Freezing fog

From the National Weather Service in Jackson, Ky.: Freezing fog occurs when the water droplets that the fog is composed of are "supercooled." Supercooled water droplets remain in the liquid state until they come into contact with a surface upon which they can freeze. As a result, any object the freezing fog comes into contact with will become coated with ice. The same thing happens with freezing rain or drizzle