If a flow is crossing over a mountain chain and therefore forced to rise, Stau cloudiness will develop if sufficient humidity is available within the flow. The Stau cloudiness mostly consists of low and middle clouds, depending on the height of the mountain range and the angle between the streamlines and the orographic obstacle. It exists as long as the necessary configuration between mountains and wind field exists.

Appearance in METEOSAT imagery

A belt or a bulge of relatively low but thick cloud forms along the windward side of a mountain barrier. Regularly there is an enhancement of preliminarily existing cloudiness in association with orography. In general, low grey cloudiness exhibiting a certain form and appearance can be seen as Stau cloud along a mountain barrier.
Sometimes Stratus and/or Fog Sheets which are slowly advected onto the mountain barrier have the same appearance as Stau cloudiness, but they are usually associated with synoptic situations with a low pressure gradient and weak winds at all levels. However Stau cloudiness is associated with a strong flow and forms after a frontal passage over mountains or redevelops within a baroclinic zone.

• IR imagery
  • Mostly a dark grey to grey cloud bulge indicating low cloud with a relatively warm cloud top
  • Light grey to white clouds form during cloud enhancement
  • Light grey to white clouds form during cloud enhancement over the main mountain ridge
  • The downwind edge of the mountain crest is sharply defined
  • Because of the strong upper level stream also lee or high fibre clouds can often be formed to the lee of the mountain chain, appearing as bright grey to white (see Lee Cloudiness ).
• WV imagery
  • Low level Stau cloudiness is usually not seen in WV images, but the enhancement of frontal cloud during its passage over the mountain chain can be well seen in WV imagery
  • Bright lee or high fibre cloudiness is also well seen in the WV image
• VIS imagery
  • The sharp edge along the downwind side of the mountain barrier is best seen in the VIS image

Stau cloudiness in Europe is often observed over the Scandinavian Mountains, the Pyrenees, the Carpatian Mountains, the Dinarian Mountains and the Alps.

Alpine Stau cloudiness

Alpine Stau clouds take different forms depending upon:

• The angle and direction of the flow against the main ridge of the mountain barrier, causing a characteristic appearance of the Stau cloudiness bulge and extension towards the windward side.
• Whether it is a Stau within a frontal cloud band or separate Stau cloudiness

About 81 cases have been investigated in detail; 53 cases show frontal Stau (12 cases Detached Warm Front) and 28 cases show separate Stau cloudiness (11 cases with Stau north of the Alps, 9 with Stau south of the Alps and 5 cases with Stau at the west Alps)

The schematic above shows the topographical distribution of the Alps with the main alpine ridge indicated (dashed blue)

1. Orthogonal (right angle) stream to the main Alpine ridge
   • The orthogonal stream has a dominating northerly component throughout all levels
   • Stau cloudiness frequently forms after a frontal passage, when remaining cloudiness is enhanced along the windward side of the mountains.
   
   The case of the 27 September 2002 shows Stau cloudiness over southern Germany and Austria from an orthogonal stream. In the IR image the Stau cloudiness can be observed as a separate feature after frontal passage (below left). the sharp edge towards the lee side of the mountains can be seen in the VIS image. The bright white cloud feature implies higher multilayered cloudiness (below right).

   27 September 2002/12.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models 02.00 - 17.00 UTC 2-hourly image loop	27 September 2002/12.00 UTC - Meteosat VIS image; SatRep overlay: names of conceptual models

   The IR image loop (above left) clearly shows the frontal passage and the development of remaining cloudiness as Stau. Further lee cloudiness forms over northern Italy at the end of loop. The behaviour of the Stau cloudiness in the loop can be seen as a stationary cloud sheet directly above the windward slopes and main mountain ridge in Austria, extending upstream towards Germany.

   Special case of orthogonal stream:

   • In this case the remains of a frontal cloud band are "tied" to the mountain chain, forming a characteristic elongated cloud band
   • During the crossing of the mountain barrier the frontal cloud band ascends within its baroclinic zone
   
   The IR image of the 02 January 2002 and the IR loop (below left) show a frontal cloud passage over Austria, orthogonal to the main Alpine chain. The remaining (and weakening) cloud intensifies again in the Stau area (bright grey in the IR image). The Baroclinic Boundary of the system lies over Germany and consists of dark grey low clouds. In the visible image (below right) the sharp southern edge of the Stau cloudiness can be observed. Also the enhancement during the Stau process can be seen in the WV image (2nd row below left).

   02 January 2002/12.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models 03.00 - 15.00 UTC hourly image loop	02 January 2002/12.00 UTC - Meteosat VIS image; SatRep overlay: names of conceptual models
   02 January 2002/12.00 UTC - Meteosat WV image; SatRep overlay: names of conceptual models

2. Southern orthogonal stream:
   • The same mechanism takes place, only with the opposite direction of the flow, and with the Stau cloud on the southern slopes
   • Stau over the southern Alps often causes heavy and persistent precipitation because of the high humidity content of the southerly flow arriving from the Mediterranean region. The resulting Stau cloud to the south is part of the flow regime of a classical Foehn situation. (see Foehn ).
   
