Basic parameters like surface pressure, geopotential height of 500 hPa and relative humidity at 700 hPa are able only to visualise the synoptic environment being favourable for fog formation. For better understanding we need some derived parameters, more suitable for boundary layer investigations. In the following we present some such parameters, which tell something about the boundary layer conditions during the fog event. Site-specific profiles of humidity and temperature will be shown in the Chapter "Vertical Cross Sections".
Parameters and parameter combinations which can be helpful in the diagnosis of the fog layer are:
Boundary Layer Height is displayed and described in a 12-hourly sequence. In radiation fog situations the boundary layer tends to be shallow; also over the land BLH typically is lower than over the sea. The fields are forecast fields (12 h forecasts).
Low Level Cloud Cover is displayed and described in a 12-hourly sequence. This parameter varies between 0 and 1 (0 = cloud-free, 1= totally cloudy). The fields are forecast fields (12 h forecasts).
2-metre Dewpoint Depression is displayed and described in a 12-hourly sequence. The value is the difference between the surface temperature and dewpoint temperature. The fields are forecast fields (12 h forecasts).
Meteorologically it is clear that the situation was within boundary layer almost as stagnant as it was in synoptic scale: moist air, at or close to saturation, was trapped within thin boundary layer having less than 200 metres vertical extent. Low-level clouds which had formed, tended to persist, in the absence of external synoptic scale forcing.