The altitude at which ice crystals and snowflakes begin to melt as they descend through the atmosphere. In cloud physics and in radar meteorology, this is the accepted term for the 0°C constant-temperature surface (see bright band). It is physically more apt than the corresponding operational term, freezing level, for melting of pure ice must begin very near 0°C, but freezing of liquid water can occur over a broad range of temperatures (between 0° and −40°C; see supercooling). See also freezing point, ice point, melting point.