Details: the drivers behind the forecast Campione

Ora & Pelér driving force (pressure gradient)

The pressure gradient is the air pressure in the Po plain (Ghedi near Brescia) minus the air pressure in Bolzano. This difference in air pressure is the primary driving force behind the Ora and Pelér winds.

How the difference occurs:

During the day the Alpine valleys get warmer than the Po plain. Slopes facing the sun receive the sunlight at a steeper angle and therefore heat up more than the plain. In addition, the narrow valleys contain less air that has to be heated. As the air warms it rises, the air pressure drops and a pressure difference develops.

Nature tends to even out the difference. Air from the plain flows into the valleys. That is the wind we all want: the Ora, a south wind blowing across the lake from the south (positive value, light, top).

At night it is the other way round. The mountains cool down more than the plain, and the cold, heavy air flows from the north through the valleys and across the lake: the Pelér, a north wind (negative value, dark, bottom).

The bigger the difference, the stronger the wind. In our measurements the Pelér at Campione blows as a windy day from about −3.8 hPa in more than half of the hours between 4 and 11 am (dashed line). The Ora blows at the Lake Garda spots from about +2.9 hPa in more than half of the hours between noon and 7 pm (dashed line).

North foehn: pressure difference Innsbruck−Bolzano

Calculated and shown is the air pressure in Innsbruck minus the air pressure in Bolzano, i.e. north minus south of the main Alpine ridge. If the value is positive (light, top), pressure is higher in the north and the air pushes over the Alps from the north. From about +3 hPa north foehn breaks through in the northern valleys of South Tyrol, from about +6 hPa it reaches Bolzano. It rarely reaches Lake Garda, and then, as usual for foehn, it is very gusty. In our measurements at Campione, from about +7 hPa there was wind of at least 11 knots from the north in 29% of the morning hours, below that in 2%.

Ora driver: sun in the mountains

The sun drives everything. Without sun there is no heating, so no pressure difference and no wind.

Shown is the forecast solar radiation in the Adige valley near Trento, north of the lake (area), and dashed the radiation that would be possible there under a clear sky. The smaller the gap, the fewer clouds. The sun in the mountains supplies the heat that creates the pressure difference for the Ora (see pressure gradient). If clouds build up there around midday, the drive is missing and the Ora weakens. In our measurements at Campione there was almost never any Ora below about 50 % of the possible sunshine (dotted line).

Temperature gradient Adige valley−Lake Garda

Calculated and shown is the air temperature in the Adige valley near Trento minus the air temperature at Lake Garda. If it is warmer in the north (light, top), air rises there and air flows in from the lake: this helps the Ora in the afternoon. If it is colder in the north (dark, bottom), the cold, heavy air flows from the mountains to the lake: this helps the Pelér at night and in the morning. In our measurements at Campione, the Ora blew in 32% of the afternoon hours when it was more than 2 degrees warmer in the north, and only in 3% while it was colder in the north than at the lake.

Upper wind 700 hPa

Calculated and shown is the wind at about 3 km altitude (700 hPa) above the lake: upwards the share from the north, downwards the share from the south, plus the direction (right axis).

The Ora comes from the heating of the valleys and needs calm conditions aloft. If a strong south wind blows up there, it hardly develops. In our measurements at Campione there was almost never any Ora with more than about 10 kn South (dashed line).

A strong north wind aloft, on the other hand, pushes the Pelér. In our measurements at Campione, from about 20 knots of north wind aloft the Pelér blew in 10% of the morning hours, otherwise in 3%.