The official term is a Basin:

https://en.wikipedia.org/wiki/Ostrava_Basin

We tend to suffer from "inversions" here, and way back in the coal times, the air quality used to be comparable to London during the Great Smog of 1953. Nowadays it is better, but still quite bad compared to rest of Europe.

In December 2024, I traveled from Ostrava to Warsaw and back via train, so through both Czech and Polish Upper Silesia. The Silesian part of the journey was a pea-souper, like riding through a yellowish cloud. (Warsaw itself had crisp chilly air.)