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WeatherBlog 7 2021/22 | Christmas weather in the west

The Christ Child brings a change in the weather

by Lea Hartl 12/22/2021
The high pressure will stay with us until tomorrow, but then the general weather situation will change to much more zonal, mild westerly weather. This means more snow, at least for the higher elevations, albeit at relatively high temperatures.

Current situation and outlook

We are still under the influence of the Omega High, which has been providing calm, sunny weather for some time. But the days of this weather situation are numbered. Tomorrow, Thursday, is likely to be much cloudier and warmer thanks to a warm front moving through from the NW. There may be a little rain in the far east, snow probably only above about 2000m and not much at any rate. Friday (24.12.) will initially be quite sunny, but the next disturbance from the west will be just around the corner and by the evening it will be cloudy again. There will be some precipitation here and there, but again not much is expected. By Friday at the latest, the last remaining lakes of cold air will probably have cleared out and the valleys, which have been cold due to the inversion so far, will be much milder than recently.

The national holiday weekend also looks mild and unsettled in the Alps, with lots of clouds, a few windows of sunshine now and then, and some precipitation from time to time, mostly in the southwest. At the start of next week, the south-west is in for a second helping, this time potentially in somewhat more substantial amounts, but there are still many uncertainties. Everywhere else it looks like fairly mild, changeable weather. The rest of the forecast is not particularly winter-friendly at the moment, but there is a lot of movement in the weather again and therefore a lot can still happen.

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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.

Advection fog usually refers to a different process, namely the sliding of warm air onto colder air in the course of a weather change, whereby the distinction to "advected radiation fog", as it often occurs in the Inn Valley, is not always linguistically clear. If you want to know exactly, you can find a complete diploma thesis on high fog in the Inn Valley here (we particularly recommend the diagram on the forecasting technique on page 12). For the Vienna Basin there is a detailed explanation by ZAMG. If you also want to know, but not in too much detail, here is a good general explanation.

We also recommend the annual forecast for 2022 from the German Weather Service. Good reading for all those who want to get a well-founded overview of the coming year!

Other than that, all that remains for the WeatherBlog today is to wish everyone happy holidays! Merry Christmas, a peaceful midsummer, a pleasant weekend! Pick something suitable for you.

This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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