There are two ways snow can begin to form clouds. When the air is very cold (< -40 C) water vapor can convert directly to ice, through a process called "homogenous nucleation." At temperatures between -5 C and -40 C snow begins in the atmosphere as water condenses on small (dust) particles forming tiny droplets that freeze. This is called "heterogenous nucleation." Because of the differences in saturation vapor pressure over ice versus water, these tiny ice embryos grow at the expense of the tiny cloud droplets and eventually grow six evenly spaced branches. As more and more water vapor diffuses onto these branches, the ice crystal becomes increasingly heavy and begins to fall from the sky. As it descends, the ice crystal encounters very complex and variable atmospheric conditions, which ultimately contributes to its unique form. If it comes into contact with warmer air, for example, the crystal may begin to melt. This melting acts like a glue, causing crystals to bond together into larger flakes (aggregation), forming what many people think of as the "classic" fluffy snowflake.

The ice crystals that make up snowflakes are symmetrical because they reflect the internal order of the water molecules as they arrange themselves in the solid state through a process called "crystallization." During this process, water molecules align themselves into the most stable, lowest energy form by maximizing attractive forces and minimizing repulsive ones. In general, the most stable crystals have the most symmetrical form. Therefore, because of their molecular structure (two hydrogen atoms joined to a single oxygen atom), water molecules form weak hydrogen bonds with one another and arrange themselves in predetermined spaces to form a very specific arrangement. These ordered arrangements result in the basic symmetrical, hexagonal shape or six-sided snowflake (i.e., two hexagonal "basal" faces (c-axis) and six rectangular "prism" faces (a-axis)), which can be characterized as either "column-like" or "plate-like."

Ultimately, it is the temperature at which a crystal forms, and to a lesser extent the humidity of the air, that determines the basic shape of the ice crystal. (Click here to see a graphic of snowflakes.) For example, at very low humidity levels, ice crystals grow mostly as simple hexagonal prisms that vary only slightly with temperature, changing from columns (-5 C) to plates (-15 C) and back to columns again (-30 C). At higher humidity levels, however, ice crystal formation is strongly temperature dependent. Thus, we see long needle-like crystals (a/c axises << 1) at -5 C, and very flat plate-like crystals (a/c axises >> 1) at -15 C. At even higher humidities, the crystals become even more structured. For example, at -15 C one sees sectored plates at intermediate supersaturation levels, and fern-like dendrites at high supersaturations (humidity slightly greater than 100 percent).

Simple six-fold symmetry is one thing, but it is the more complex symmetry within "individual" crystals that gives them their special beauty. The complex shape of a single arm, for example, is also determined by the atmospheric conditions experienced by the crystal as it falls. A crystal might begin to grow in one manner and then minutes or even seconds later slight changes in the surrounding temperature or humidity causes the crystal to start growing in another manner. Although the hexagonal symmetry is maintained, the ice crystal (and its six arms) may branch off in new directions. Because each arm experiences the same conditions, they all tend to look alike, producing large-scale, intricate, six-fold symmetric snow crystals. However, since snow crystals all follow slightly different paths (and thus encounter slightly different atmospheric conditions), individual crystals all tend to look uniquely different. In the end, there are all kinds of forms that can arise: everything from prisms and needles to the familiar lacy snowflakes. With this, we can now pretty much explain why there's such a rich diversity of snow crystal shapes in nature.

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*Text on this web page courtesy of NOAA
URL: http://www.noaa.gov/questions/question_120601.html

Images on this web page courtesy USGS
URL: http://www.anri.barc.usda.gov/emusnow/Selected/Select1.htm