Mountain ranges have important effects on the airflow across them. One such local effect is the föhn or chinook. It is a strong, gusty, dry and warm wind, which develops on the lee side of a mountain range when stable air is forced to flow over the barrier by the regional pressure gradient.
In November 1982, the Alps were affected by an exceptionally marked föhn event that resulted in strong winds in Austria, Germany and Switzerland. The observed pressure gradient across the Alps reached a record of 20 hPa causing winds of up to 190 km/h across the Gottard Pass. While Locarno observed an air temperature of +1°C with mixed rain and snow, skies above Zürich were clear with a temperature of 25°C.
The Meteosat-2 infrared satellite image dramatically shows the föhn situation in the morning of 7 November 1982. There was an anticyclone over the Balcans (1035 hPa) and a deep low over the Atlantic (960 hPa). The resulting southerly wind was forced to rise as it met the Alps.
The low clouds on the southern slope of the mountains is not clearly visible in the image because the cloud top temperature is nearly the same as that of the surrounding terrain. However, on the northern slope of the mountains a darl stripe can be clearly seen that represents the terrain heated by the strong, warm Föhn flow.
In the picture below the
synoptic surface chart for the above situation is shown:
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