Between the Jura Mountains and the Swiss Alps lies the Swiss Plateau [Swiss Midlands]. It is some 50 km wide and extends 300 km from Lake Geneva in the extreme southwest to Lake of Constance in the extreme northeast. It is by far our most densely populated region, and the most important with respect to economy and transportation.
In the winter half-year, the air on the Swiss Plateau can stay still, building a lake of cold air with a ceiling of high fog. These clouds look like an ocean of fog when seen from above [usually above 800 m]. Hence, it is called the ‘Nebelmeer’ [Nebel = fog; Meer = sea]. Sometimes, this sulky, ‘inversion’ weather lasts for days or even for weeks, during which the neighboring regions of the Alps and the Jura may have bright sunshine.
Hiking just above the fog line I find really fascinating. The ‘Nebelmeer’ is highly dynamic and changes its appearance constantly.