The intricately shaped, six-sided crystals of ice that form in the Earth's atmosphere and fall to the ground as fluffy white snowflakes have intrigued scientists for centuries.
Why, for example, does snow look white? After all, an ice cube or an icicle may appear transparent.
"It's because of scattering. All the light is scattered back because there are so many surfaces to scatter it," said Charles Raymond, a geophysics professor and specialist in glaciology at the University of Washington.
Snowflakes and other loosely packed snow formations have many surfaces that are exposed to air. A considerable amount of the light that strikes these surfaces is reflected. They appear white because light of all wave lengths is reflected. In contrast, a block of ice contains fewer surfaces, and more light penetrates.
How do snow crystals and flakes form and why do they assume their elaborate, hexagonal shapes?
Snow usually forms around tiny particles, known as nuclei and often composed of clay and similar substances, according to Peter Hobbs, an atmospheric sciences professor at the University of Washington in Seattle. The tiny particles about which snow crystalizes often have hexagonal shapes themselves. "Really, the drop [of water] is fooled into thinking it has a little bit of ice in it," Hobbs said.
The ice crystals take on hexagonal shapes because, at normal atmospheric temperatures and pressures, the hydrogen and oxygen atoms that compose the ice are most stable in such arrangements, scientists say.