WeatherBlog 7 2021/22 | Christmas weather in the west

High fog

Before the high pressure phase comes to an end, let's take a look at the associated inversion, or rather the cloud cover that often lies above the inversion as high fog. The WeatherBlog is mostly based in Innsbruck and the high fog in the Inn Valley is also a recurring, popular topic of conversation among meteorologists. If the basic conditions for radiation high fog are met, i.e. an inversion, sufficient moisture in the inversion layer and low-wind, high-pressure weather, the further forecast is quite complex. The last few days have once again shown that fog can sometimes form and sometimes not, despite seemingly identical weather conditions. Local wind systems related to the topography are decisive here. Accordingly, local factors must be taken into account in the local high fog forecast, which differ depending on the location. Different criteria apply in the Vienna Basin than in Zurich or Innsbruck.

The pressure difference between the Munich Riem and Innsbruck Airport stations plays an important role in the high fog forecast practice in the Inn Valley. Always assuming that the initial situation is suitable (inversion, sufficient humidity in the air, radiant weather), the timing of the inflow or outflow from the valley towards the Alpine foothills is usually decisive. The smaller volume of air in the valley warms up faster after sunrise than the larger volume of air in the Alpine foothills. This causes the air pressure in the Inn Valley to drop and creates a pressure gradient towards the Alps (pressure in Munich is higher than in Innsbruck). Air and high fog flow into the valley from the foothills of the Alps. As soon as the sun goes down, the effect is reversed. The air in the valley cools down, the pressure rises. Air flows out of the valley. So much for the more or less usual valley wind system.

Slight changes in the larger-scale situation, such as slight differences in humidity or pressure, or the arrival of slightly warmer or colder air masses can influence whether, when and to what extent the valley wind system kicks in. On some days, the outflow may be stronger and the air up the valley drier, so that there is no longer enough moisture in the air for the next round of high fog. Or the side valleys and their valley wind systems have a good or bad day, interfere in the main valley and ensure that the wind in the Lower Inn Valley only turns an hour later. The inflow and outflow of cold air close to the ground can also influence the height of the high fog layer.