DURING winter, some people are surprised to see surreal ice images on their cars. These patterns are called "fern frost". Metal is a good conductor of heat, so the car's bodywork is often the first surface to drop below freezing as temperatures fall. Like the windowpane, the paintwork encourages the growth of long ice crystals. Each of these crystals interlock with the last. The overall pattern is determined by tiny scratches, dust and other imperfections on the surface. The patterns are symmetric, governed by the geometry of the water molecule.