This weblog is graciously provided by Mike Smith. Mike Smith is CEO of WeatherData Services, Inc., An AccuWeather Company. Smith is a Fellow of the American Meteorological Society and a Certified Consulting Meteorologist. He is a recipient of the American Meteorological Society’s Award for Outstanding Contributions to Applied Meteorology and WeatherData has received the Society’s Award for Outstanding Services to Meteorology by a Corporation.
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In order to understand the effect of soot, the concept of “albedo” has to be explained. The definition of albedo is “The ratio of the outgoing solar radiation reflected by an object to the incoming solar radiation incident upon it.” Fresh, pure white snow has an albedo of nearly 100% — in other words, just about all of the solar energy striking the snow is reflected back into space. Since the heat is reflected rather than absorbed, the solar energy has relatively little melting effect on pure white surfaces.
However, if the ‘color’ of the snow is darked by soot, the albedo drops dramatically. Since the soot absorbs some of the radiation that otherwise would have been reflected, heat transfers from the soot into the snow resulting in an accelerated rate of melting. It is important to state that this heat transfer can cause melting to increase even if the ambient temperature remains constant.
I conducted a backyard demonstration on Christmas Eve 2007.
Here is a photo of fresh snow cover in my backyard over which I had tossed some eight month-old fireplace ash under a totally blue sky.
Keeping in mind this demonstration is occurring just two days after the winter solstace (meaning the albedo effect is less than it would have been under clear skies in February or March), in just one hour, the greater melting in the ash-covered areas is already apparent:
After four hours, the ash-free area has a depth of 5.5 inches
At the same time, the ash-covered areas have a depth of about 2.5 inches
The areas without soot melt about 0.5 inches of snow during this 4-hour period while the soot-covered areas melt 3.5 inches.
Any discussion pertaining to melting glaciers or icecaps must consider the accelerated melting caused by soot pollution in addition to any contribution from changing ambient temperatures.
Photos: Copyright 2007, Michael R. Smith