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Snow Crystals, Snowflakes and Snowfalls
"I should hardly admire them more if
real stars fell and lodged on my coat."
--Henry David Thoreau, 1856
As we head into Fall and Winter we will experience changes in our weather. These changes vary according to where in the world we live, but in the temperate areas of North America and Europe we must expect lower temperatures, showers, rainstorms, hail, snow and fog whenever there is sufficient moisture in the atmosphere.
After a bitterly cold spell followed by a warm-up and snow shower in last winter in Scotland the question "Why does it get warmer when it snows?" was overheard. Well, of course, it doesn't. The warm-up comes before the snow.
In very cold weather air does not hold much water vapor. However, warm air can hold more moisture than cold and when warm, moist air cools as it rises or moves into and mixes with colder air, its moisture, at temperatures between -5 C and -40 C, condenses on small (dust) particles and freezes into snow crystals.
The temperature at which a snow crystal forms and, to a lesser extent, the humidity of the air, determines its basic shape. For example, at very low humidity levels, snow crystals grow mostly as simple hexagons. At higher humidity levels, snow crystal formation is strongly temperature dependent. At -5 C long needle-like crystals tend to form. At –15 C very flat plate-like crystals occur. When the humidity is very high, the crystals become even more structured. For example at -15 C when the air is supersaturated complex crystals form with fern-like dendrites. A snowflake is an agglomerate of many snow crystals.
Each snow crystal is symmetrical because its structure reflects the internal order of the water molecules of which it is constructed so that it eventually grows six evenly spaced branches. As more and more water vapor diffuses onto these branches, the crystal becomes increasingly heavy and begins to fall from the sky. As it descends, it encounters very complex and variable atmospheric conditions. A snow crystal might begin to grow in one manner and then minutes or even seconds later slight changes in the surrounding temperature or humidity causes it to start growing in another manner. This results in each snow crystal having a unique design.
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