Foehn is a special weather situation which appears on the lee side of a mountain chain in the situation where an air stream is crossing it perpendicularly. It is a weather situation characterised by very strong, dry and warm winds. There are also features in satellite imagery connected to these processes which appear as typical modifications of pre-frontal and frontal clouds. Four case studies stemming from half a year are investigated in detail in this chapter; all from the Alpine region.

• The satellite images show a Cold Front or Occlusion cloud band crossing mountain ranges
• During the passage a typical modification of the leading parts of the frontal cloud takes place
• In the IR image:
  • Darker grey shades develop on the windward side connected to the Stau cloud
  • a cloud free wedge/band develops within the mountain area; sometimes some Orographic Convergence Lines, in the form of dark grey cells/lines can be seen in the cloud free area;
  • Very bright cloud with a sharp boundary at the "outermost" mountain range extends downstream; this is high Lee Cloud;
  • Lee Cloud sometimes becomes detached in due course.
• In the VIS image
  • a bright cloud field appears on the windward side representing Stau cloud
  • a cloud free wedge/band develops within the mountain area; sometimes some Orographic Convergence Lines, in the form of white cells/lines can be seen in the cloud free area.
  • On the lee side different types of clouds are possible:
    • No clouds or only sporadic small scale low cloud on the lee side with some fibrous cloud superimposed where there is high Lee Cloud;
    • dense frontal cloudiness (white to light grey) with smooth structures where the high Lee Cloud is superimposed upon lower cloud.
• In the WV image:
  • The WV image shows the same features as the IR image.

The channels used for the images below show very well many of the features summarised above: the different grey shades (in the VIS and IR image) of the cloud on the windward and lee side (in this case S and N of the Alps); the generally cloudfree band within the mountain chain; the similarity between IR and WV image.

Foehn at the Alps

As has just been demonstrated in the example above, one mountain range which frequently produces Foehn events is the Alps. Intensive Foehn appears north of the Alps when there is a distinct southerly stream ahead of a frontal cloud band approaching from the W; under such circumstances a life cycle can be observed:

• Approach of a (broad) multilevel frontal cloud band from the W to SW sector; Foehn substructures appear when the frontal cloud band reaches the W. Alps and Switzerland;
• Lowering of the cloud tops S of the Alps;
• Development of a cloud free wedge extending from Switzerland to E. Austria;
• Development of the high Lee Cloud especially over E. Switzerland and Austria extending downstream into Germany;
• Development of a cloud free (very dark in IR) small scale area N of the Alps over W. Austria and Bavaria;
• This development takes place at the same time as the separation of Lee Cloud;
• W-E movement of all these features as the front moves east.

The IR image loop shown below is typical of a case where the mesoscale changes appearing within the multi-layered frontal cloud band are very distinct in the IR. The lowering of the cloud tops south of the Alps can be seen as well as the development of a cloud free band over the mountain chain combined with the development of high Lee Cloud to the north. A bit later an even darker small scale area develops over W. Austria moving eastward.