In the Plaine d’Alsace (Alsace plain), autumn is a season for the formation of a thick fog that fades as you climb the Vosges mountains. Built on a rocky spur at an altitude of 757 metres, the Haut-Koenigsbourg castle in the Bas-Rhin offers a panoramic view of the plain, the Vosges and the Black Forest, even when the fog stagnates in the valleys. Meteorologists call this phenomenon thermal inversion. Heavy and humid, the cold saturated air remains at the bottom of the valleys forming fog while the warmer, drier and lighter air clings to the heights of the Vosges mountains.