These two effects, Stau and Foehn, are the reason for relatively cold air on the windward side and relatively warm air on the lee side of the mountains (at surface levels).
Convergence at low levels leads to a high pressure area within shallow cold air on the windward side, causing a characteristic ("nose like") horizontal pressure gradient across the mountain chain. This convergence is compensated by divergence above. A similar mechanism under opposite sign, leading to an area of low pressure, occurs on the lee side of the mountain chain.
Ascent of moist air is responsible for the enhancement or formation of Stau cloudiness along the mountain slope. The appearance, form and the extension of such a cloud pattern depends on the flow crossing the mountain chain. The direction of the flow to the mountain chain, e.g. if it is orthogonal, tangential, causes different cloud formations (see Cloud structure in satellite images). As mentioned above, the change in wind direction and velocity with height will be responsible for the intensity of the Stau event, either in the enhancement of the cloudiness or in the amount of precipitation. For example: if there is a northerly flow at low levels, and a strong westerly stream above, there will only be shallow cloudiness with weak precipitation on the windward side of the mountain chain.
The physical processes behind Stau cloudiness also show the concurrence between two main situations:
The Chen - parameter is an indicator for the tendency of blocking and deflection (flow around):
Values of the Chen - Parameter around 1 provide a non - blocking and complete over flow of air with a stable layer above the mountain ridge and geostrophic flow. Values <1 cause a blocking case with strong deflection of the flow at low levels. There is a split in the flow, with some flowing around the mountain chain.
Consequently the splitting/blocking has an effect on the intensity of the cloudiness and the precipitation of a Stau event: The flow splitting causes a more diffluent stream at middle and lower levels, leading to a decrease of convergence and lifting on the windward side.
The example below shows the case of 29 May 2002. Driven by the westerly flow a narrow Stau cloud forms along the western edge of the Alpine mountain ridge. In the wind field at 850 hPa (below left) a slight splitting of the flow around the mountain barrier of the Alps is seen. The hourly IR image loop from 06.00 UTC to 15.00 UTC shows the growth of the Stau cloud at the western Alpine ridge over eastern France. During the day the northern part of the flow is deformed and forced to propagate along the northern slopes of the Alps, also forming a narrow Stau cloud. The cloud texture of the Stau over southern Germany reflects the deformation and diffluence of the stream.
At the southern flank of the Alps marked cloud dissipation can be observed. The upper flow is indicated by the appearance of Lee Cloud fibres over northern Italy.
29 May 2002/12.00 UTC - Meteosat VIS image; magenta: wind vectors 1000 hPa, green: wind vectors 850 hPa
29 May 2002/06.00 UTC - Meteosat IR image