Snow, transparent ice crystals formed around dust or other small particulates in the atmosphere when water vapor condenses at temperatures below the freezing point. Partly melted crystals usually cling together to form snowflakes, which may in rare cases grow in size up to 7 to 10 cm (3 to 4 in) in diameter.
Structurally, elemental crystals of snow occur in any of various hexagonal forms, depending upon exact atmospheric temperatures during formation. Among these six-sided, basically symmetrical shapes are needle, columnar or stud, platelike, and star-shaped crystalline types. Because of the infinite variability of weather conditions, every snow crystal is unique in its precise configuration, and it is the large number of reflecting surfaces of the crystal that make snow appear white. The longer rays that constitute the arms of the six-rayed stars are generally hollow tubes; they are evidently built up by additions to the edge of an original crystal.
Snowfall measurement is usually stated as depth in centimeters, or other unit, of newly fallen snow; it is also measured in terms of the depth of the layer of water that would result if the snow were melted in place; 25-30 cm (10-12 in) of snow melts to 2.5 cm (1 in) of water.
