Originally aired during the week of January 15, 1999
If you hear of someone described as "pure as the driven snow," it's not necessarily a compliment. The seed of every snow crystal is a tiny airborne speck of dirt. In very cold air, water vapor can freeze around these particles. The size and shape of a snowflake depends on how cold the air temperature is: the higher up you go, the colder it gets. Those wispy cirrus clouds you sometimes see on a winter day (some people call them "mares tails") are really collections of ice crystals drifting way up there in sub-zero temperatures. In warmer air, further down in the atmosphere, ice crystals grow more slowly and their shapes are less intricate. Most snowflakes have six sides - they are hexagons, and each has its own distinctive design. Native Vermonter William "Snowflake" Bentley is famous for capturing the beauty and variety of these shapes using a camera and a microscope.
Sedimentary rock forms when small pieces of mineral, carried by water, settle and harden. Limestone, for instance, is rock formed very gradually from the skeletons of tiny sea animals that settle onto the ocean floor. The same kind of sedimentation occurs in winter when new-fallen snow covers the ground. Take a close look at the top layer of fresh snow. You can often see individual snow crystals leaning against one another. Even if nothing disturbs the snow, the flakes will lose their crystal structure and become granular. The snow compacts together and settles. Dig into a snowbank late in the winter with a shovel, and you might notice the snow has definite layers, each one representing a different snowstorm. It's sort of like a history of the winter snowfall! We see the same layering effect when we look at walls of sedimentary rock.
Grab a handful of snow, pack it tightly together, and you have the ultimate winter weapon. And perhaps a ticket to the principals office. Making a snowball is also a "cool" science experiment because it has a lot in common with geology. Sedimentary rock changes into a different form when it undergoes heat, pressure or other forces. For instance, limestone, which is formed as sediment gathers on the ocean floor, will "morph" into marble when its under enough pressure. Making a snowball is an example of the metamorphic process, because we're using force to compress the snow tightly in our hands. The outside of the snowball will melt a little bit from the warmth of your hands. Cut open the snowball, and you'll see the fluffy handful of flakes you picked up are now packed tightly together. It's more than a snowball - it's metamorphic, man!
After snow has been on the ground for a while, it goes through lots of changes. The snowpack may settle or drift from wind or changing temperatures. Freshly fallen snow can be described as an "ice skeleton" - a loose pile of crystals that traps huge volumes of air. Cullen Murphy, in a recent article in
Atlantic Monthly magazine, described an experiment showing the ratio of snow to air in a fresh snowfall. A big loaf of Wonder Bread was compressed to its smallest possible size (a two-inch cube!). That's sort of what happens to the snow after it's been around a while. You may see distinct layers of snow when you slice through a snowbank with a shovel. An early season snowfall of six inches might be compressed into a layer one inch thick. If the snowpack doesn't melt entirely, it constantly changes in density, mass, and texture.
When molten rock cools and hardens, it is referred to as igneous rock. Granite is a common example of igneous rock. Water can be thought of as a mineral in molten form - and when it hardens due to freezing temperatures, it becomes ice. Ice and snow, like igneous rock, will eventually weather and erode over the course of the winter. Of course, rain, which we've had a lot of this winter, will destroy the snow's structure. Wind can also move and carve the snow in the same way it blows around grains of sand. You might notice the wind causing snowdrifts, or creating wave patterns in the snow. The geologic cycle takes millions of years to complete - igneous rock erodes, creating sedimentary rock, which, after heat and pressure is applied, turns into metamorphic rock. This cycle can be observed in the snowpack in your backyard every winter.
