An innovative set of six observed predictors of fog formation by radiative cooling is proposed, which describes: 1) a central process of fog formation which is the water uptake by aerosols at surface level; 2) the fog development along the vertical into a few tens of meters, as well as the cloud cover; 3) the temporal evolution of the parameters describing the water uptake by aerosols and the cloud cover. The visibility is used as a signal of atmospheric and surface processes affecting the water uptake by aerosols. The vertical fog development is described by the vertical thermal gradient along a 30-m meteorological mast. The cloud cover is described by an association of a ceilometer for low and mid clouds above the instrument, and satellite data for clouds in the pixel and in a larger 9x9-pixel region around the site. The set of predictors can be observed in operational conditions, as on airport fields. Data acquired at the SIRTA platform (Paris, France) during four autumn and winter seasons were analysed, and the cloud cover classification derived from the SEVIRI instrument on the METEOSAT Second Generation satellite by the EUMETSAT/NWCSAF program was also exploited. The training data set was acquired in November 2011 while the validation data set extended over 12 months.
All situations were discriminated between favourable and unfavourable scenarii of radiative cooling fog formation, with a factor of 10 in probability. 246 moderate visibility events (visibility included between 5 and 10 km) were observed under clear-sky in 12 months at SIRTA. While developed fog formed according to one scenario, thin fog formed according to six scenarii. On the contrary to thin fog, developed fog did not form when cirrus were observed at visibility < 10 km. Fog formation probability varied from 12 to 43%, and even reached 100% for one thin fog. As the training data set was acquired in autumn, the predictor set seems more appropriate to nowcast fog in humid autumn conditions, and the fog formation probability was multiplied by ~2 in autumn. The predictor set could be enriched of predictors proposed in literature, and tests suggest that relative humidity would improve the fog formation probability in winter. Five predictors identify 65 no-fog over the 217 no-fog events counted in 12 months, with ~3 h anticipation time. If visibility decreased below 5 km, 88 other no-fog events were identified. In this case, the fog formation probability was 35% for developed fog, and 24-60% for thin fog, but with an anticipation time reduced to ~1.5 h. Only 1 fog, which represents 3% of the fogs, would be missed with such a predicting scheme.