The formation of a thunderstorm
Thunderstorms occur when three key conditions come together: high humidity in the lower layers of the atmosphere, an unstable stratification of the troposphere (warm at the bottom, cold at the top) and a triggering impulse that causes the air to rise. Such "triggers" can be thermal updrafts over sunlit slopes, convergences or approaching frontal systems.
As soon as warm, humid air rises, cumulus clouds form first. If these continue to grow vertically, a towering cumulus (TCU) is formed, which eventually develops into a full-blown cumulonimbus cloud (CB) - the typical thundercloud. This goes through three phases: Development (only updrafts), maturity stage (simultaneous updrafts and downdrafts, often with heavy precipitation) and dissipation (dominant downdrafts, decreasing precipitation).
An important feature is the wind shear. If the flow is very different at different heights, long-lasting and organised thunderstorm complexes can develop - including supercells with a rotating updraft core. If this shear is missing, the thunderstorm usually remains short-lived and localised.
Typical triggers: From the mountain flank to the cold front
Thunderstorms are particularly common due to thermally induced convection over highly heated surfaces such as mountain slopes. If these processes are additionally supported by high-altitude currents, the thunderstorms can also move into the neighbouring low-lying areas. Cold fronts, which can bring linear bands of thunderstorms, are also typical triggers. Convergence lines and outflowing cold air from older thunderstorms (so-called outflows) are also known to trigger new thunderstorm cells.
Thunderstorms can occur not only in the afternoon, but also at night and in the early hours of the morning - triggered by trough axes or cold air drops in higher layers of air. In many cases, several of these mechanisms act simultaneously.
Distribution of thunderstorms in Switzerland
The distribution of thunderstorms varies greatly from region to region. They occur most frequently in Ticino and in the foothills of the Alps, where thunderstorms can occur on 20 to 40 days per year. The frequency is also high along the Jura. In contrast, the Central Plateau and, in particular, the inner Alpine regions record significantly fewer thunderstorm days. In addition to the number of thunderstorm days, the density of lightning strikes is also a relevant indicator that varies greatly from region to region.
Dangers for visual flying
Thunderstorms are associated with considerable risks for VFR aviation. Even TCUs contain turbulence that can be dangerous for small aircraft depending on its strength. Downdrafts close to the ground, known as downbursts, are particularly threatening and can drastically change flight behaviour on approach or during take-off. These wind shears often also occur at some distance from the storm - especially in mountainous terrain, where wind structures behave in a channelled and unpredictable manner.
Hail is another serious risk. It occurs in thunderstorm cells with particularly strong updrafts. It should be noted that hailstones can also be transported outside the visible cloud areas by strong winds. Anyone flying below a thunderstorm anvil may be exposed to increased danger without realising it.
Heavy rain can also massively impair visibility. A VFR flight can become uncontrollable within seconds under such conditions. The combination of loss of visibility, turbulence and structural stress can have fatal consequences.
Strategies for flight preparation and execution
Anyone flying in potential thunderstorms should consciously prepare for the situation. The general rule of thumb is to fly well clear of thunderstorms. A minimum distance of 20 nautical miles is recommended at the rear of the thunderstorm. On the front side, the distance should ideally be based on the wind speed: at least one nautical mile distance per knot of wind - for example 30 NM at 30 kt.
An alternative route should already be planned during flight planning, additional fuel reserves should be taken into account and potential alternative airfields should be marked. It also helps to plan your time as flexibly as possible so that you can wait on the ground if necessary.
Various products are available to assess the weather situation: SIGMETs warn of dangerous phenomena such as thunderstorms or hail. SWC maps show the vertical extent of thunderclouds. TAFs, GAFOR, satellite images and webcams also provide valuable information. However, it is particularly important to keep an eye on the weather. The first signs such as rapidly growing cumulus clouds or the formation of an anvil should never be ignored.
Conclusion
Thunderstorms are among the most spectacular, but also most dangerous weather phenomena in aviation. Their development can be understood, but their effects always require increased attention and respect. If you keep a close eye on weather developments, are well prepared and strictly adhere to safe distances, you can fly safely and confidently even in summer. Vigilance, prudence and flexible decision-making are the most important companions in the cockpit.
Source references:
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