Due to the laws of thermodynamics -- none of which we Why Filers even pretend to grasp -- water vapor crystallizes most easily on a particle of, say, dust. Once the process starts, it cascades: water vapor in the air easily crystallizes onto existing water crystals.

Because of the geometry of bonds between water molecules, snow flakes are all based on hexagons. Still, depending on temperature and humidity, the bonding can result in any number of shapes. And since conditions can change as the flake gets bounced around in a cloud, one shape can change into another; witness the plate-dendritic flake shown above.

(BTW, those images were made by Wilson "Snowflake" Bentley, a Vermont farmer-scientist who pioneered the study of snow by shooting fresh flakes during winter. These days, scientists capture snow flakes on a liquid polymer that creeps together to form a 3-D mold around the flake.)

All unique?
Preamble done, let's settle a pressing question: Do all snowflakes have different shapes? We asked Pao-Kuan Wang, an atmospheric physicist who studies flakes -- and ice -- in clouds at the University of Wisconsin-Madison, and he told us it depends on how hard you look. "With a microscope, going down to the molecular level, of course they're all different," he says. "But without a microscope, at the superficial level, they may look alike."

He suspects that the popular belief that every snow crystal looks different arose "as a kind of appreciation for the complicated design -- and because they're so beautiful."

Wang's actual excuse for studying frozen, airborne water is to figure out how it will affect global climate. Clouds, especially the high-flying cirrus clouds, are essentially formations of airborne ice, and they can have a complicated effect on the Earth's heat budget. Clouds reflect some incoming sunlight back to space, but they also reflect heat back to Earth.

In other words, clouds could either amplify or dampen global warming. Wang says there are too many variables to know the correct answer, particularly since, as his graduate student Emily Liu just learned, the exact effect depends to some extent on the shape of the ice crystals.