Louisiana and Southeast Texas |
1) Radiation Fog |
This is the type of sounding that occurs with a purely radiation fog event. This sounding is never found with advection or marine fog and rarely with frontal fog. If the height of the lowest inversion is 100 feet, then that will the maximum height this fog may reach. Although, radiation fog does not always follow this rule and it may be well beneath this height. It does not move much and develops from ground up. It is never very deep which means it burns off faster. The wind is calm to very light. This type of fog usually takes place as post-rain events up to 36 hrs or moisture increases during the day. When there is not sufficient moisture depth, this fog develops near the ground and stays within 2 feet of the surface. The radiation caused inversion starts almost immediately off the ground to 100 feet on average. Only when enough moisture is present will this fog get to 1/4 of a mile in visibility. Usually this requires some type of moisture advection process from the gulf or warm marsh and lake waters during daylight hours. Clear skies will always be needed as well. Radiation fog rarely envelopes the downtown areas of a city. The heat island effect is normally strong enough to keep radiation fog from forming. Radiation Fog Needs: 1) Winds 3kt or less 2) Rainfall within 36 hrs or moisture advection during the day. 3) Neutral or negative omega 4) Weak to no boundary layer positive vorticity. 5) Clear skies 6) Outside downtown areas
This fog is extremely vulnerable to existing atmospheric conditions. If any parameter changes, it will disperse readily. It may also be the second hardest to forecast since it can be a localized phenomenon. Although when widespread, this fog usually stays for the duration until sunrise.
|
2) Frontal Fog |
These two sounding types may be found with all fog events but they are most common with marine, advection and frontal fog. The moisture depth is most noticeable. The moisture depth is also necessary for any type of advection fog. The depth of the lowest inversion will tell how deep the fog potential will be. Environmental air from above the inversion should not mix with the air flowing below it. If this happens, a decoupled layer at the inversion will not be evident and drying of the moist layer will begin. Decoupling at this interface will not allow this to happen. Frontal Fog Needs 1) Moisture advection or rain within 36 hrs 2) Neutral or negative omega 3) Clear skies or very high ceiling
Frontal dynamics may cause other types of fog, such as radiation, marine or advection fog, to form. If a front will be responsible for enhancing fog associated with these types, then those conditions for forming each will also have to exist. Most times that fog forms, the front will be moving 10kt or slower and even more often it will be stalled within 250 miles. A pre-frontal trough doesn't have to pass but if it does it will add to the subsidence and moisture profiles. |