Southern Wisconsin experienced freezing rain and freezing drizzle just before the start of the holidays.
When particles falling from clouds reach the surface as precipitation, they do so primarily as rain, snow, freezing rain or sleet.
Where we live, most precipitation particles are frozen at one time during their formation. What determines the precipitation type at the surface is the temperature between the cloud bottom and the ground.
In winter, precipitation usually begins falling out of a cloud as ice particles. If the temperature underneath a cloud stays below freezing all the way to the ground, the ice crystals never melt and snow falls.
If the temperature is above freezing below the cloud bottom to the ground, the frozen particles melt into liquid droplets that reach the surface and this is called rain.
Ice storms occur when precipitation particles melt and then fall through a layer of subfreezing air near the ground.
Freezing rain forms when a very shallow layer of cold air is at the surface, causing the freshly melted raindrops to freeze on contact with exposed objects on the ground, which has a temperature below freezing.
Sleet results when the layer of subfreezing air at the surface extends upward far enough so that raindrop freezes into a little ball of ice.
When sleet hits the surface, it bounces and does not coat objects with a sheet of ice. Freezing rain covers everything in a sheet of ice, creating shimmering and treacherous road conditions.
Freezing rain is the precipitation type with the highest rate of accidents and death during the weather event. The number of deaths due to accidents is larger for snow events, but that is because snow is more common than freezing rain. Southern Wisconsin averages only about 20 hours of freezing rain a year.
