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types of snowflakes

Simple Prisms

A hexagonal prism is the most basic snow crystal geometry.  Depending on how fast the different facets grow, snow crystal prisms can appear as thin hexagonal plates, slender hexagonal columns (shaped a lot like wooden pencils), or anything in between.  Simple prisms are usually so small they can barely be seen with the naked eye.

Stellar Plates

These common snowflakes are thin, plate-like crystals with six broad arms that form a star-like shape.  Their faces are often decorated with amazingly elaborate and symmetrical markings.

Sectored Plates

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.

Stellar Dendrites

Dendritic means "tree-like", so stellar dendrites are plate-like snow crystals that have branches and sidebranches.  These are fairly large crystals, typically 2-4 mm in diameter, that are easily seen with the naked eye.

Fernlike Stellar Dendrites

Sometimes the branches of stellar crystals have so many sidebranches they look a bit like ferns, so we call them fernlike stellar dendrites.  These are the largest snow crystals, often falling to earth with diameters of 5 mm or more.  In spite of their large size, these are single crystals of ice -- the water molecules are lined up from one end to the other.

Hollow Columns

Hexagonal columns often form with conical hollow regions in their ends, and such forms are called hollow columns.  These crystals are small, so you need a good magnifier to see the hollow regions.

Needles

Needles are slender, columnar ice crystals that grow when the temperature is around -5 C (23 F).  On your sleeve these snowflakes look like small bits of white hair.

Capped Columns

These crystals first grow into stubby columns, and then they blow into a region of the clouds where the growth becomes plate-like.  The result is two thin, plate-like crystals growing on the ends of an ice column.  Capped columns don't appear in every snowfall, but you can find them if you look for them.

Double Plates

A double plate is basically a capped column with an especially short central column.  The plates are so close together that inevitably one grows out faster and shields the other from its source of water vapor.  The result is one large plate connected to a much smaller one.  These crystals are common -- many snowflakes that look like ordinary stellar plates are actually double plates if you look closely.

Split Plates and Stars

These are forms of double plates, except that part of one plate grows large along with part of the other plate.  The picture at right shows all eight ways to make a split star.  Split plates and stars, like double plates, are common but often unnoticed.

Triangular Crystals

Plates sometimes grow as truncated triangles when the temperature is near -2 C (28 F).  If the corners of the plates sprout arms, the result is an odd version of a stellar plate crystal.  These crystals are relatively rare.

12-Sided Snowflakes

Sometimes capped columns form with a twist, a 30-degree twist to be specific.  The two end-plates are both six-branched crystals, but one is rotated 30 degrees relative to the other.  This is a form of crystal twinning, in which two crystals grow joined in a specific orientation.

Bullet Rosettes

The nucleation of an ice grain sometimes yields multiple crystals all growing together at random orientations.  When the different pieces grow into columns, the result is called a bullet rosette.  These polycrystals often break up to leave isolated bullet-shaped crystals.

Radiating Dendrites

When the pieces of a polycrystal grow out into dendrites, the result is called a radiating dendrite (also called a spatial dendrite). 

Rimed Crystals

Clouds are made of countless water droplets, and sometimes these droplets collide with and stick to snow crystals.  The frozen droplets are called rime.  All the different types of snow crystals can be found decorated with rime.  When the coverage is especially heavy, so that the assembly looks like a tiny snowball, the result is called graupel.

Irregular Crystals

The most common snow crystals by far are the irregular crystals.  These are small, usually clumped together, and show little of the symmetry seen in stellar or columnar crystals.

Artificial Snow

Snow machines shoot a mixture of water and compressed air out of nozzles.  The water comes out as fine droplets, and the air cools as it decompresses, causing the droplets to freeze.  A fan blows the ice particles onto the slopes.



You can see from the picture at right that artificial snow is made of frozen water droplets, with none of the elaborate structure found in real snow crystals.


excerpts from http://www.its.caltech.edu/~atomic/snowcrystals/class/class.htm