Rain, snow, freezing rain, and sleet all generate hazardous traffic conditions. Freezing rain, and the less intense freezing drizzle, can create the very treacherous road condition referred to as “black ice.” A freezing fog may similarly coat objects in ice while also reducing visibility. Black ice is so named because the affected roadway appears dark, just like wet pavement. Black ice creates nearly zero friction conditions with vehicle tires so that correcting a skid in such conditions can be nearly impossible.
Sleet consists of translucent balls of ice that are frozen raindrops. It occurs when a layer of subfreezing air at the surface is deep enough for raindrops (usually freshly melted snowflakes) to freeze as they travel through the layer. Freezing rain forms when a very shallow layer of cold air is at the surface, causing raindrops to freeze on contact with exposed objects on the ground, objects whose temperature is below freezing. Thus, both freezing rain and sleet form when there is a temperature inversion near the surface – that is, when the air temperature increases with increasing altitude. Perhaps because of this underlying similarity, the difference between sleet formation and freezing rain formation is quite small, although the two precipitation types do not look alike at all. When sleet hits the surface, it bounces and covers flat surfaces such as roads and driveways with millions of icy ball bearings as opposed to the sheet of ice left in the wake of a freezing rain event.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
