Details: the drivers behind the forecast Urfeld

Thermal drive: air pressure Munich minus Innsbruck

On sunny days, the wind at Walchensee is a thermal. It forms because the Alpine valleys heat up more during the day than the flat Alpine foothills. The slopes facing the sun receive the sun’s rays at a steeper angle and therefore heat up more than the flat terrain. (Due to the steeper angle of incidence of the sun’s rays on south-facing slopes, the same amount of solar energy is concentrated on a significantly smaller area than on flat terrain.) In addition, there is less air in the narrow valleys that needs to be heated. Air pressure drops over the mountains, and air from the foothills flows into the mountains to replace it. At Walchensee, the wind comes from the north to northeast via Urfeld and usually begins around noon.

The chart shows the air pressure in Munich minus the air pressure in Innsbruck, a pressure gradient. If it is higher in Munich (light, top), the air is drawn into the mountains: favourable. In our measurements at Urfeld, from about +2.3 hPa there was wind in more than half of the hours between 11 am and 6 pm (dashed line). If the pressure is higher in Innsbruck (dark, bottom), it works against the thermal: then there is wind in only about 4% of the hours between 11 am and 6 pm.

Thermal drive: sunshine at the lake

Without sunshine there is no thermal wind: only the heating of the valleys builds up the pressure difference. Shown is the forecast solar radiation at the lake (area) and dashed the radiation that would be possible under a clear sky. The smaller the gap, the fewer clouds. Sunshine all day is the best condition. If clouds build up from midday, the thermal weakens. In our measurements at Urfeld, from about 90 % of the possible sunshine there was wind more than twice as often as with more cloud (dotted line).

Upper wind 850 hPa

Shown is the wind at about 1.5 km altitude (850 hPa) above the lake: the line shows the strength, the dots the direction (right axis).

For the thermal, the direction matters most. If the upper wind comes from north to east (green), it supports the north wind over Urfeld: our measurements then showed wind in about 60 to 65% of the hours between 11 am and 6 pm. From the northwest (grey) it was just under half. Upper wind from southeast to west (red) works against the thermal: only a good third of the hours.

South wind aloft is usually foehn and hinders the thermal. Only when it is very strong (over 15 knots) does the foehn itself reach the lake. Then it is windy, but from the south and gusty.

Stratification: a lid over the valley?

For a thermal wind the warmed air has to be able to rise. Shown is the temperature difference between the ground at the lake and about 1.5 km altitude (850 hPa). If it is much warmer below, the air rises well: favourable. If the difference is small or it is even warmer above (inversion), there is a lid over the valley.

What matters is where this lid lies. If the cold air lies only in the foreland below the Kesselberg, it often pushes a north wind over the Kesselberg to Lake Walchensee (see Inversion and fog in the foreland). If the lid lies at the height of the Herzogstand, it also sits over Lake Walchensee and slows the thermal. The weather model resolves this height only roughly, so the value is just an indication.

Inversion and Fog in the Foothills

The graph calculates and displays the temperature difference between Lake Kochel (valley, about 600 m) and the Herzogstand (just over 1,000 m), as well as—on the right axis—the extent to which the Alpine foothills are cloudier than the lake.

Normally, it is warmer in the valley than higher up (positive value). If the value is below about 1 degree (dotted line), it is cold down below: an inversion. In such cases, fog or high fog often settles in the foothills, while the sun shines over the lake.

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.