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.
How Freezing Rain Occurs
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.
In Flight. The best response is avoidance: aircraft should be routed away from forecast/reported freezing rain horizontally and/or vertically. When in the vicinity of a warm front in winter, if performance, ATC, and regulations allow, consider flying above the freezing level, where the ice crystal precipitation will not have a chance to melt.
On the Ground. Check information on runway braking action before taxiing for take-off or attempting a take-off or landing. Generally airline and passenger/freight carrying operations are suspended during periods of freezing rain. Even without being prohibited by the operators manual, operations in freezing rain are difficult as the applicable Hold-Over Time from an anti-icing treatment is very limiting in freezing rain. This is because the ice accretion rates due to freezing rain can be much greater than those associated with other forms of winter precipitation. A typical hold over time can be as little as 15 minutes in light freezing rain, which can often mean that it is not possible to receive treatment and then take off within the allowable time frame. It should be noted that Holdover Time (HOT) Tables are not provided for operations in conditions of moderate or heavy freezing rain as the anti-icing fluid is not capable of providing protection under those circumstances. Ice pellets at surface level may indicate freezing rain aloft.