Thursday, November 8, 2012

Fog envelopes downtown Memphis on a frosty morning

Downtown Memphis was treated to a somewhat rare sight early this morning, especially if one had a vantage point from above.  A thick cloud of fog formed over the Mississippi River and drifted over downtown streets!

Photos taken by Jonathan May (Twitter: @jmay11) from the 29th floor balcony of One Commerce Square downtown (shown below) captured the essence of the surreal sight. Note that even the Hernando-DeSoto Bridge carrying I-40 traffic is nearly completely enveloped in the fog.  The bottom photo is taken looking southeast, away from the river, and shows that a "branch" of the fog had separated from the river and drifted east of the FedExForum towards the eastern portions of downtown.

Process of steam fog creation, courtesy USA Today.
What caused the fog?  On calm, chilly fall mornings, "steam fog" or "evaporation fog" is a common occurrence over relatively warmer bodies of water.  The necessary ingredients are the warm water below and cold, somewhat dry, air above, which are found in combination most frequently in the fall.  If you've ever driven by a pond or other small body of water, especially in an outlying or rural area on a cold fall morning, it's not unusual to see low-lying fog over the water.  This is called steam fog and it forms when cold air moves over warm water. As the warm water evaporates, it rises into the cooler air just above, cooling the rising air.  The cooling process changes the water vapor created by evaporation into visible fog.  In this case, the warm water of the Mississippi River produced steam fog on a grand scale!

Air temperatures over the water were in the mid 30s, while the temperature of the water was likely 20 degrees warmer. Fortunately, temperatures stayed just above freezing downtown, but had they been a few degrees colder, steam fog can quickly produce "freezing fog," which leads to black ice, particularly on bridges.  A few degrees cooler and this spectacle could have become a nightmare for commuters!

How can you create fog in the same manner steam fog is formed?  Exhale outside on a cold morning! See your breath?  You just mixed warm moist air into cold dry air.  You made steam fog!

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