Denser layers below can be stronger than fluffier layers above, and become increasingly so as the weight above increases. Analogies for this are hard to come by, since snow has such unique physical properties. It is as if bread dough, spread over a tilted cutting board and adhering to the wood, began to stretch downward until the gluten bonds snapped and sent the released half crashing to the floor. This is particularly true given the wide variety in the shapes and sizes of individual snow crystals. 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.