December 02, 2010

Snowfall

The pond at the Nature Garden
Snow has been falling steadily for two days and nights now,
and lies about five inches deep in our garden.
Since last night there's been a steady sift of the tiny white
grains known as graupel. These accumulate gradually on the ground to form a soft, dense layer, quite unlike the wet white blanket that results from the usual large clusters of falling flakes.
Briefly last night there was even a shower of snow pellets: brittle white balls that bounced very slightly as they reached the ground and could just be heard, like faint static, as they hit the windowpanes.

The beautiful, star-shaped crystals we're familiar with from picture-books are known as stellar dendrites. There are a range of basic forms, each one decorated differently, but they usually have six branches. This is dictated by the shape of the water molecules from which they are formed. Snow crystals also come as twelve-pointed stars, prisms, flat, sectored plates, ice needles, tubes or spool-shaped capped columns. Their shape is determined by conditions in the upper atmosphere, where they form; the basic rule is, the faster they form the more intricate the shape.

A snow crystal can be up to 5mm across, although the 'snowflakes' one sees drifting down are nearly always agglomerations of dozens of individual crystals. As they fall, they may melt slightly, ice up or join together, until what we see, melting on our glove, is a tiny miracle of physics, a living illustration of the natural forces going on five miles or more above our heads.