Advection Fog

Advection fog is caused by a strong temperature gradient between the open Gulf waters and the inland water areas adjacent to the coast. This temperature gradient extends from Appalachicola, Florida to the Mississippi River.  The temperature gradient is attributed to cold outflow from the rivers and bays along the Gulf Coast.  There is a potential for a fog bank to form off the immediate coast when the temperature of Pensacola Bay is at least 5 degrees F colder than the Gulf.  The fog bank is produced by the return flow of continental polar air. The lower layers of air are modified by a 24-48 hour trajectory over the Gulf. 

The formation of a fog bank can be detected by an increase in the radar ground return pattern east of KNPA during the afternoon and in the visual GOES image. Keesler AFB (KBIX), Tyndall AFB (KPAM), and Hurlburt Field (KHRT) can also observe coastal fog banks. 

There must be a sustained onshore surface wind from 110 degrees to 250 degrees at 6-10 knots for a fog bank to move over immediate coastal areas. When the onshore winds are less than 6 knots or calm to light northeasterly during the early evening, the fog bank will remain offshore until early morning. 

An advection fog cycle usually begins 3-4 days after a frontal passage and does not dissipate until the next frontal system moves through.  Early in an advection fog cycle, the fog bank over land will dissipate by mid-morning due to normal heating. In a true advection fog situation, when the surface winds are sustained at 6-10 knots, the fog bank will remain adjacent to the immediate coast over water. When the land areas begin to cools, the fog bank will move back over land areas. Later in the advection fog cycle, dissipation occurs later in the day. By the third day 300' ceilings and 1 mile visibility occurs around 1100 local.  During the early afternoon, brief periods of VFR conditions are observed when the stratus layer based at 600-900' becomes scattered.  Zero-zero (0-0) conditions with occasional periods of 200-1/2 from 1800 until 2400 local is a good forecast.  0-0 conditions from 0000 to 0900 local usually prevail.  Visibility will improve briefly to 1-3 miles in prefrontal rainshowers and thunderstorms. 

Concept Mapping Toolkit
Insitute for Human and Machine Cognition
The University of West Florida