There seems to be a fair amount of confusion surrounding the term "freezing rain". Part of this comes from local weathermen, most of whom are not meteorologists, using the term interchangeably with the word "sleet". However, the two are not the same at all.
With sleet, raindrops fall from a cloud in a relatively warm air mass. But when they get near the ground, they encounter thick layers of atmosphere that have an air temperature below freezing, causing the rain to freeze so that when it hits the ground, it is in the form of tiny bits of ice. This is also called Pellet Snow, since it appears to be many small pellets of ice. Sleet is not uncommon any time of the year. The air near the ground need not be freezing; there simply must be a layer of air that the rain passes through, that is.
Freezing rain is entirely different, and is a far more dangerous weather phenomenon. It also took more research to determine the cause.
Freezing rain only occurs under certain unusual situations, usually in late fall, though occasionally in very early spring. It occurs when a very cold air mass moves through an area, generally very slowly. Surfaces such as cars, roofs, fencerows, and power lines become super cooled. That is to say, the surface takes on a temperature that is far below freezing. This sets the stage for the next situation; a relatively fast moving warm and damp air mass.
Since the air temperatures rise above freezing, as rain begins to fall, it is in liquid form. The surfaces, though, are still super cooled, so as the moisture of the falling rain strikes them, it instantly freezes. This creates layer upon layer of very heavy ice that can crush rooftops, coat roads with ice that is so slippery that it is hard to even walk on, and can snap power lines.
This isn't the end of the danger, however. If the warm air mass flows over the top of a shallow cold air mass, rain can still fall, but since it doesn't have the time to freeze as it passes through the cold air, it still strikes the ground as rain. Yet the cold air that is close to the ground maintains the super cooled state of the surfaces, not allowing them to warm up to above freezing. Ice simply continues to accumulate.
Even more dangerous is when the warm air mass that is capping the cold air mass happens to stall; the severe conditions can last for days. This is what happened many years ago in Southeast Canada and the Northeast US. Emergency vehicles had trouble getting around, car accidents happened in such profusion that many roads were closed, water mains and pipes burst, roofs collapsed, and power lines broke in so many locations that the main transmission line for the power grid in the entire area was in danger of completely failing. Many of the huge towers that held the lines collapsed under the weight of the ice. The storm even came in so suddenly that birds sitting on tree branches were frozen solid where they perched, coated with ice.
It was only providence and a good deal of luck that the stalled weather system finally moved on, preventing what would have been an even more tragic catastrophe, though it was still tragic and many people and animals lost their lives.
Little can be done to prevent damage from being done when there is a freezing rain, but we can now predict them with fair accuracy, and warnings can be issued so that people can prepare, keep their roofs clean, and so forth. As important as what we do is what we do not, though. When freezing rain starts, stay off the roads. Not only is your life and the lives of your loved ones too important to risk, but emergency personnel are going to have a bad enough time as it is just helping those in need.
Freezing rain may sound like a benign weather pattern, but often it can be far worse than it what we envision. Don't take chances with your life. Respect what it means when freezing rain is being forecast by NOAA (National Oceanographic and Atmospheric Administration). Knowledge and common sense can be the keys to survival.