Föhn gap

B. Geerts


A Foehn gap is a cloud-free area just downstream of a mountain crest, between the cloud layers on the upstream side of the mountain range, and the lee wave (or lenticular) cloud on the downstream side (Fig 1). A rotor cloud is sometimes found below the leading lenticular cloud, but usually the air on the lee side is too dry to manifest this evidence of strong turbulence and updrafts.

Fig 1