Fog during the 2004–2005 winter season in the northern mid-atlantic states: spatial characteristics and behaviors as a function of synoptic weather types

Fog is a major factor for airport operations and understanding the factors that generate widespread versus localized fog under varying synoptic weather patterns is of importance to forecasters and airport terminal managers. Therefore the occurrence of fog during the 2004-2005 winter season (December through February, 90 days) was examined based on the frequencies of fog occurrence for 14 stations in the northern Mid-Atlantic States. Data were obtained for each site from monthly climatic summaries available online to determine both the frequency of fog occurrence days (i.e., at least one station reporting fog on that date) and those days when dense fog was also reported. The intent was to identify the spatial characteristics and behaviors of fog as a function of synoptic weather patterns. This information would be valuable to forecasters and would also allow assessment of the feasibility and ability to employ fog dispersal techniques.

An initial examination of the data indicated 75 fog events (or days) occurred (83% of study period) with at least one station reporting dense fog during 34 of those events (45%). Total frequencies of fog plotted by station showed a clear pattern of maxima in select coastal and inland regions. The data were then considered with regard to the frequency of three basic synoptic weather types: high pressure (34), low pressure (16), and frontal situations (22); with only three events not meeting these criteria. Within each synoptic weather type subtypes were also identified according to the location of the pressure and frontal systems (e.g., over, to the west-northwest, et cetera) with regard to the centroid of the study area (south-central New Jersey). High pressure accounted for 45% of all events with 32% of these experiencing dense fog, low pressure 21% of which 63% included dense fog, and frontal 29% in which 55% of the events included dense fog.

Consideration was also given to the spatial distribution of fog with regard to patterns of maxima and minima and the spatial coverage by event and synoptic types. When more than 10 of the 14 stations (i.e., more than 71% of all study region stations) reported fog, the event was defined to be widespread. The event was defined as discontinuous when 4 to 10 stations reported fog (i.e., 29% to 71% of stations) and as localized (or isolated) when less than 4 sites observed fog (i.e., less than 29% of stations). When examined according to these criteria, high pressure events experienced widespread fog only 29% of the time, discontinuous events 30%, and localized 41% of the time. Low pressure (and frontal) systems experienced widespread fog 69% (50%) of the time, discontinuous 12% (27%), and localized 19% (23%). Applying the same definitions to dense fog events revealed that all high pressure events were localized, 25% of low pressure events were discontinuous (75% localized), whereas for frontal events only 14% and 9% of the occurrences of dense fog were considered to be discontinuous and widespread.