The flake's progress

Snow is hot.

For centuries, naturalists have marveled at its pristine beauty. Mathematicians and physicists have dulled pencil tips over it. Others ski on it, throw it, or grumble over having to shovel it.

But during the past two decades - and in some cases only in the past four to five years - research into snow and ice has gathered the momentum of an avalanche. Discoveries touch on processes ranging from the formation of giant planets and their magnetic fields to materials science, ecology, mountain-building, and climate change.

"Twenty years ago, snow research was a sleepy backwater," acknowledges Matthew Sturm, a geophysicist with the US Army Corps of Engineers' Cold Regions Research and Engineering Laboratory office in Fort Wainwright, Alaska. But that changed, he adds, with growing concerns about global climate change.

Yet for all the increased interest, scientists say, results so far barely scratch the surface of what snow can teach about its role on our planet and the properties and formation of crystals in general.

The journey along the road to answers begins with a single snowflake.

Somewhere in a frosty wintertime cloud over Thoreau's patch of New England, a microscopic mote of dust floats through air over-saturated with water vapor. To the naked eye, the dust is invisible. But to a water molecule, the dust grain is enormous. If the molecule were the size of a human, the dust grain would present a potential home the size of an asteroid tens of kilometers across.

Like water added to a glass already full and spilling over the rim, the water molecules in the "supersaturated" cloud seek their own form of equilibrium and find it by freezing to the dust grain. Each molecule bonds to the next, first forming a six-sided plate of shimmering ice, then building outward from the plate surfaces and the points of the hexagon. At the appropriate temperature, the arms springing from the points begin to grow faster than the ice building on the plate surface. The flake takes on the stellar form that ultimately graces Thoreau's coat.

For Cal Tech physicist Kenneth Libbrecht, the birth and evolution of an ice crystal carries more than enough mystery to keep a lab busy.

The anatomy of ice surfaces and how they influence the growth of ice crystals are "all pretty basic stuff," he notes. "But we are surprisingly ignorant of even these fundamental issues."

His lab has been focusing on growing ice crystals under tightly controlled conditions, then measuring how an ice crystal's size and shape change with time. One intriguing result indicates that while a crystal's growth rate depends on temperature and the extent of supersaturation, other gases in the air may also affect its growth.

Experiments using a vacuum chamber filled with nothing but water vapor yielded crystals that were little more than simple prisms, while in air, the crystals grew in a variety of plate-like and needle-like shapes.

All this is of interest to atmospheric chemists looking to track the life cycle of pollutants and their constituent gases in the atmosphere.

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