Graupel grain, polarised light micrograph. Normally frozen water is very weakly birefringent. Due to the complex formation process, however, quite strong birefringence can be seen in these structures, represented by intense interference colours. Graupel is defined as a precipitation, usually of brief duration, consisting of crisp, white, opaque ice particles, round or conical in shape and about 2 to 5 mm in diameter. Formed in temperatures below 7 degrees Celsius as snow, then is rimed in layers by supercooled liquid from updrafts into showers. Graupel, or snow pellets, usually occur when the lower atmosphere is very instable. Microscopic contrast method: polarisation contrast. Magnification: 20x at a print width of 10 cm.