Unusual Snow Crystals

... Some exotic examples from the snow crystal menagerie ... In addition to the normal snowflake forms (see Guide to Snowflakes), there are also many unusual shapes that can be found floating from the sky. Some of these are described here, with photos from the Rasmussen&Libbrecht collection (see Photo Collections). And, of course, you can find even more in our new book.... Not Your Ordinary Snow ... 12-Sided Snowflakes

A bit of snowflake watching may turn up some 12-sided snowflakes, as these occur along with the normal 6-sided variety. They're not real common, but you can spot them if you look. Some snowfalls bring quite a few twelve-siders, although no one really knows what weather conditions are best for making them. Triangular Snowflakes

You won't find any 4-, 5-, or 8-sided snowflakes in the wild, but you may spy some 3-sided crystals. As with the 12-siders, these crystals appear along with the more common hexagonal variety. And again, their origin is still something of a mystery. Bullets

Bullets form when several columnar crystals grow from a single nucleus, a beginning that is similar to the spatial dendrites (see Guide to Snowflakes). Bullets always start out in clusters, but the clusters can break up to yield individual bullet crystals. Arrowhead Crystals

Crystal twinning is a phenomenon whereby two crystals grow out of a single seed and have a specific crystallographic orientation relative to one another. One example is found in arrowhead crystals, which occur along with columns. The unusual shape results from the way the two crystal halves are joined.

The seam between the crystals can be seen when some of the ice evaporates, leaving an evaporation groove, as indicated in the picture at right. Twin Prisms

Another example of crystal twinning occurs when twin columns grow from a common center, one rotated 60 degrees relative to the other. The growth looks like an ordinary column, but again we can find an evaporation groove that indicates twinning.

Twelve-sided snowflakes also form by columnar twinning; in this case the two columns are rotated 30 degrees and the assembly grows into a capped column (see Guide to Snowflakes). Pyramidal Crystals

At very low temperatures one can find snow crystals with pyramidal facets in addition to the usual basal and prism facets (see the Snow Crystal Primer) (photo from Tape [1]). Sometimes seen at the South Pole, these crystals are exceedingly rare in more benign climates. [1] W. Tape, Atmospheric Halos, Antarctic Research Series, Vol. 64, (American Geophysical Union, 1994).