Usually, when we think of snow, we are envisioning that beautiful, white, fluffy stuff that falls silently over the forests and plains. That is just one form of frozen precipitation that visits us in winter, and I started wondering what makes snow, versus hail or sleet. Like most simple questions I ask myself and bother to research, I learned that the answer was pretty interesting, and I found some really great web sites to help me.
Snow happens when three conditions are true:
- Water vapor is present in the atmosphere.
- Minute particles of dust or bacteria, or sea salt crystals, etc. are present.
- The temperature is freezing or below freezing.
Crystals form around the particle that are hexagonal prisms, then more vapor builds on this shape. The most common crystals that form are six pointed stars. If you would like to see some marvelous photos that show the beauty of these crystals click on Snow Crystals, a site created by Kenneth G. Libbrecht, Caltech. As these crystals fall, they are modified by wind or by bumping into each other or some other obstacle. The soft, fluffy snowflakes we enjoy are collections of the stellar crystals that have hooked together. When they pile up, they can transform even the simplest things into little scenes of beauty. Other crystals may become icy needles, columns, capped columns, rounded pellets we know as sleet, etc. Hail is large round ice balls. I have seen them as big as grapefruits and they did a tremendous amount of damage. When super-cooled water vapor (usually in fog) encounters a surface, it freezes to it almost immediately. This is known as rime and it can be quite beautiful on trees and bushes, but if it is very thick, it can also break limbs and take down power lines.
Snowpack
Over winter in the Colorado mountains, snow builds up, layer upon layer, and each layer is probably different from the previous layer, depending on the wind and temperature in the area when it fell, and many other factors. This snow pack has strong layers that bond together tightly and weaker layers that are poorly bonded. Like many other things, this snowpack is only as strong as its weakest link. Changes that place stress on the load can be very dangerous, and cause large areas to break off suddenly and slide down steep slopes. This, of course, is an avalanche.
Avalanche Photograph courtesy of Doug Abromeit, Director - U.S. Forest Service National Avalanche Center
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