First autumn fogs in Europe

By Jochen Kerkmann (EUMETSAT)

During the first two weeks of October 2005 the weather in Central Europe was dominated by a high pressure system that led to fine, sunny weather with temperatures reaching over 20°C. However, this was also the time for the first autumn fogs to develop. Forecasting these first fog events is quite difficult as the forecasters tend to follow the persistency rule, i.e. they often miss the first fogs that develop over night, especially in the river valleys.

An example of such a development is given in the following images and animations. With the help of the IR3.9 and IR10.8 channels, it is possible to monitor the formation of fog and low stratus during the night-time (see upper left image and corresponding animation). Despite the large satellite viewing angle, one can see how the fog/low stratus develops during the night over southern Sweden and how it dissipates again during the following morning (see HRV animation).

It is interesting to note that Lake Vänern in southern Sweden is the last area to be covered by fog/low stratus. The reason for this is probably that in October the lake is still relatively warm and therefore the air above the lake remains unsaturated. Only at the end of the period the fog/low stratus is advected over the lake.

During the same night, patches of fog also developed in the upper Rhine valley, including Darmstadt. However, because of high-level cloudiness the fog is not visible in the RGB composite image (upper left). Only by looking at the animation can one actually see patches of greenish colour (indicating fog/low stratus) underneath the high-level clouds.

Finally, over Italy one can see an old low pressure system, which caused heavy rainfall (see also Genoa Cyclone and severe convective storm over the Mediterranean Sea (4 October 2005)). In particular, one may note the severe convective development that takes place to the south-east of Sicily, over the warm waters of the Mediterranean Sea (sea temperatures of about 25°C).