Gases, Liquids and Solids

Abstract

A microscopic speck of dust, trapped in a molecule of water vapour, rides the buffeting winds inside a winter storm cloud. As the particle is frosted with droplets of supercooled water, it begins the six-mile plunge to Earth. The falling ice crystal is then sculpted by the varying temperature and humidity—lengthening here, a spiky branch pushing out there—until it grows into a shape as individual as a fingerprint. The resulting snowflake may look like Cleopatra’s needle or a fern or a chunky hexagon, but it will be different from every other snowflake around. And although such particulars are understandably meaningless to people digging out from billions of flakes, they are becoming crucial to scientists who are trying to predict everything from avalanches to metal failures. Only now, after centuries of befuddlement, are scientists beginning to make sense of snow.