The diagram below shows the new situation. One can think of the initial development of the projections as an instability. Any little protrusion that occurs by chance can grow faster than the flat part, so it becomes more and more pronounced as it "robs" water molecules from the air and slows growth on the neighboring area of the ice surface. The hexagonal corners provide natural starting points for the projections because they already protrude.
A well-formed dendritic snow crystal (like that shown in the first illustration) has six main arms that are 60 degrees apart, branches at 60 degrees from each arm and, sometimes, smaller branches on those branches. The most prominent branches in snow crystals often occur at the same distance from the center of the crystal on each main arm, and probably are started by an abrupt increase in the humidity of the air through which the growing crystal is falling.
The minor branches, however, are not usually as symmetrical. They are aligned along the 60-degree crystal axes, but they start more randomly. In other words, perfectly symmetrical drawings are not entirely realistic.
Answer originally published January 18, 1999.