Becky captured an image of this snowflake with her digital camera
during the first snowfall of the2006-2007 winter season, in late November
2006.
This
snowflake
is
an example of a "stellar dendrite" and has a shape
most commonly associated with snow. However, snow crystals come in
a variety of structures
including plates, needles, hollow columns and solid prisms.
While the structures are different, snow crystals are all based on
hexagons, because water molecules bond only at 120 degrees. A snow
crystal is formed when water vapor in a below-freezing atmosphere is
deposited on a ice nuclei. Initial growth is generally in the shape
of a hexagonal prism. Branches or arms develop at the 120 degree angles
of the hexagon.
The type of crystal that forms depends on the temperature and relative
humidity of the air. Dendrites are common at temperature between -12C
and -16C, while plates form at both colder and warmer temperatures.
Shapes are more simple with low relative humidities and more complex
at higher relative humidities.