How Snow Crystals Form
Ice, like water, consists of two hydrogen atoms attached to one oxygen atom. When water vapor freezes, the three-fold symmetrical molecular structure of the water molecules creates a hexagonal prism. The outer surface of this prism has two hexagonal basal faces (top and bottom, so to speak) and six rectangular prism faces (think of them as sides). Where faces meet ("corners"), branches may grow, creating the shapes we think of as snowflakes.
Snow Crystal Shapes
Different environmental temperature and humidity conditions create different snow crystal shapes. The most basic form is the hexagonal prism, the branchless crystal, which is flat near the freezing point (28 degrees Fahrenheit) and columnar at colder temperatures (23 degrees Fahrenheit). Star shapes form around 28 degrees Fahrenheit, and, with plate-like prisms, form again when it gets colder, about 5 degrees Fahrenheit. Needles are made at about 23 degrees Fahrenheit. Both plates and columns form at even colder (-22 degrees Fahrenheit) temperatures.
Snow crystals tend to remain simple when the humidity is low and form complex shapes when it's higher. The most unstable shapes--long needles and large plates--appear when the humidity is highest.
Skiing Snow Crystals
A single molecule of ice is too small for the naked eye, and small snow crystals are barely visible. For cross-country skiing competitions, a waxing technician examines the actual snow crystals on the course. From this examination, he selects waxes and decides which portions of the racer's skis will receive which wax.
For the recreational skier, the process is simpler. Looking at and feeling the snow is a good way to decide whether to use grip or glide wax and whether to pocket just one kind or both. One indicator is the shape of the snow crystals: fluffy, branched crystals mean sticky snow, so take along glide wax.