Characteristics and Observations of “Black Ice” in the Atlanta Metropolitan Area

Over six million people reside within the Atlanta metropolitan area, many of whom commute daily, resulting in significant traffic, even on days with no weather impacts. When unanticipated icy road conditions develop, severe travel impacts can result. Ice often develops on roads from freezing or frozen precipitation; however, on some occasions, roads become icy in the morning hours from other factors, such as the refreezing of meltwater or of liquid runoff on the road surface. The colloquial term “black ice” has been used by meteorologists, the public, and the media to describe these situations. This study attempts to characterize meteorological conditions at the surface and aloft associated with a series of black ice events that developed from the refreezing of liquid water from 1991 through 2015 in the Atlanta metropolitan area.

It was found that these refreezing black ice events occur approximately once a year in Atlanta, typically in the early morning hours, with an average length of two days. Decreasing temperature trends were strongly related to black ice development, with a significant swing from above freezing to below freezing temperatures often seen at the beginning of the event. A combination of increased cloudiness during the day and clear skies at sky was noted during prolonged black ice events, reducing daytime heating while enhancing radiational cooling overnight.

In the wake of two major icing events which struck the Atlanta region in early 2014, a set of Road Weather Information System (RWIS) sensors was installed for the first time. Data from these sensors, as well as METRo model forecasts of road temperature and condition (Crevier and Delage, 2001), were used to analyze a refreezing black ice event on 16 January 2015. The RWIS observations showed better ability to monitor developing black ice conditions compared with traditional airport-based 2m above ground observing equipment, while further work is needed to determine if the METRo forecasts were effective for anticipating black ice events. Continued research into these areas will enable local forecasters and transportation officials to better anticipate and the mitigate the hazardous impacts of future black ice events.