A simple layer of fog, as benign as it may appear, can have dramatic effects on the region to which it afflicts. For some parts of the world the prediction of fog can be almost as clear cut as a precipitation forecast. Unfortunately, the Mississippi coast is not one of those areas. Under the influence of changing synoptic, regional, and local variables as well as multiple and varying fog formation processes, the prediction of this event along the Mississippi coast has become one of weather's more challenging tests. Fog, simply put, is a surface based cloud formed through cooling or humidification processes. Suspended water droplets form on small particles of dust, salt or sand known as cloud condensation nuclei as the layer of air reaches its saturation point. This can occur for many reasons resulting in the classification of fog types corresponding to the synoptic regime and geographic region to which the fog forms. Along the Mississippi coast, fog formation occurs as radiational, advective/sea or in a combination of types. Radiational fog occurs under stable atmospheric conditions when air near saturation at the surface is cooled to its dewpoint. This cooling is reached as the earth radiates out the heat absorbed during the day. Clear skies allow for a faster cooling rate, due to a lack of atmospheric insulation, while light winds preserve saturation by hindering any mixing of this near surface air. Wet soils as well as damp vegetation also help to influence this type of fog formation. Both evaporate water into the low lying levels of the atmosphere while vegetation also evapotranspires moisture through transpiration. The effect of these phenomena is added moisture and therefore shrinking dewpoint depressions near the surface. A high dewpoint as well as light precipitation formed over stable conditions also help aid in radiational fog formation. When a number of these factors are grouped together synchronously, the chances of radiational fog development greatly increase. When warmer air overrides cooler ground surfaces, advection fog forms. This can be a very troublesome type of fog along the Mississippi coast. When the low level wind direction switches to an onshore flow, especially during the summer, warm, moist Gulf of Mexico air is brought in over the cooling land surface. The addition of moisture as well as the lands influence on the relatively warmer air temperatures can cause the layer to reach saturation. Warm air advection, from this southerly component, also causes rising air. Many times, this slight uplift alone is enough to cause fog formation along the coast. This is especially true due to the moist nature of the warm Gulf air. Sea fog is also a form of advection fog. When warm, moist air is transported horizontally over a relatively cooler area of water, saturation occurs, usually on the seas own excellent hydrometer; salt. Unfortunately for coastal Mississippi, this particular event can impact not only the offshore regions, but as southerly winds continue to transport this air northward, the fog can move ashore. Many times with advection fog, winds continue to blow at speeds typically not conducive to fog formation allowing even experienced forecasters to bust forecasts. The cool season, in conjunction with morning hours seem to present the greatest number of fog days for the Mississippi coast. Synoptic scale features, such as the passage of mid-latitude cyclones, are much more common during these months as the influence of the subtropical high is not quite as great. As a mid-latitude cyclone approaches from the west, Mississippi will come under the control of the warm front. This influence stimulates a southerly wind flow allowing warm, moist Gulf air a swift ride towards the coastline. Under stable conditions associated with the warm front and the likelihood of dramatic air mass modification before frontal passage can occur over the coast, fog can become a persistent problem during these months. Transition seasons also provide an excellent atmosphere where land, sea and air all find the same magic temperature resulting in air saturation. And regrettably, the dominance the Bermuda high displays during the summer months coupled with the near daily afternoon rain shower or thunderstorm allows not for the warm season to be spared from this forecast challenge. Year round local economies, travelers, sportsmen, and personal safety are plagued by the impacts of this phenomenon. Coastal fog is not merely a nuisance to travelers, air, land and sea, but can become a serious safety issue regarding the reduced visibilities and therefore reaction times. When traveling by automobile, a significant impact will be noticed upon the presence of foggy conditions. Delays and even accidents are quite common. Air travel many times is delayed or cancelled as certain instrument flight rules must be reached for safe take off and landing. Military airborne and marine training sessions are often times sacrificed to the fog. Even travel by rail can be dangerous with recorded accounts of disasters due to reduced visibilities. Many do not realize the effects that coastal fog has on the energy costs some are so quick to complain about. When dense fog forms over the coastal ports, many times marine pilots cannot travel to or from the offshore oil rigs, let alone dock ashore. And when the movement and transport of new, time sensitive equipment cannot be executed, the entire country feels the effects. Forecasting coastal Mississippi fog events can be difficult for a number of reasons. Occurrence, extent, duration, and thickness can be very difficult to forecast and differ greatly in terms of time and space. Variables that impact fog formation also occur on many different levels making its diagnosis very difficult by computer models. For instance, synoptic-scale circulation would allow one to see fog formation as a macroscale incident however regional characteristics present mesoscale features while additional interactions are influenced through microscale processes (Croft 547). The Mississippi coast is a melting pot of these features forcing forecasters to look at a multitude of scenarios that may not be obvious for that particular morning. Unable to rely on computer models, the forecaster is required to look at an assembly of data, climatology, and personal experience to compile an accurate assessment of fog formation. Even then, if fog does occur, duration and thickness many times is busted on account of the variability and inconsistence of attributing factors. Experts have performed many local and regional studies in order to attempt development of better fog forecasting models, techniques and checklists. Alas, forecast models continue to miss and miss-forecast fog due to insufficient parameters and gross assumptions (Croft 548). Therefore, the use of satellite imagery, numerical modeling and climatology used only as guides, complete local and atmospheric analysis and in depth local rules of thumb have been considered some of the best methods of successfully forecasting fog over this particular area. Using these methods, absorbing the knowledge of those more experienced, and learning to be flexible with the ever changing effects along the Mississippi coast will help better forecast this event. Until a conceptual model intended to ultimately serve as a collection of "surfaces" in space and time that encompasses varying atmospheric conditions resulting in varying extents, intensities, and durations of fog is developed, in depth analysis of all available tips, tools and climatology will remain the best forecast method (Croft 549). Though fog may not serve as the fanciest or most popular weather event, its effects can still impact lives as dramatically as a thunderstorm or high wind event. With so many factors associated with its formation and the Mississippi coast a jackpot of these processes, the prediction of fog is not always a local success story. While experts strive to improve forecast tools and techniques, it remains the forecaster's responsibility to dedicate the necessary time required for such a delicate forecast. It may not be a prestigious challenge, yet forecasting fog along the Mississippi coast is undoubtedly a challenge reserved for those who can take a busted forecast or two. REFERENCES: - Bergot, T. and D. Guédalia, 1994: Numerical forecasting of radiation fog. Part 1: Numerical model and sensitivity tests. Mon. Wea. Rev., 122, 1218-1230. - Croft PJ, Pfost RL, Medlin JM, Johnson GA (1997) Fog Forecasting for the Southern Region: A Conceptual Model Approach. Weather and Forecasting: Vol. 12, No. 3 pp. 545-556. -ImpactWeather, Inc Meteorologists: http://www5.impactweather.com/ -Meteorologist Jeff Haby: http://www.theweatherprediction.com/habyhints/ |