Freezing Rain

Different types of winter precipitation (freezing rain, sleet, and snow) form in the same fundamental way. The main difference among these precipitation types lies in the temperature profile, or how temperature changes with height. The temperature profile associated with freezing rain has particular features. For freezing rain to occur, there needs to be a deep layer of air above freezing over a shallow layer of air below freezing. The depth of the layer above freezing is crucial because it determines how much melting may occur. When the warm layer is deep, the frozen precipitation melts completely, resulting in liquid. As the rain falls back into the air that is below freezing but since the depth is shallow, it does not have time to freeze into sleet and upon hitting the ground or objects such as bridges and vehicles, the rain freezes on contact. Some of the most disastrous winter weather storms are due primarily to freezing rain.

For freezing rain to occur, there needs to be a deep layer of air above freezing over a shallow layer of air below freezing. The water becomes super cooled and freezes upon impact with the ground.

Freezing rain is common ahead of warm fronts in winter. Serious icing occurs when the aircraft is flying near the top of the cold air mass beneath a deep layer of warm air. Rain drops are much larger than cloud droplets and can therefore create significant structural icing such as clear ice.

Some of the most dangerous icing situations occur within freezing rain, ice pellets, or wet snow at or near the ground. Some important things to remember are listed below:

  • The worse icing conditions generally occurs in the vicinity of fronts
  • Moderate to severe icing is often widespread and not easily escaped
  • There may be multiple freezing levels. Aircraft may not be able to descend into above-freezing temperatures, and low powered aircraft cannot climb into above-freezing temperatures (if present)

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