We’ve all seen it: a branch, a fence post, a sign where the snow that fell upon it seems frozen in a perpetual state of falling off, never quite letting go. How does it do that? It’s not like snow is endowed with abs of steel, or, like a snake, has near mythical suspension abilities thanks to overlapping scales. Or does it?
I’ve been taking the time this winter to read up on, well, winter itself. To most folks snow is merely frozen rain, end of story, but there are people out there who make it their life’s work to study snow, in all its glory, and they have discovered some pretty amazing things. I’m not just talking about how every snowflake is different, or how when water freezes it expands and floats. That’s old hat. I’m talking about the hidden characteristics of snow, those little details that make or break winter survival for small mammals and alpine skiers. It’s pretty amazing stuff.
Snow, as it turns out, has three different phases of metamorphism: destructive, constructive and melt. Each one has its own set of characteristics that influence the snowpack around us. We’ll start with the first and work our way through them.
Let’s say it’s a typical winter day here in the Adirondacks. Flakes of snow drift lazily towards the ground, eventually landing to add their fluffy mass to all those that have gone before. From the moment a snowflake forms, it is acted upon by myriad outside forces, wind and temperature not the least among them. Their delicate forms, be they needles or plates, are battered, melted, and reshaped. Ultimately what remains is a rounded grain of ice. Smaller grains are absorbed into larger grains until all the grains of ice in the snowpack are comparable in size. The warmer it is, the faster this happens.
As you can probably guess, each time a small ice grain melds into a larger one, the associated pockets of air around the grains decrease. Bit by bit, the snowpack gets denser. The grains bond together more strongly. As the bonding increases, so does the mechanical strength of the snow. This is what allows piles of snow to ooze over the edge of a supporting structure while never actually letting go, like in the photo above.
Next comes the constructive metamorphism, alternatively referred to as the creation of a temperature gradient within the snowpack. Basically, what happens is this. The snow at the bottom of the snowpack is warmer than the snow on top. This makes sense, since the earth is warm and the air is cold. Because it is warmer below, the snow at the bottom of the pack begins to sublimate, turn from a solid directly into a gas, in this case from ice to water vapor.
It’s a basic property of physics (the Second Law of Theromdynamics) that stuff migrates from areas of high concentration to areas of low concentration. Therefore, the water vapor produced at the lower portions of the snowpack moves upwards towards colder and drier regions. As the vapor cools, it re-condenses on the ice crystals around it, increasing their size and thus their strength. Meanwhile, sublimation continues below, shrinking the size of the ice crystals at the bottom, reducing their strength, and creating what is known as depth hoar, a very loose and fragile type of snow that makes travel easier for the small mammals that run about under the snowpack while at the same time making it more treacherous for alpine travelers – think avalanche.
The final change that occurs in snow is the melt metamorphism. This may seem pretty straight forward: the air warms up, the snow (and ice) melts. Ah, but the devil is in the details, and this is no different with snow.
Fresh snow is nature’s best reflector of shortwave (solar) energy. Translation: it keeps away the heat of the sun, thus reducing the likelihood of melting. However, as we all know, snow rarely stays in that pristine white condition. Road salt and sand, bird seed and old leaves, branches, bits of bark, cone scales…all sorts of stuff gets onto the snow, changing it from the perfect reflector into something that readily absorbs longwave energy, what we commonly think of as heat. This heat is coming from earthbound objects, like trees. Trees are dark; they soak up the solar energy (shortwave) and release it (longwave) to the surrounding environment. This is why trees (and rocks, and buildings) are often seen with a dearth of snow at their bases – it has all simply melted away.
The details of melt metamorphism have to do with energy exchange: the energy released when a solid becomes a liquid, and the energy required for liquids to refreeze, and how this affects the snowpack as the liquid moves downward through the layers. Suffice it to say that at the end of this process, the entire snowpack has reached a uniform temperature.
Rain and fog continue the melting process. Again, we are looking at energy transfer, but this time from the atmospheric moisture to the snowpack. This is directly influenced by the air temperature and the amount of moisture that is penetrating the snow. In the final analysis, as water evaporates and condenses, energy (heat) is lost, which melts more of the surrounding ice, which evaporates, and condenses…until all the solid water (ice) is gone (either liquid or vapor).
What all this sums up to is the simple fact that the snow beneath our feet is constantly, and I mean constantly, changing. From moment to moment it is never the same. This gives whole new meaning to the phrase “there is nothing permanent but change.” So, we should all go outside and enjoy the freshly fallen fluffy snow while we can, for tomorrow it may be gone.