Snowflakes give clues about ozone depletion.

• Position: YOUR LIFE

There is more to the snowflake than its ability to delight schoolchildren and snarl traffic. The structure of the frosty flake also fascinates ice chemists. "A lot of chemistry occurs on ice surfaces," notes Travis Knepp of Purdue University, West Lafayette, Ind., a doctoral candidate in analytical chemistry who studies the basics of snowflake structure to gain more insight into the dynamics of ground-level, or "tropospheric," ozone depletion in the Arotic.

Knepp is studying snow crystals and why sharp transitions in shape occur at different temperatures, which, among other things, explain why no two snowflakes are identical. "On the surface of all ice is a very thin layer of liquid water," Knepp explains. "Even if you're well below the freezing point of water, you'll have this very thin layer of water that exists as a liquid form. That's why ice is slippery. Whenever you slip, you're not slipping on ice, you're slipping on that thin layer of water."

This thin, or quasi-liquid, layer of water exists on the top and sides of a snow crystal. Its presence causes the crystal to lake on different forms as temperature and humidity change. For instance, the sides of a crystal growing in a warmer range of 27-32[degrees]F expand much faster than the top or bottom, causing it to take on a platelike structure. Between 14[degrees]-27[degrees], crystals look like tall, solid prisms or needles. "As you increase the humidity...