Research Studies

 Louisiana and Southeast Texas
Fog Research and Modeling
(continued)

3) Marine Fog

Marine fog (or sea fog) is the deepest of all, reaching heights of 1000 feet or more as shown here in soundings A) and B). If sounding A) is some number above 1000 feet, then that number will most likely be the height of the marine fog. If sounding B) is 500 feet, then that will be the maximum height of the marine fog. This type of calculation may be used for all fog events except radiation fog.

Marine fog may move onshore as early as sunset to as late as near midnight. Marine fog ALWAYS needs a southerly component to the wind of at least 4kt to as much as 12kt. It is also known as advection fog but since it is produced and supplied by the marine environment, hence the name marine fog. Most of the time, marine fog will start out as a low level deck anywhere from 200 to 1000ft and move onshore during the late evening or around midnight. The ceiling will gradually lower to the ground with visibilities falling sharply. It is very unusual to see this fog move onshore on the surface and even more so while it is still daylight. Marine fog is the hardest to dissipate because it is so deep. It is normal for it to stay around until late morning and sometimes, though very uncommon, all day. In research, the largest marine fog producer was an arctic front. As an arctic front plunges through the gulf, strong northerly winds push the top warm water away from the coast. This is then replaced by upwelling of cooler water. A very sharp contrast in temperature begins to set up where the upwelled cooler water meets the warmer water offshore. At this interface, if conditions are right, fog in huge plumes will be created when the next front begins to bring return flow.

Marine Fog Needs

1) Southerly winds of at least 4kt and not more than 12kt

2) No moisture advection (it is the moisture advection)

3) No or negative omega

4) Clear or very high ceilings. (Not including the low level marine deck)

Marine fog events can occur under a strong subsidence inversion or a weak high level inversion as shown in soundings A) and B). The sounding will show a much deeper moist layer than radiation or frontal fog shows. The fog itself will be deeper as well.

4) Advection Fog

Advection fog is also a very deep fog and shows the same sounding profile as marine and frontal fog events as shown here. Marine fog that moves or spreads from one location to another is a type of advection fog. Here we will discuss another type of advection fog other than marine. Fog that forms to the east or west and moves into the area during the night will be looked at here. Most of these events are from the east. This would make sense because there is still moisture at the surface in the many marshes, swamps, lakes and rivers. The gulf has some to do with advecting moisture with southerly winds turning more easterly once over land. This fog has to have winds of at least 4kt but not greater than 12kt at the surface. It is very rare for this fog to move in as a cloud deck before reaching the ground such as marine fog. Advection fog rarely if ever moves in from the north.

Advection Fog Needs

1) Winds of at least 4kt but not more than 12kt

2) Moisture advection will be induced by the fog

3) Neutral or negative omega

4) No or very high ceilings. (Not including the low level deck that occasionally produces the fog.)

Advection fog needs almost the same conditions as marine fog except one. The wind direction must be to where the fog will advect into the area. This would be accomplished by a NE, E or Westerly wind. Advection fog can move in from the southwest or west. This most often occurs when the wind fetch is southerly out of the gulf and immediately shifts to the west over the coast. This type of wind structure may come from weak high pressure centered just off the northwest gulf coast. Again there is not much to dissipate this fog once it sets in. Advection fog acts like marine fog and normally has a marine association. The same times relate to set in and ending times as marine fog.

5) Conglomerate Fog

Conglomerate fog is just as it suggests; it's a mixture of different fog-forming conditions. This type is what we see the most. For example, a marine deck wouldn't normally lower to the ground if there were not some early evening boundary layer radiational cooling. Frontal fog will form at night after having advected moisture in from the gulf and some radiational cooling. Radiation fog will not form unless enough moisture has pooled in the boundary layer from rainfall or gulf moisture advection and the list goes on.

There is one type of fog that is difficult to understand. It forms in the morning as a thin ribbon no more than a mile meridionally hugging the coast and can extend to heights of 500 to more than 1000ft. Conditions existing at the time are those which would promote radiation fog. This fog also develops in the Calcasieu ship channel as well as rivers. Visibilities will range from 1/4 of a mile inside the fog to 10 miles outside. Without any observations or soundings within the fog, it is hard to tell exactly what is going on to cause it to form. This fog is very hard to find since it occurs in a relatively small area but it creates a large hazard to those trying to traverse it. The result is lost time for helicopters, ships and crewboats until it lifts enough to improve visibilities.

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