The Formation of Snow Crystals
Subtle molecular processes govern the growth of a remarkable variety of elaborate ice structures
Whether it is a tray of cubes in the freezer or the surface of a lake in winter, ice takes on the shape of its container. But when it comes to snowflakes, the same simple act of freezing water has a completely different result, producing a stunning diversity of complex patterned forms. The variety of shapes is so remarkable that it easily supports the old adage that no two snowflakes are exactly alike. Even a casual look on a snowy day brings about the kind of wonder that prompted Henry David Thoreau to comment: "How full of the creative genius is the air in which these are generated! I should hardly admire more if real stars fell and lodged on my coat."
Water is such a common substance that one might expect that everything was already known about Thoreau's "creative genius"—how snowflakes develop into their complex structures. In fact, a great deal about the growth of these diminutive ice masterpieces remains maddeningly difficult to explain, even at the qualitative level. The growth of snowflakes is a highly nonlinear, nonequilibrium phenomenon, for which subtle processes at the nanoscale can profoundly affect the development of complex patterns at all scales. Understanding their formation requires a rich synthesis of molecular dynamics, surface physics, growth instabilities, pattern formation and statistical mechanics. Even though they fall from the winter clouds in vast numbers, we are only now on the verge of understanding why snowflakes form their distinctive shapes.