The entire Alpine region was able to enjoy fantastic radiant weather with sunshine and stable conditions over the past week. If you looked dreamily down into the valley while taking a breather on the ascent or enjoying a cappuccino on the sun terrace, you could often see a brown-yellow haze or even a low-lying layer of cloud.
When high pressure persists, large-scale subsidence of air masses occurs. Relatively warm air then rests on colder air close to the ground, which forms in stable weather thanks to nocturnal radiation and often lies like a lake in basins in winter. As the sun is largely too weak in winter to cause thermal lifting processes, the cold air remains on the ground without mixing with the air above. As a result, the air in the valley collects more and more exhaust gases and particulate matter and appears yellow when viewed from above.
Radiation fog
In the lowlands, persistent high fog often forms in such weather conditions and remains for a long time. The moisture contained in the air condenses at the temperature inversion and forms high fog. The Vienna Basin and the Swiss Plateau are particularly susceptible to this. If there are many condensation nuclei in the air due to industry and other sources of particulate matter, it can easily snow or rain from the fog. In mountain valleys, such as the Inn Valley, high fog also occurs in winter, but it often dissipates during the course of the day due to the onset of slope wind circulation.
High fog forecast
This type of lifted radiation fog is difficult to predict, as the models do not resolve the classic winter inversions or are usually of the opinion that the inversion sinks almost to the ground. The upper limit of the fog is located at the boundary between the layers of warm and cold air. The exact location of this can be seen in radiosonde ascents, for example, and the upper limit can therefore be specified quite accurately. The stronger the influence of high pressure and thus the sinking, the deeper the high fog lies. If there are no topographical wind systems involved, high fog usually only dissipates when the weather situation changes and, for example, a foehn wind blows away the cold air on the ground, as is currently the case.
High fog-like clouds
On the other hand, shallow convection, which is capped at the top and also produces clouds, is reasonably predictable (with well-resolved CAPE maps). In the weather forecast, this is usually referred to as a "high fog-like cloud cover". In contrast to "real" high fog, the clouds are then more tower-shaped and do not form such a uniform layer, and the upper limit is often higher.
Weather outlook
And what's next for the weather? The lakes of cold air in the valleys should have cleared out everywhere by now thanks to the strong Föhn wind. While it is snowing rather unproductively in the south, hardly anything is likely to arrive in the north and it will remain mild at least until the beginning of next week, with generally changeable weather. Next week there is potential for an impressive return of winter, at least in terms of temperature.
