Details: the drivers behind the forecast Trimini

Foehn

Calculated and shown is the air pressure in Bolzano minus the air pressure in Innsbruck, i.e. south minus north of the main Alpine crest. If the value is positive (top), pressure is higher in the south and the air pushes over the Alps from the south. On the northern side it drops into the valley as a warm, dry and gusty foehn.

From about +3 hPa (dashed line) the pressure difference is enough for foehn. Whether it breaks through to the lake also depends on the upper wind and on dry air. On real foehn days the value usually reaches about +5 hPa during the day.

At Lake Kochelsee the foehn is one of the main winds, usually early in the morning between about 3 and 9 am. At Lake Walchensee it reaches the water less often; there the chance of wind only rises clearly from about +7 hPa. If the value is negative (bottom), the higher pressure lies in the north: no foehn.

Patscherkofel upper wind

Calculated and shown is the wind at the main Alpine crest above the Patscherkofel near Innsbruck at about 2 km altitude (800 hPa), speed and direction (right axis). A strong south wind there shows that air is flowing over the Alps from the south, a precondition for foehn at Lake Kochelsee. On its own it is not enough: the pressure difference (foehn chart) and dry air are needed too. In our measurements at Kochelsee, in the morning there was wind about four times as often with upper wind above 15 knots as with weak upper wind.

Upper wind & dryness

Calculated and shown are the wind at about 1.5 km altitude (850 hPa), upwards the share from the north, downwards the share from the south, and the humidity at the lake (right axis).

Foehn is dry air that comes over the mountains from the south and sinks. If a strong south wind blows aloft and the humidity drops clearly, the foehn is more likely to break through to the lake. In our measurements at Lake Kochelsee the morning wind came almost only with south wind aloft: with more than 10 knots from the south in 30 to 50 % of the hours, with north wind aloft in only 1 %. With humidity below 50 % there was wind in the morning in a good quarter of the hours, above 80 % almost never.

Cold air & thunderstorm outflow

Calculated and shown are the thunderstorm energy (CAPE) and the rain in the mountains south of the lake (right axis). The most common summer wind at Lake Kochelsee is not foehn but cold air: thunderstorms in the mountains send their cold outflow down into the valley, and after hot, clear days the cooled air drains towards the lake by itself in the evening and at night. On cold-air wind days in summer, the thunderstorm energy south of the lake reached about 500 J/kg in our data and about 8 mm of rain fell there, on calm days almost nothing. These winds usually come in the evening and at night.

Front (Alpine foothills)

Calculated and shown is the wind in the Alpine foreland, about 90 km north of the lake.If the wind picks up significantly there, a front often arrives from the west or northwest—the third major wind-generating factor at Lake Kochel, after the Föhn and cold air. The value alone doesn’t mean much: Only when there was strong wind in the foothills (over 16 knots) did the likelihood of wind at the lake increase noticeably in our measurements.