Believe it or not, water comes in many forms, or "phases." What we recognize as water is, in fact, the liquid state of a combination of molecules of hydrogen and oxygen -- 2 hydrogen atoms and 1 oxygen atom, or H20. But water can also be found in the form of a gas (steam) and in a solid state (ice, snow, crystals).
At this time of year, most of us in the northern hemisphere experience lots of solid water. We usually begin by getting some snow flurries, perhaps some overnight frost, and finally, for those who like to ski and otherwise have a disastrous winter adventure, SNOW! If we're fortunate, this solid water will behave itself and come in manageable amounts -- just enough to cover the ground with a nice "warm" blanket and just often enough to keep everything clean and pretty. Usually, however, it prefers to come down in big blobs so as to keep the road crews busy making piles in the streets.
The Deep Freeze
Pure water freezes at 0°C (Celcius), and 32°F (Fahrenheit). Incidently, it boils (at sea level) at 100°C and 212°F. This is true for "pure" water at "sea level" because dissolved solutes in water or pressure at various altitudes will have an affect on these temperatures. Just ask anyone who has tried to boil water high in the mountains!
Anyway, one of the more interesting things about water is its comparative weight in different states. When ice crystals form in a cloud, they fall because they are heavier than the air. But the ice on a pond (or in your favorite drink) floats because the solid state of water is lighter than the liquid state. Now think about this for a minute. Imagine what would happen if ice were heavier than liquid water. First off, any ice forming on the top of a pond would sink. Soon, the fish and other life forms in the water would either be frozen solid in blocks of ice, or would be crushed under the weight of the ice on the bottom of the pond. Not very conducive to making it through the winter!
Underground Tunnels
I mentioned above that snow can cover the earth like a "warm" blanket. This property of snow is very important in many areas of the north, where animals need to be able to get around during the winter to get food and, perhaps, a little exercise. Tightly packed snow provides excellent insulation. Wind cannot get through, although oxygen and carbon dioxide can. This is why people in extreme weather conditions have been able to build houses out of the ice/snow -- igloos -- and survive with the heat from a small oil-lamp and human bodies.
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