Freezing rain is rain that freezes once it reaches the ground surface. The rain may briefly runoff after striking the ground which will promote the development of icicles once the water does freeze. There are two features of the troposphere that promote freezing rain and they are (1) a layer aloft that completely melts frozen precipitation into liquid precipitation and (2) a surface ground temperature at or below freezing. The diagram below shows a typical situation that produces freezing rain. First, snow forms aloft and falls toward the surface. The snow then falls through a layer of warm air (called the Elevated Warm Layer (or EWL)) that completely melts the snow to rain. The rain then reaches the ground and freezes on ground that is at or below freezing. To determine if the snow will completely melt to rain aloft, the 850 to 700 mb thickness can be used. If the 850 to 700 mb thickness is above 1,555 m, then it is likely that the EWL is deep enough and warm enough to completely melt snow. Once the snow is completely melted, it does not matter too much if it falls through a sub-freezing layer aloft again. The reason for this is that there are no longer any ice nuclei that the water drops can freeze on. Thus, even if the water drops fall below freezing they will remain supercooled water drops. Supercooled water drops are drops of water that remain liquid even though they are below freezing. Supercooled water drops will freeze very quickly once they reach a ground surface that is at or below freezing because when they reach the ground they have a surface they can now freeze upon. Freezing rain is perhaps the worst of the winter weather hazards. Even small amounts of freezing rain can have a dramatic impact on driving conditions. Large amounts of freezing rain can be catastrophic because it can lead to widespread and long power outages. It can also bring down all sizes of trees branches and destroy vegetation.  |