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Fog consists of minute water droplets suspended in the air. When visibility is less than 1 km, this is referred to as fog. It occurs in low-lying areas on the northern side of the Alps, especially in the autumn months.


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Fog is formed when water vapour condenses in the layer of air closest to the ground. In terms of its physical properties, fog is no different to low stratus or cloud. They all consist of minute water droplets that float in the air and reduce visibility.

Fog rests on the surface of the earth. The term "fog" is used when ground-level visibility is less than 1 km. Fog layers can be a few metres to several hundred metres thick.

Low stratus – when the fog is not at ground level

The term "low stratus" is used when the fog layer is resting a little way above the earth's surface. There is no reduction in visibility at ground level with low stratus. The thickness of a low stratus layer is usually 100-500 m. Weather forecasts include the height of the stratus top.

When the stratus top is around 2,000 m.a.s.l. or higher, meteorologists usually refer to low stratus-like cloud and do not give the height of the stratus top.

When is a foggy day deemed to be a fog day?

A fog day is when fog has been present at one of the three daily observation times (morning, noon, evening). This means that days on which fog dissipates in the early morning, followed by sunshine, are also counted as fog days for statistical purposes. In contrast, days with low stratus are usually not counted as fog days as the visibility is often greater than 1 km. In the low-lying areas on the northern side of the Alps, fog occurs particularly frequently in the autumn months.

Fog in autumn

Fog: a detailed explanation

The phenomenon of fog gives rise to many questions. Why is there often fog or low stratus on the Swiss Plateau in autumn and winter? Why is it so difficult to forecast fog? Meteorologists from MeteoSwiss have compiled answers to these questions in a dossier of articles originally published in the MeteoSwiss blog.