Cold fronts and the squalls that precede them should be avoided when flying. The situation is different with the BACK of the front, i.e. what the cold front brings in its wake. This BACK can mean the best flying conditions for the lowlands, while rain falls in the inner Alps and the foehn blows south of the Alps. So much weather variety in a small area calls for an attempt at an explanation if you want to be in the right place at the right time.
Warm and cold air meet
Switzerland lies in the westerly wind zone and our weather/weather changes therefore come, not surprisingly, from the west. Westerly wind conditions (there are various) are also the most common large-scale weather conditions in Switzerland. Westerly wind weather is changeable - imagine typical April weather for example - and is characterized by a change between warm air masses from the south and colder ones from the north. Where different air masses meet (fronts), there is turbulence, things come and go, the weather changes. These frontal passages (i.e. warm and cold fronts) lead to different weather north and south of the Alps, in the lowlands, the foothills of the Alps and in the inner Alps - and flying conditions ranging from good to miserable.
Cold air makes its way
Perhaps not quite rightly, cold fronts are not valued much in aviation circles. There is no doubt that the passage of a cold front and the gust lines in the air that may precede it by a few hours can be dangerous. The speed of a front (30-80 km/h) can be measured and its "schedule" can therefore be predicted well; even if the cold front moves in without any far-reaching cloud harbingers, it can only surprise those who fly headlong and without preparation. The actual passage does not last long, 30-60 minutes. If you include the pre- and post-frontal phases that are tricky for aviation, it is nevertheless an event of foreseeable duration, around 3-6 hours. Over the course of the year, cold front passages in the summer months are a more dramatic affair and are associated with more obvious side effects (thunderstorms, squalls, drops in temperature, etc.) than in winter. In any case, however, a cold front does not pass unnoticed and brings with it upheaval - revolution! -; Stable layers are destabilized, the weather cards are reshuffled. And that is why the cold front is so important to appreciate: despite all the fanfare and the hours when it is impossible to fly, a cold front brings fresh, unstable and therefore updraft-prone air.
From the back side
Immediately after the cold front passes through, i.e. on the back side, the air mass sinks over a large area and the clouds disperse. This is known as post-frontal subsidence, and a short-term (a few hours) improvement in the weather occurs. In this phase, the sun hits the moist, unstable air, which causes evaporation and very quickly forms cumulus clouds with deep bases, and the blue sky gradually disappears again. The cooling associated with the incoming cold air intensifies this process (cold air is saturated with moisture more quickly than the warm air that is receding), the cumulus clouds grow and showers form. In our latitudes, cold air generally originates in the north, warm air in the south. In keeping with this fact, with the influence of the cold air on the back side, the wind direction changes from pre-frontal and frontal SW/W to post-frontal NW. The cold front is often stormy and even on the back side, the wind speed remains sporty for the time being. The change in direction to NW/N pushes the moist, unstable air mass against the Alpine arc, where it is massaged and lifted: this leads to a build-up of air and precipitation in the foothills of the Alps and the Alps. Regardless of the difference in air pressure, it is now expected that a northerly foehn situation will establish itself on the southern side of the Alps, at least for a short time. In order to be able to cover the timing of this post-frontal event, it can be assumed as a rough estimate that all of this will happen in the first 12 hours after the cold front passes through; depending on what follows, it will last longer.
What next?
After the low-pressure tumult of the cold front, there is always a rise in pressure (cold = heavy air = higher pressure), but this does not mean that high pressure is present, not yet. The further course of the weather can vary greatly. We are ignoring the southern side of the Alps for now; the potentially foehn-like/unflyable situation there will not be alleviated for the time being, regardless of the subsequent scenarios. One such scenario, the classic and most common, is the influence of high pressure. The low that brings the fronts moves further east and the high pressure area that follows gradually establishes itself, the weather calms down, the wind turns noticeably clockwise from NW to NE, subsidence sets in, and it becomes bluer again. If this high pressure influence is only short-lived (two days of good weather, for example) and the next low is foreseeable, it is referred to as an intermediate high. In a very lively, stormy westerly wind situation, it can happen (this is another subsequent scenario) that fronts pass through very quickly and one after the other, so that no rear side forms and the front is followed by the front; unflyable, squally weather dominated by low pressure prevails.
