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Glossaire

Above Freezing Layer

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NOAA NWS

Caption: This graphic shows a hypothetical vertical temperature structure (green line) with an AFL located between 2,000 m and 700 m above the earth’s surface. As a snow crystal falls from the base of a cloud (left side of diagram), it will eventually melt and transition to rain as it descends through the AFL. Since the air temperature changes to subfreezing temperatures below 700 m, water droplets become supercooled and freeze on contact when they reach the ground (freezing rain). If the depth of the surface-based cold layer were greater, it would give water droplets enough time to freeze completely while in the air, and reach the groundas ice pellets.

An above freezing layer (AFL) is a horizontal layer of air at some altitude in the atmosphere with a temperature above 0 C. It lies between two layers of sub-freezing air. AFLs are a common result of atmospheric subsidence and incoming warm fronts overriding cold air entrenched within valley bottoms. A temperature inversion is always present at the bottom of the layer.

Note that warm air at valley bottom is not referred to as an above freezing layer because air is normally warmest at low elevations.

Elevated layers of above freezing air play an important role in the type of precipitation observed at a given altitude. Above an AFL, precipitation falls as snow. It then transitions to rain within the AFL (assuming the layer is of sufficient depth), and then to ice pellets or freezing rain below the AFL.