Unusual Snow Crystals

When trying to understand a complex phenomenon, it is often useful to look not only at the most common occurrences, but also at unusual cases. Likewise there is something to be learned by looking at uncommon snow crystal forms, which appear in both the natural and artificial varieties. Some of these are described here.

Triangular Plates.

A number of observers have seen simple plate-like snow crystals shaped like triangles, often with truncated tips, in contrast to the much more prevalent hexagons. The image at the right is from Tape's book, and additional examples can be found in Bentley's and Nakaya's books. Triangular crystals are also readily produced artificially, especially in free-fall growth. The cause of these triangular plates is not known.

Pyramidal Crystals.

Another uncommon form is shown at right, again from Tape's book. These crystals exhibit pyramidal facets in addition to the usual basal and prism facets, and are occasionally seen in great numbers in natural snowfalls. The conditions under which they grow are not well understood, but it's likely that they form only at very low temperatures and supersaturations.

12-Sided Stellar Dendrites

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A brief period of snow crystal watching is likely to turn up some 12-sided snow stars (provided the conditions are right for the production of normal 6-sided stellar dendrites), such as the example at left. These crystals are prevalent enough in natural snow that Nakaya created two separate morphological types (out of 41 total) for 12-branched forms (see Classifications). It is believed that these crystals represent rotational twins of normal crystals, and Kobayashi and Furukawa have examined the misorientations of the branches using a Coincidence-Site Lattice theory.

Spatial Dendrites and Bullet Combos.

A substantial number of natural snow crystals take the form of spatial dendrites (Nakaya's type P5a; see Classifications), such as the example at right. The angle between neighboring components of these polycrystals has been found to be strongly peaked near 70 degrees. Bullet combinations (Nakaya's type C2a) also often form with a 70-degree angle between the different needles. These structures have been modeled by assuming a cubic ice structure in the junction region, and there is evidence that the cubic form of ice may often nucleate as a metastable state under normal atmospheric conditions.

Twin Prisms.

A twin prism is composed of two single prisms in juxtaposition, in which the c-axis and one of the a-axes are identically shared in common by each single prism. It appears identical to a normal single prism, except for a groove which appears when the crystal is evaporating. Twin prisms appears to be the result of a rotational twinning about the [0001] axis, again indicative of a cubic structure in the ice nucleus.