black ice

Black ice, sometimes called clear ice, is a thin coating of glaze ice on a surface, especially on streets. The ice itself is not black, but visually transparent, allowing the often black road below to be seen through it. The typically low levels of noticeable ice pellets, snow, or sleet surrounding black ice means that areas of the ice are often next to invisible to drivers or people walking on it. Thus, there is a risk of slippage and subsequent accident due to the unexpected loss of traction. Definitions The term "black ice" in the United States is defined by the National Weather Service as "patchy ice on roadways or other transportation surfaces that cannot easily be seen. It is often clear (not white) with the black road surface visible underneath. It is most prevalent during the early morning hours, especially after snowmelt on the roadways has a chance to refreeze overnight when the temperature drops below freezing. Black ice can also form when roadways are slick from rain and temperatures drop below freezing overnight." The World Meteorological Organization definitions of black ice are: A popular alternative for the term "glaze". A thin sheet of ice, relatively dark in appearance, may form when light rain or drizzle falls on a road surface that is at a temperature below freezing point or, alternatively, when water already on the road surface subsequently freezes when its temperature falls below freezing point. It may also be formed when supercooled fog droplets are intercepted by buildings, fences, and vegetation. Thin, new ice on freshwater or saltwater, appearing dark in color because of its transparency, a result of its columnar grain structure. On lakes, black ice is also commonly overlain by white ice formed from refrozen snow or slush.