Now and then, the southerly foehn is accompanied by very little to no precipitation on the south side of the Alps. When a typical southerly foehn occurs, the influencing low-pressure area and its associated cold front lie somewhat further away from the Alps.
Southerly foehn also occurs without any precipitation whatsoever when there is a high-pressure area over south-eastern Europe (anticyclonic foehn). Dry air flows over the Alps from the southeast, which means that on the south side of the Alps, conditions are often practically cloudless, with no precipitation. This results in extremely low relative humidity in the foehn valleys on the north side of the Alps. With typical foehn, humidity is usually in the range of 30-40%. During anticyclonic foehn, the relative humidity can drop to 10-20%, and in rare cases even lower.
Northerly foehn – origins and typical characteristics
The typical northerly foehn is triggered by an area of high pressure over the Western Europe/Atlantic region. It brings cool, humid air from the North Sea area towards the Alps. The air masses cross the Alps from north to south, and the area of accumulation, with clouds and precipitation, lies over the northern slopes of the Alps. The thickness of the cloud layer decreases rapidly the further away it moves from the Alps. By the time it reaches the Jura, the cloud cover is usually already broken up.
With a northerly foehn, the largest amounts of precipitation are recorded on the central and eastern slopes on the north side of the Alps. The precipitation, however, is not as heavy as in the case of southern orographic lift on the south side of the Alps. Weather activity is lower in western Switzerland than in eastern Switzerland, as the high-pressure influence is more noticeable in the west. In Valais and Graubünden the sky is mostly cloudy, accompanied by the occasional shower, especially in Graubünden.
South of the main ridge of the Alps, the cool, heavy air from the north descends into the valleys. As it does so, it warms up and the clouds dissipate. The northerly foehn generally brings clear, fine weather and sometimes a considerable rise in temperature on the south side of the Alps.
Lee waves also form during a northerly foehn, with their familiar lenticular clouds and rotors. A strong northerly foehn can reach wind speeds of 80 km/h or more while still in southern Ticino, sometimes flowing out into the Po Valley. On rare occasions, the northerly foehn has reached as far as the Gulf of Genoa.