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Black ice is a deposit of ice which forms in a way which causes it to be transparent. Because the ice is totally clear, it is often invisible, which makes it very dangerous. Motorists are often at risk from these deposits because they don't see them on the roads, but they can still skid on them. Pedestrians and cyclists aren't immune to the dangers of black ice, as anyone who has fallen on seemingly safe pavement in the winter can tell you.
The formation of black ice begins when rain, fog, or mist deposits ice on pavement in cold weather. The latent heat of the pavement slows the freezing rate, so these droplets of water run together before they freeze. As a result, the air bubbles in the water are forced out, and the frozen sheet of ice which forms is almost completely clear. The ice often blends in with deposits of rain on a road, and it can also form with a matte appearance which makes the road surface look clear and dry.
Due to tricks of the wind and winter weather, black ice can form when temperatures are above freezing. In extremely cold regions, frozen ground and roadways can freeze when ambient temperatures are above the freezing point. It can also form on bridges and elevated walkways, because the wind cools these surfaces from both sides, making them much colder than the rest of the road.
Because black ice forms in a single sheet, it is extremely slick. Cleated shoes and tires can still slide on it, as they may have difficulty grabbing hold to get the necessary traction. These ice deposits form commonly in the morning and early evening, and can melt away as quickly as they form. Some winter traffic accidents are blamed on black ice which has done just that, when all the evidence suggests that a car encountered a patch of ice, even though the ice may have vanished by the time responders arrive on the scene.
Some regions are more prone to black ice than others, and motorists in these areas may be aware of this. In some cases, signs are posted to alert people to the danger. There isn't much that drivers can do, other than driving slowly in cool weather, since this ice blends in with the roadway perfectly. If you do encounter a patch of black ice while driving and you skid, take your foot off the gas and steer in the direction of the skid until you regain control of the vehicle.