Inversion & fog
CSENInversion and fog
Why a November basin stays grey from dawn to dusk while the sun shines a thousand metres above it.
Normally temperature falls with height — the higher you climb, the colder it gets. In a temperature inversion the opposite happens: through some layer of the lower atmosphere, temperature rises with height instead. Cold air stays at the bottom, warmer air lies on top of it, and because cold air is denser it has no reason to go anywhere. The layers simply stop mixing.
The most common version here forms by radiation. On a clear night the ground radiates its heat away to space and cools the air in contact with it. If there is almost no wind, nothing stirs that chilled surface layer into the air above. Colder air is also heavier, so it drains down the slopes and collects in basins and valleys — a literal lake of cold air.
Fog is just the visible surface of that lake. Cool the air far enough and it reaches its dew point, the vapour condenses, and you get a cloud whose base is the ground. Meteorologically it is stratus; from a summit you look down on it as a sea of cloud. That is exactly why fog sits in valleys and beside water while a few hundred metres higher the air is clear.
The crucial difference from a summer morning mist is how easily it clears. In summer the sun warms the ground, convection stirs the layers, and the fog is gone within a couple of hours of sunrise. In autumn and winter the sunlight is weak, the day is short and the inversion layer is deep — the sun may not break it at all. An inversion can therefore last several days, exceptionally weeks, and what usually ends it is the wind of an incoming front.
In Czechia inversions are most frequent where the terrain is most enclosed: the Ostrava basin, the foothills below the Ore Mountains, and the Prague basin. That has an unpleasant corollary — whatever is emitted into the air collects under the inversion just as the fog does, so inversion episodes tend to be poor air-quality episodes too.
The practical rule that follows is simple. During an inversion there is no point hunting for sun in the lowlands, because it is above them. The gap between a summit station and a lowland one tells you whether the drive is worth it, and a mountain webcam confirms it in pictures.
FAQ
Why is it warmer on the summits than in the valley during an inversion?
Because cold air is denser and drains downhill, where it pools, while the warmer layer stays above it. The layers do not mix, so the contrast persists — ridges can be ten degrees warmer than the basin below them.
When does morning fog clear?
In the warm half of the year usually within two hours of sunrise, once the ground warms and the layers mix. In autumn and winter the sunlight is weak and the fog can last all day, or for several days running.
Is fog the same thing as low cloud?
Physically yes — both are stratus made of fine droplets. The only difference is where you are standing: if its base is the ground and you are inside it, it is fog; if it sits a little higher, you are looking at low cloud.
Why does air quality get worse during an inversion?
Because the inversion layer acts as a lid. The air beneath it does not mix, so dust and emissions from traffic and heating accumulate. A longer inversion episode therefore usually means higher particulate concentrations too.
How do I know it is worth driving above the inversion?
Compare a summit station with a lowland one. If it is warmer up top, the inversion is holding and it will be clear above it. The Fog and inversion page works this out continuously from measured ČHMÚ station temperatures and links the nearest mountain camera.