 49 inches of snow so far this fall and winter, an inch above average but still 16 inches below last year's record to this date.
There is a problem with that information, however. We love to focus on snowfall totals, I suppose because they inform outsiders, and equally important, ourselves how we tough it out here in the Miami of the North - or is it the Point Barrow of the South? But snowfall totals and inches on the ground don't convey some important information. The light fluffy snow that we have had so far this year does not compare with the kind of water-filled snow we often get when conditions are different.
Hydrologists use the term water equivalent of snow, which is the depth of a layer of water having the same mass and upper area, in other words the depth of water that snow would produce when melted. That same information is conveyed by what meteorologists call precipitation. So far this season we have had a little less than six inches of precipitation, which means that the density of our new snow was only about 12%.
Over time new snow is affected by two factors. Sun and wind causes some of it to sublimate and blow away. Sublimation is a physical process by which a solid changes directly to vapor, skipping the usual intermediate step of becoming a liquid. The rest of the snow settles under its own weight, some of it melting and refreezing, until its density reaches 30-50%.
During that melting and refreezing, the individual grains of snow become little ice balls and begin to adhere to each other, thus making the snow mechanically stronger. That is why you see overhanging snow at the edge of your roof.
Also as this is happening, a temperature gradient develops within the snowpack, the temperature near the ground higher than that at the surface. Sublimation occurs near the ground, as well as at the surface, creating air spaces called pukak, passageways through which voles and other small mammals make their way to find food.
I once watched a handsome red fox stop in the middle of a snowfield to listen intently, then suddenly leap up to dive nose-first into the snow to grab one of those mice. Owls hunt that way as well. What amazing hearing they must have. Surely the raucous music of today's youth that shocks my own reduced hearing would deafen such sensitive animal senses.
The temperature difference within the snow causes another effect: water vapor moves upward causing more freezing near the snow surface but at the same time increasing the fragility of the snow near the ground. One result of this in mountainous areas is avalanches, whole snowfields sliding off to leave a bare upper slope, accumulating more snow like those roll-up balls that make snowmen but to less happy purposes as they wreck havoc scouring the slopes below.
Finally, and none too soon for us as the incidence of cabin fever increases, the snow melts. This process is driven by sunlight, rain and fog. By that time the snow is no longer the pristine white we see at this time of year and the dirtier the snow the more it absorbs heat rather than reflecting it.
There will come a day, I promise, when the only snow we'll find will be those few drifts on north slopes.-- Gerry Rising