Commonly, and in aviation terminology, the lowest altitude in the atmosphere, over a given location, at which the air temperature is 0°C; the height of the 0°C constant-temperature surface. This simple concept may become slightly complicated by the existence of one or more “above- freezing layers” formed by temperature inversions at altitudes higher than the above-defined freezing level. In cloud physics terminology, this is more accurately termed the melting level, for melting of ice always occurs very near 0°C, but liquid cloud drops may remain supercooled to much colder temperatures. See icing level.