Dense fog has slashed visibilities to fractions of a mile west and south of the city since late Wednesday. The tiny, supercooled (sub-freezing but still in liquid form) droplets which comprise the fog can freeze on contact with colder outdoor surfaces and care is urged while walking or driving west and south of the city where icy patches have developed overnight. The fog is trapped by a layer of warm air and resides within a shallow layer of cold, dense air not much more than a thousand feet deep. Melting snow is behind the haze and fog of recent days and nights. This atmospheric set-up, which meteorologists refer to as a temperature inversion because the normal decline of temperature with height has been inverted, is having another effect beside the haze and fog build-up. Air quality, as measured by Illinois EPA, neared the unhealthful range in Chicago Wednesday and actually moved into the unhealthful range in the Quad Cities.
--By Tom Skilling, WGN-TV Chief Meteorologist
WGN-TV Chief Meteorologist Tom Skilling and the WGN Weather Center staff provide daily coverage of weather in the Chicago area.