   The case of the 15 March 2002 is shown below. After a Cold Front passage, remaining low cloudiness can be seen within the south western Alpine bow over northern Italy. A sharp edge to the cloud can be seen along the main Alpine crest in the IR image.
   15 March 2002/00.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models

   
3. Westerly stream
   • A distinct cloud bulge forms along the western Alpine bow associated with a strong westerly flow towards the western boundary of the Alps.
   • A secondary tangential thin branch of Stau cloudiness forms along the main north and north eastern alpine ridge.
   
   After a frontal passage from E. France, across S. Germany and W. Austria Stau cloudiness within a westerly flow is clearly separated from the most extensive post frontal Cold Air Cloudiness over western Europe. The Stau cloudiness extends tangentially to the main Alpine ridge with a pronounced cloud bulge over the western Alpine bow.
   The post frontal low level Cold Air Cloudiness forms a distinct sharp edge on the lee side (below left) which can also be well observed in the VIS image below right. The IR image loop below left shows the frontal passage and the development of the Stau cloudiness which remains long after frontal passage. The most pronounced area of the Stau cloudiness can be seen over the western Alpine bow, but the inner eastern Alps are also affected.

   29 May 2002/12.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models 06.00 - 22.00 UTC 2-hourly image loop	29 May 2002/12.00 UTC - Meteosat VIS image; SatRep overlay: names of conceptual models

4. Stau appearing within frontal cloud bands
   • The only indication of Stau within the front is the enhancement of the frontal cloud band over the Alpine ridge as a result of forced ascent along the windward side. The sinking motion along the lee side results in cloud dissipation (see schematic below)
   • Only embedded Lee cloudiness can be observed with this type
   
   In the IR image below left, the case of the 24 April 2002 at 18.00 UTC shows a pronounced area of cloud enhancement at the northern border of the western Alps and an area of descent at southern edge. The maximum cloud enhancement occurs over Switzerland and southern Germany. The WV image also shows enhanced and diminished grey shades.

   24 April 2002/18.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models	24 April 2002/18.00 UTC - Meteosat WV image; SatRep overlay: names of conceptual models

   One common synoptic situation associated with the formation of long lasting Stau cloudiness is the approach of a Detached Warm Front (see Detached Warm Front ). Here the downwind edge is not as sharply defined as usual. From experience, some cases of Stau show a shift of the Stau cloudiness somewhat to the lee side. This often seems to occur with Warm Fronts, with a marked southward displacement of high cloudiness.

   In the IR and VIS image from the 26 July 2002 the enhancement in the area of Stau associated with the frontal zone of a Detached Warm Front over Germany and Northern Austria can be seen. In the W. Alps Stau cloudiness develops from the residual low cloudiness after the frontal passage (below left and right). In the IR image loop it can be seen that the descent along the lee side becomes more pronounced. A slight shift of the Stau cloudiness somewhat to the lee side is also observable.

   26 July 2002/12.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models 00.00 - 22.00 UTC 2-hourly image loop	26 July 2002/12.00 UTC - Meteosat VIS image; SatRep overlay: names of conceptual models

Scandinavian Stau Clouds

Scandinavian Stau appears on both sides of the Scandinavian mountains. These areas are indicated in the schematics below. On the typical western or southern Stau event (left schematic) the clouds are associated with a more humid air mass due to the underlying Norwegian or North Sea, while for the eastern Stau (right schematic) the conditions are generally drier. The examples for both flow types are shown above. The AVHRR image on the left shows pronounced Stau Cloudiness on the western coast of Norway. The AVHRR image on the right shows Stau Cloudiness in Northern Sweden and Finland. Some Stau cloud is also seen in middle parts of Scandinavian mountains.

Wind directions that are favourable in Scandinavia are S-W stream in southern Norway and the W-N stream over northern Scandinavia. Even NW-jets passing over the southern tip of Norway have caused Stau Clouds. On the western side, the Stau cases often appear together with Lee Clouds and Foehn on the eastern side of the mountains. Foehn is common even for the easterly cases. In southern Norway Stau clouds sometimes grow larger covering a larger area than the coastal areas of south-western Norway.

In the easterly cases over the eastern side of the Scandinavian mountains, easterly low-level jets with wind speeds 30 knots or more are common. If the winds increase and back north, the Stau Cloud usually stretches southwards along Sweden. Easterly cases may be quite dry and therefore only quite thin stratocumulus develops. With the easterly winds, there is often Foehn on the western Norwegian coast. Even in southern Norway northerly winds have caused Stau Cloudiness that sometimes stretches down to the coastal region.

Stau at other European mountain ridges

In the set of images below typical Stau situations over the main central European mountain ridges can be seen (schematic below left). Below right (IR image) the Stau case from 30 July 2002 along the Pyrenees and the northern coastal mountains of Spain is displayed. Post frontal low level cloudiness over the Bay of Biscay is enhanced over the mountain barrier, well seen over the north coast of Spain. A deflection of the low level flow around the Iberian Peninsula is also indicated by the cloud pattern.

2nd row below left shows Stau cloud over the Carpatian and Dinarian mountains on 06 November 2002. Low stratus sheets are situated over the whole area, only slightly enhanced in the Stau zone.

In the case of 16 March 2003 (2nd row below right), Stau cloudiness over the Dinarian, Carpatian and over the Italian mountains is clearly visible. A distinct gap within the zone of sinking air over the Adriatic sea, with a second zone of Stau over Italy, develops during the process.