Any given layer of snow can be 85 to 95 percent air, with the interlocking arms of individual crystals forming a kind of web around it, but the hardness of a wind-packed layer of old snow may be 50,000 times that of light, fluffy snow. Snow crystals can last an instant and melt. Snow cover varies with virtually everything that influences its fall, from the temperature, moisture, and wind speed of the air to the geography, latitude, and altitude of the ground on which it falls. And since snow falls on roughly half of the North American continent, its characteristics and personality are as varied as the people it falls upon. The places where snow falls most heavily are the world's mountain ranges, which make up about 20 percent of the earth's continental land mass, and which, by holding weather systems in place, become some of the wettest places on earth.
Individual layers can be thick or thin, frozen solid or soft and damp.
Usually formed by nighttime moisture precipitating onto a cold snowpack, surface hoar is beautifully constructed of feathery crystals, makes for great ski conditions, and is one of winter's most delicate features. Under a microscope, surface hoar crystals are often revealed to take the shape of hexagons, each with sharp angles formed by two opposite sides longer than the other four. This geometry means that surface hoar is also a significant factor in avalanche formation, since hexagons, their angular sides scraping up against each other rather than locking together, have trouble bonding, especially on the horizontal plane. If a layer of surface hoar becomes covered by subsequent layers of snow, it may be able to withstand compression from above but will slide instantly once set in motion. One beautiful, utterly common layer of feathers, laid down in a single night, can mean a month of danger.