Snow crystals grow when water vapour levels in clouds reach super-saturation.  Super-saturation is a metastable state, meaning water vapour will condense onto any readily available object.  The conditions where snow crystals begin, with low pressure and low temperature, are such that water vapour condenses directly into ice, not into water.  This ice forms onto a dust particle, which is called the nucleation site.

Two forces, both of which are heavily affected by temperature and pressure, determine the shape of the crystals. Crystals come in many different forms, yet they all have the same basic structure, illustrated below. (picture from www.its.caltech.edu/~atomic/snowcrystals)

The three a-axes are spaced 60º apart. The c-axis is perpendicular to the a-axes.  The hexagonal structure occurs because the prism faces grow very slowly.  The faces grow slower because on a molecular level, some surfaces of the crystal have many hanging chemical bonds, so the water molecules can attach to these surfaces more readily. This results in a hexagonal prism, with 6 prism faces and 2 basal planes.  This process is called crystal faceting. 

This controls the basic plate shape of the crystal, but many crystals also have complex branches and lace-like patterns. Molecules of water have to diffuse through the air to reach a growing snow crystal. If they have to diffuse further, they reach the crystal later and the crystal grows slower. But if a bulge forms on the surface of the growing crystal, the water molecule has less distance to travel to reach the bulge, reaching the crystal sooner, and the bulge grows faster. As the bulge grows larger, more molecules can join on, and the bulge grows faster still. The bulge has become a branch. Bulges on this branch become side-branches, and the snow crystal grows into a complex stellar dendrite. This is called branching instability.

There are several types of snow crystals.  Some are common, while others are rarer. Stellar dendrites—with 6 symmetrical treelike branches—and sectored plates—flat, thin plates that are divided into 6 parts—are the most commonly known types, but hollow columns are the most common. Needles, spatial dendrites, capped columns, and the various other irregular forms are much less common.  Changes in temperature and pressure greatly affect the type of crystal that forms.  Other things that can change the shape and appearance of crystals include wind, metamorphism, and rime.  Metamorphism is the changing of the crystals over time, and droplets freezing onto a falling snow crystal form rime.

The growth of crystals is a delicate balance between crystal faceting and branching instability, and slightly different conditions can create very different snow crystals. 

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