We couldn't ski without it, but how many of us have ever paid really close attention to that amazing thing called snow? Reduced to basics, snow is the precipitation of ice crystals. But snow is far from basic. Compressed into glaciers, snow quite literally formed our continent. It fertilizes fields and generates electricity. It's a building material, and has even been used as a weapon: During World War I, armies in the Tyrol caused deliberate avalanches, killing about 60,000 soldiers.
The miracle of snow begins in the "troposphere," the first layer of the atmosphere, where moisture from the oceans and sweaty skiers collects. If it's cold, that moisture finds an attractive "condensation nuclei" (minute bits of sea salt, volcanic ash or dust) and goes through a process called sublimation. To Freud, sublimation had to do with, say, converting one's urge to have constant sex into an urge to build giant skyscrapers. To scientists, it's what happens when water vapor becomes a solid without passing through the liquid state first. Gas to ice, hold the water. Sublimation works in the opposite direction, too; it's what causes ice cubes to shrink in your freezer.
Once a snow crystal is born, a skier's on-mountain happiness is dependent on the temperature. If those crystals blow around in a cold, moist troposphere for 12 hours, they can grow half an inch wide. If they then fall through sub-freezing air, several hundred crystals might join together to form flakes that are inches in diameter, causing skiers to jump for joy. But if they fall through warm air, they land as rain. And sometimes snow does neither: It disappears altogether.
If it's cold on the ground, crystals keep their shape and the snowpack stays fluffy. If the ground is warm, the edges of the crystals melt, moisture gravitates toward their centers, and they turn into round granules. If you look at old snow, you won't find many puffy six-sided flakes; it looks more like sugar. Snow from a gun resembles tiny ball bearings. A thick snowpack usually contains a "temperature gradient," meaning the upper layer is colder than the layer closer to the ground. This speeds up the granulating process, gradually creating that spring skiing delight called "corn." If snowpack doesn't melt away, these granules will gradually bond together into ice or even glaciers.
In 1652, a Bavarian court found witches to be the cause of avalanches and recommended that they be put to death. Needless to say, this didn't solve the problem.
Those Bavarians might have done better to peer down deep under the snowpack at something called "depth hoar." If the temperature gradient in a snowpack is high enough, the bottom doesn't firm up. No, our old pal sublimation reappears, causing the growth of fragile ice walls and spaces, which creates an unstable base and an easy trigger for avalanches.