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Precipitation that exists in supercooled liquid form below temperatures of 0°C and freezes on contact with surfaces that have a temperature lower than 0°C.
Freezing Rain occurs when precipitation, in the form of rain, passes from a warm air mass into a relative cold air mass of an air temperature less than 0°C. The rain maintaining its liquid state in sub-zero temperatures renders it super-cooled. These super-cooled rain droplets freeze when they come into contact with the ground or other exposed surface, if the surface temperature is below 0°C. By definition, freezing rain has either droplets of a diameter greater than 0.5 mm (0.02 inch) or, if droplets have less than this diameter, they must be, in contrast to (freezing) drizzle, widely separated.
The winter storms which give rise to the phenomenon of freezing rain are often referred in North America as Ice Storms. The cloud type generally associated with freezing rain is Nimbostratus.
Freezing Rain can cause significant accumulations of Clear ice on upper surfaces which the aircraft anti/de-icing systems, being primarily designed to deal with ice accumulation caused by horizontal movement of the airframe, may not be able to cope with. Aircraft on the ground during freezing rain may become covered in thick layers of ice which will be difficult to remove; airfield operating surfaces and runways will also be affected, and all aircraft movement on the ground may be severely disrupted.
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