Freezing rain is caused when you have a warm mass of air in the middle altitudes between the ground and the cloud deck, followed by a mass of freezing air near the surface. When the precipitation falls from the cloud, it will generally be snow. As it encounters the warm air, it will melt into the usual rain. But right before it reaches the ground, it enters the below-freezing air and quickly turns to ice. On impact, it usually freezes to whatever it lands on. That can turn streets into skating rinks in no time.
SEATTLE - Although not that common in Western Washington, freezing rain is one of the most dangerous types of winter-time weather around, as it can turn road and sidewalks into a sheen of ice in a very short time.