A rare and strong south foehn in the Alps, in which the strong foehn winds reach the ground only in the lower parts of the valleys and the plain or foreland. Under these conditions the sea level pressure difference between the south and north side of the Alps is greater than 12 mb. In the upper portions of the valleys, the surface winds are light and variable, as the strong winds do not penetrate to the ground. This effect has been attributed to the persistence of a cold-air layer in the upper valleys, decoupling the surface winds from the flow aloft and making the flow unable to follow the terrain in this region, or to the position of this region with respect to the long mountain wave that may accompany this foehn. The foehn wall and the precipitation area extend beyond the crest across the almost calm surface region in the upper valleys. This condition “is known as ‘dimmer-foehn,’ meaning blocked or dammed-up foehn. ”