Dissipation of Fog

December 15th, 2006 |

The long nights of winter can be ideal for the formation of fog. Clear, calm conditions are conducive to strong radiational cooling, and if the cooling occurs over a moist surface, cooling to the dewpoint is likely and fog may be a result. Once fog has formed over a location, when will it lift? In large part that depends on the distance to the fog edge, as radiation fog will frequently erode from the outside in. Consider this example from December 15.

Fog image with isobars and plotted visibility

Visibilities are plotted and mean sea level pressure values (both from 1300 UTC) are contoured on this visible image from 1332 UTC on 15 December 2006. Very low visibilities (in statute miles) correspond to the region of fog and low clouds in central North Carolina. The sounding from Greensboro from 1200 UTC that morning (click on the thumbnail below) confirms that the saturated layer is confined to the surface and was likely a result of strong raditional cooling in the absence of any strong winds (as evidenced by the weak pressure gradient and the surface winds in the Greensboro sounding). The region of fog is centered on the Pee Dee River valley east of Charlotte.

GSO rawinsonde
What happens during the day? A thin layer of fog will dissipate as it mixes with the dryer air above it. For that to happen, mixing in the vertical that is driven by insolation must begin. Insolation underneath a dense fog is very minimal, but as you approach the edge of a region of fog, the fog thins and insolation increases, allowing more vertical mixing that entrains dry air above the fog into the fogbank, hastening evaporation. Because of that, isolated radiation fog banks such as this erode from the outside in.

Note in the visible image loop from 1332 UTC to 1815 UTC below (at 15-minute intervals, with some gaps) that this fog bank initially expands to the east. It is likely that regions east of the fog bank continued cooling to their dewpoint after sunrise. Cooling to the dewpoint at night was inhibited by the presence of upper level cirrus associated with a jet streak propagating up the East Coast.

The development of this fog at night was detectable using the method described here.
Loop of Fog Erosion

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