Freezing rain is a meterological phenomenon that causes rain to freeze upon impact.
Freezing rain happens when frozen water (in the form of hail or snowflakes) enters a warm pocket of air at around 10,000ft and changes state into a supercooled liquid. As the rain approaches the surface it enters a pocket of cold air, where the temperature drops to just below freezing. When the water hits the earth's surface, it freezes instantly.
In graph form1, the process looks like this:
_______________________________
| Temperature |
| Below | Above |
| Freezing | Freezing |
_______________________________
| | |
| \ | |
| \ | |
| \ | |
| \ | |
| \| |
| | - 10,000ft |
| |\ |
| | \ |
| | / |
| |/ |
| | - 2,000ft |
| /| |
| / | - Surface |
_______________________________
In order for freezing rain to occur, the surface temperature must be below freezing point. This encourages the rapidly cooling liquid to freeze at the moment that it strikes the ground. The impact helps perturb a phase transition; ice crystals form rapidly on the surface. The chilled, shallow air keeps the ice frozen.
Freezing rain plunders everything in its path. The rain moulds itself to every surface as it freezes, which creates a blanket of ice across roads and footpaths. The "glaze" of ice is more condensed and tightly-packed than usual, which makes it harder to break and melt. Telegraph wires, power lines and tree branches often snap under its weight.
If freezing rain is predicted for your area, move all of your possessions inside, clear away any dead or dying branches from nearby trees, stock up on candles and good books, and stay inside during the storm. Apparently the formation of freezing rain is beautiful to watch; they say that it is pretty, in a deadly sort of way.
The most notorious case of freezing rain occured in Canada in 1998 and is often known as the Great Ice Storm.
1. This chart was kindly provided in graphic form by the Shreveport Weather Service and can be viewed at http://www.srh.noaa.gov/shv/FzRain.htm