High fog in Switzerland
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The Stratus cloud form, known as high fog , can occur in the Swiss Plateau due to the trough location throughout the year. Typically, such locations are weather-effective between September and April: The effect is so great in Switzerland that the annual sunshine duration in the Alpine region is higher than in the densely populated Central Plateau despite the frequent congestion in the Alps and pre-Alps (with cloud formation only along the mountains).
frequency
While the air stratification for high fog in the Central Plateau generally also occurs in summer, the rate at which the high fog dissipates differs over the course of the year: From March onwards, the rather stable seas of fog with an upper limit of 1500 meters dissolve around 80 percent during the day. At the beginning of January the rate was 30 percent, at the end of November it was only 18 percent. The length of sunshine and the position of the sun, i.e. the angle of incidence of the sunlight, are responsible for this difference. Statistically, 25 days of blanket fog in the Swiss Plateau can be assumed.
The perceived duration of the fog period differs from the measurable frequency due to the difficult weather forecast, since the presence and the dissolution of the fog cover does not function reliably.
The symbol for the high fog in the weather report is accordingly to be found much more frequently. From September 1 to November 29, 2011, the symbol was found on 52 days in at least one part of the country. In 2012 the high fog symbol appeared in the weather report for the first time on September 6th, in 2013 SRF Meteo mentioned local high fog on August 21st. This means that it is no longer a matter of rapidly dissolving morning fog.
effect
The high fog location is characterized by a temperature reversal: cold air lies under warmer air and cannot escape from the "lake" that forms in the trough of the Swiss Plateau during a low-wind high pressure situation. High fog is created like fog when the lower layers of the atmosphere are damp or cold through
- night cooling
- Evaporation after heavy rain
- Supply of cold air ( Bise )
or a combination of these effects. The temperature difference between the layers can quickly be up to 25 degrees Celsius.
The topography is the decisive factor, the waters play less of a role or even have a balancing effect. According to the foggy days map in the Atlas of Switzerland, there is much less fog on Lake Zurich than in the further Limmat Valley west of the city of Zurich.
Thin high fog dissolves into haze in the course of the day, while in high pressure weather conditions and inversion conditions in the winter half-year it stabilizes over several days or in the case of an omega position over weeks and slowly becomes thicker and tougher. In densely populated regions with more emissions , it even begins to drizzle or, if it's cold enough, to snow lightly . Despite the influence of high pressure, there can be no more talk of sunshine and drought.
Due to the lack of air exchange, the fine dust limit values are regularly exceeded in foggy areas.
The temperature difference between the layers can easily be 25 degrees Celsius, for example on December 3, 2013, when the temperature in Gais was −15 degrees at 900 meters and a temperature of +10 degrees on the 700 meters higher Kronberg .
The height of the high fog is around 800 meters when the pressure distribution is weak. In the case of southwest winds, the high fog limit falls, but in the case of bise it rises. And the higher the level of fog, the stronger the bise.
Regional occurrence
The main fog areas can be seen well on the permanent sunshine map of a winter month (especially well in the slightly warmer February) and cover the entire Swiss Plateau. Areas particularly affected by fog are the Reuss valley below Muri, the Bernese Seeland, Unterthurgau, as well as the city of Lucerne and the trough behind Lake Lucerne from Buochs to Sarnen. With an upper limit of 800 meters, the high fog does not succeed in penetrating other valleys in the foothills of the Alps . This leads to the presence of a sometimes very abrupt edge of fog and leaves most of the lower valleys of the Alps fog-free.
If the upper limit is higher, around 1500 meters, MeteoSwiss expects the mist to penetrate into the large valleys of the main Alpine ridge. However, there is simply no statistical data on the fog, as Felix Blum from SF Meteo explained on the radio on October 25, 2012. However, there is a tendency to shift the location of the respective high pressure areas from Russia in the northeast to areas more in the north, which brings with it a stronger bise and thus higher fog limits and a deeper penetration into the Alpine valleys. This particularly affects the Lake Thun and Lake Brienz region to Meiringen or the Rhine Valley to Chur .
illustration
- Formation of high fog in the St. Gallen Rhine Valley on October 7, 2010
07:45 am, ground fog moves north to south (Bise) into the Rhine Valley, Sargans in the middle of the picture. Masts of high-voltage lines protrude from the fog.
08: 19h, the fog flows over the Rhine.
08: 24h, the fog is rising.
09: 21h, the fog continues to rise and by 11: 30h it reaches 900 meters, i.e. 450 meters above the valley floor.
11:55 am, the fog reaches over 1000 meters (the photo location 9:21 am in the fog), but cannot penetrate the Seeztal near Sargans.
14: 25h, the warmth of the day burns a few gaps in the fog.
16: 19h, the gaps are almost invisible from a flat angle at approx. 1100 meters, but the accumulating wave on the left slope is visible.
16: 49h, view under the blanket of fog with the sun penetrating through the gaps.
17:27, the wall of fog from Sargans, which remained fog-free all day.
information
In the city of Zurich, the trams carried a plaque on the front for many years when the fog was high, when the local mountain Uetliberg protruded from the fog; «Uetliberg bright». This inscription, which is difficult to see for outsiders, was sometimes supplemented with the inscription " Forch hell " on line 11 , if the pan handle was just as free. The tablets used to be carried on the trams, but have not been used since around 2000. Such a plaque can be seen in the Tram Museum Zurich .
The upper limit of the high fog is mentioned in the respective weather reports.
Individual evidence
- ↑ Sunshine duration - average - 1961-1990 - annually on MeteoSwiss ( Memento of the original dated December 2, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Dissolution of the high fog due to the position of the sun in March ( Memento from December 2, 2011 in the Internet Archive )
- ↑ Simone Lippuner: The "soup" has the Seeland firmly under control . In: Berner Zeitung . September 26, 2009. Archived from the original on October 30, 2011. Retrieved October 30, 2011.
- ↑ The weather report can be found on the last page of the 20 minutes
- ↑ 20 minutes from September 6, 2012 (PDF; 24.7 MB)
- ↑ a b 25 degree temperature difference from Gais to the Kronberg. ( Memento of December 14, 2013 in the Internet Archive ) Meteo-Switzerland Archive, December 3, 2013.
- ↑ http://www.meteoradar.ch/forum/forum_uploads/incoming/20060224_132343.jpg ( Memento from September 4, 2012 in the web archive archive.today ) Fog day map from the Atlas of Switzerland
- ↑ Insufficient air exchange and accumulation of air pollutants in the Tages-Anzeiger
- ↑ Meteo Switzerland: Where is the upper fog limit? , November 16, 2018
- ↑ Average monthly sunshine duration in January 1961-1990 on Meteo-Switzerland
