Types of Snowflakes
A hexagonal prism is the most basic snow crystal geometry (see the Snowflake Primer). 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 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.
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.
Stellar dendrites are clearly the most popular snow crystal type, seen in holiday decorations everywhere. You can see these crystals for yourself quite well with just a simple magnifier.
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.
Some snowfalls contain almost nothing but stellar dendrites and fernlike stellar dendrites. It can make quite a sight when they collect in vast numbers, covering everything in sight.
The best powder snow, where you sink to your knees while skiing, is made of stellar dendrites. These crystals can be extremely thin and light, so they make a low density snowpack.
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.
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.
The first picture on the left shows a double plate from the side. The second picture shows a double plate with the microscope focused on the smaller plate.
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.
You may have to stare at these pictures a bit to see how the two distinct pieces fit together. Note how in each case the crystals are connected in the center with short axles.
Plates sometimes grow as truncated triangles when the temperature is near 28 F (-2 C). If the corners of the plates sprout arms, the result is an odd version of a stellar plate crystal. These crystals are relatively rare.
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.)
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.
These crystals are quite rare, but sometimes a snowfall will bring quite a few. The picture on the right shows a 12-sider where the two halves are widely separated.
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.
Sometimes a bullet rosette can become a capped rosette, as shown in the example on the left.
When the pieces of a polycrystal grow out into dendrites, the result is called a radiating dendrite (also called a spatial dendrite).
The first example on the right shows radiating plates. The second example shows a fernlike stellar dendrite with two errant branches growing up out of the main plane of the crystal.
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.
The two pictures have relatively light rime coverage.
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.
The images and descriptions are taken from and used with permission from the California Institute of Technology snowflake website at http://www.its.caltech.edu/~atomic/snowcrystals/class/class.htm