The two sides of the back
The following two images were taken at the same time, on April 30, 2022 at 10:10 a.m. Two hours earlier, a weak cold front passed through Switzerland. The first image shows the view from Üetliberg Kulm over the Auegstertal and Säuliamt towards the south: post-frontal subsidence, the sky becomes a patchy blue, the sun heats up, the precipitation evaporates and cumulus clouds form with deep, differently high bases. That looks promising! (Rountshot Üetliberg) The second picture captures the view from Weissboden in the Schächental over the Haldi to Hoch Fulen in the south and the main Alpine ridge: a closed cloud cover with showers and few signs of a short-term improvement in the weather. We are in the wrong place here! (Roundshot Biel-Kinzig)
The high low
In any case, the decisive factor in whether flyable weather will develop after the cold front is the low pressure area that belongs to the low pressure area. As long as the low pressure area has not yet crossed Switzerland, is lagging behind the low pressure area and is above the Alps, the weather will not calm down. The low pressure area continuously supplies cold air to the upper atmosphere, and the strong destabilization and vertical circulation of the air mass never ends: after the cold front has passed (i.e. after the low pressure area has moved on), it remains thundery, spooky and windy. To use the technical term, this is called a cyclonic back pressure area. So only when the low pressure area moves away and warmer air flows in at higher altitudes does the weather calm down and improve, the revolution is over, we have anticyclonic, flyable back pressure area.
The back flies
An anticyclonic (high pressure determined) backside is also unstable and a temporary weather situation. Precipitation activity in the Northern Alps remains and the cloud bases only gradually rise. Where the sun penetrates, very good thermal conditions can be expected. It is therefore important to stay in or as close as possible to the sun-favored lowlands. If you want to regulate this, you could say: the further north, the better. The Black Forest, the Vosges and the adjacent areas on Swiss soil would be ideal. The Zugi and the Tritt are known for great backside flying conditions in central Switzerland and, as soon as the wind strength decreases, the Üetzgi (thermal, without a bise). While the lowlands generally only become thermally relevant from April/May, flying on a backside can be surprisingly good as early as March. With ongoing high pressure influence, the lowlands stabilize and the hotspots move ever closer to the Alps. The south side of the Alps is not recommended in weather on the back side - even if sunshine is favourable, foehn winds must be expected. Due to the thermally very active air mass with a tendency to overdevelopment and the gradually decreasing wind speeds, the fresh back side can be crispy flying conditions, perhaps too crisp for some. According to old aviation wisdom, the second day of the back side/after the front is the better day to fly; the remaining moisture has evaporated, the wind is weaker and nothing stands in the way of trying to fly in the flatlands. Another piece of wisdom: the faster the clouds clear after the cold front passes through, the more unstable the weather that follows. And one last thing: the longer the text, the lower the concentration and ability to absorb information. So please turn the pages now! Have fun and enjoy your flights on the next back side!
Gleich nach der Kaltfront und vor dem Regen. Blick vom Zugi in Richtung W und tiefbasigen Schauerwolken. Dahinter zeichnet sich jedoch Hoffnung ab.
Blick vom Tritt über Einsiedeln in Richtung NO, wo formierte Wolken der durchgezogenen Kaltfront auszumachen sind.
Gleicher Moment, gleicher Ort, Blick in Richtung SW und Klein und Gross Mythen, frisch gezuckert.
Klare Sicht, Cumuli mit tiefer Basis, und Wind aus NW: die frische Rückseite und einer der es wissen will.
3 h nach dem Durchzug der Front: Hike & Fly vom Chli Aubrig, postfrontale Subsidenz und erstes Aufklaren am Sihlsee.
Flug von Rigi Kulm mit Blick in Richtung SO, Lauerzersee und Chli und Gross Mythen. Die Front zog am Nachmittag des Vortags durch, noch sitzt die Restfeuchtigkeit in den Hängen, der Hochdruck macht jedoch seinen Einfluss geltend.
Selber Flug, entgegengesetzte Blickrichtung. Der Zugersee in Watte gepackt.
Im Flug vom Zugi, Blickrichtung NO. Der Hochdruck ist bemerkbar, noch kontrolliert und stabilisiert er jedoch nicht genug und Überentwicklungen sind an der Tagesgangordnung.
Der zweite Tag der Rückseite und Traumbedingungen im Flachland: Sonne, klare Luft und 1-3/8 Cumulus humilis. Blick- und Flugrichtung Albis und Zürich, mir nach!
