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Snowflakes - Page 2

© Keith C. Heidorn

columns; some look like oak leaves; some are shaped like dinner plates.

Snow Crystal Classes

Normally, however, such perfectly formed crystals do not survive the fall to earth. Most crystals break off arms as they are buffeted by winds within a cloud before they grow to sufficient size to fall, producing irregular and unsymmetrical shapes.

Snow Crystal Formation

Scientists puzzled over why snowflakes have six sides for centuries. In 1611, famed astronomer Johannes Kepler took time away from peering at the heavens to look into the small snowflakes of Earth. Writing in an essay "On the six-cornered snowflake," Kepler postulated that snowflakes were made up of globules of condensed and frozen moisture - their symmetry arising from geometrical efficiency because they were well-packed arrays of tiny spheres.

It took more than three centuries to fully correct Kepler's concept. We now understand that the shape of a snowflake is first determined at the molecular level by the atoms composing the water molecule. Each molecule contains two atoms of hydrogen joined to one atom of oxygen - H₂O. In water, oxygen has a more powerful hold on the electrons that the two elements share in their water-making bond. As a result of oxygen's tighter grip, the oxygen atom side of the water molecule becomes slightly more negative while the hydrogen side becomes slightly more positive. Since opposite charges attract, the positive hydrogen atom of one water molecule tends to stick to the oxygen atom of a neighbouring molecule.

In liquid and vapour water phases, water molecules jostle and dance around their neighbours, forming few stable links between molecules. But when a mass of water molecules loses enough heat energy - that is, as the water cools down toward the freezing point -the bonds (called hydrogen bonds) between water molecules form more readily and break less often.

Upon freezing, everything seizes up. Now, each water molecule is surrounded tetrahedrally by four others to which it bonds. The oxygen atoms are arranged hexagonally in layers. When a new ice forms on the growing crystal's sides, it spends less energy than if it were to add a layer on its top or bottom. The side faces advance more quickly, and the crystal structure forms. This is what provides the underlying sixfold symmetry of the crystal lattice, that grows to become the visible snow crystal or platelet of ice.

Ice crystals are sensitive to the conditions under which they form, particularly the air temperature, and the excess relative humidity expressed as the supersaturation level - both within and without the clouds. These factors not only affect how the crystals grow but the basic shape they take. The colder the temperature, the

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