E

       F

       G

BLEMISHES

Deformed snowflakes contain a miniature history of weather conditions from the point of their formation to the place where they land. Wergin and Erbe have used their low-temperature scanning electron microscope to infer what happens when falling ice crystals run into fogs of supercooled water droplets on their way down, a common occurrence.  Some of the droplets attach to the falling crystals and freeze there like flattened little pimples, just thousandths of an inch in diameter. At first, local air currents cause these accretions to stick to the trailing face of the crystals as they descend through a region of cloud droplets. If the crystals receive only a moderate coating—so that they remain recognizable as needles, columns, plates, or dendrites—the combination of the ice crystals and their blobby accoutrements is known as rime (E). During its passage through the atmosphere, one side of an ice crystal often gets thicker and more heavily rimed (F) than the other. That side can become more elongated than the cleaner side. The coating changes the crystal’s aerodynamics, causing it to flip. Further riming unfolds on the new trailing edge, leading to a thicker coating with more elongated topography. If the crystal encounters additional water droplets on the way down, riming can continue to the point at which the entire crystal is enveloped in an icy armor. These reshaped particles take on a mossy form known as graupel (G).