Freezing rain, also known as rain ice, is a term used to describe rain that will freeze on impact with a surface – particularly the skin of an aircraft. This can be:
Both will create ice.
The classic situation used to explain freezing rain is in the cold (sub-zero) air underneath an approaching warm front in winter. Nimbostratus clouds form along the warm front, with rain forming in temperatures above 0°C but falling down into the sub 0°C air below.
An aircraft flying in the cold air can have an airframe temperature below 0°C and when rain hits the aircraft’s skin, it turns to ice. The rain has a few seconds to spread back across the wing before freezing as clear ice.
If freezing rain is encountered, the simplest solution is to descend to an altitude where the temperature is above 0°C. However, due to ground temperatures, terrain heights etc, this may not always be possible. A higher performance aircraft may be able to climb above the rain and clouds but for lower performance aircraft sometimes the only solution is to avoid freezing rain in the first place.