Stellar plates often show distinctive ridges that point to the corners between adjacent prism facets. When these ridges are especially prominent, the crystals are called sectored plates.The simplest sectored plates are hexagonal crystals that are divided into six equal pieces, like the slices of a hexagonal pie. More complex specimens show prominent ridges on broad, flat branches.

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Surprisingly, no one knows why snow crystals grow into these three-fold symmetrical shapes. (Note however that the molecular structure of triangular crystals is no different from ordinary six-sided crystals. The facet angles are all the same.)

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By growing snow crystals in the laboratory under controlled conditions, one finds that their shapes depend on the temperature and humidity. This behavior is summarized in the "morphology diagram," shown above, which gives the crystal shape under different conditions.

The morphology diagram tells us a great deal about what kinds of snow crystals form under what conditions. For example, we see that thin plates and stars grow around -2 C (28 F), while columns and slender needles appear near -5 C (23 F). Plates and stars again form near -15 C (5 F), and a combination of plates and columns are made around -30 C (-22 F).

Furthermore, we see from the diagram that snow crystals tend to form simpler shapes when the humidity (supersaturation) is low, while more complex shapes at higher humidities. The most extreme shapes -- long needles around -5C and large, thin plates around -15C -- form when the humidity is especially high.

Why snow crystal shapes change so much with temperature remains something of a scientific mystery. The growth depends on exactly how water vapor molecules are incorporated into the growing ice crystal, and the physics behind this is complex and not well understood. It is the subject of current research in my lab and elsewhere.