The most basic form of an ice crystal is a hexagonal prism , such as that shown at right. The reason these facets exist derives from the molecular structure of water, and how water molecules arrange themselves into a crystalline lattice . Before answering this, it is useful to define what a snow crystal is. These grow directly from condensing water vapor in the air, usually around a nucleus of dust or some other foreign material.
Typical sizes range from microscopic to at most a few millimeters in diameter.
These can grow to large sizes, up to about 10 cm across in some cases, when the snow is especially wet and sticky. For example, one often sees small droplets of rime on large snow crystals. A simple observation on a snowy day, with a low-power microscope or hand magnifying lens, quickly reveals a great variety of snow crystal shapes. More crystal types can be listed, as are described under Classification schemes . These other forms are mostly variations and combinations of the above basic types, such as plates with dendritic extensions, capped columns, etc.