These are a kind of precipitation, which consists of white and opaque grains of ice. These grains are spherical or sometimes conical; their diameter is about 2-5 mm. Grains are brittle, easily crushed. They do bounce and break on hard ground
Transport Canada, TP 10643 -When in Doubt. Small and Large Aircraft - Aircraft Critical Surface Contamination Training for Aircrew and Groundcrew, Glossary 7th Ed., Ottawa, ON, Canada 2004, http://www.tc.gc.ca/eng/civilaviation/publications /tp10643-glossary-1116.htm, (Avn Sfty - Canad) {652}