Guest article by MeteoSwiss: Summertime means thunderstorms

Thunderstorms are fascinating weather phenomena, but they pose a variety of hazards for aviation. From extreme turbulence and squalls to heavy rain, hail, and lightning strikes, many mountaineers and pilots have been caught off guard. Even if stormy weather conditions are clearly identified, it is not possible with today’s weather models to predict the exact location and time of a thunderstorm.

This guest article by MeteoSchweiz first explores the question of how thunderstorms form. It then goes on to show where thunderstorms occur most frequently in Switzerland and what effects they have on air traffic. The aim is to enable pilots to recognize and assess stormy weather conditions and take the appropriate precautions.

How do thunderstorms form?

The following conditions must be met for thunderstorms to develop:

• Sufficient moisture, especially in the lower layers of the atmosphere (high dew points)
• Unstable stratification of the troposphere (warm at the bottom, cold at the top)
• Triggers for initial lifting

A trigger forces warm, moist air to rise. This causes increasingly large cumulus clouds to develop, with the towering cumulus (TCU) already reaching a considerable vertical height. The mature thundercloud is known as a cumulonimbus (CB).

Figure 1: Life cycle of a thunderstorm

a) Towering cumulus TCU: only updraft and later precipitation formation b) Mature CB: both updraft and downdraft and precipitation c) Dissipating CB: downdraft and precipitation. Graphic: National Weather Service

In order to identify weather conditions with an increased tendency to thunderstorms, it is important to distinguish between the various triggering mechanisms. The best-known trigger for thunderstorms is undoubtedly convection, i.e. the rising of warm air on a sunny day above a heating surface such as a mountain flank. With the right wind conditions, mountains also cause uplift. In some cases, thunderstorms can move from the mountains to the lowlands with the right upper air flow. Apart from this, thunderstorms in flat areas are often triggered by fronts. Cold fronts in particular are known for their line-like CBs. However, convergences are also good triggers for thunderstorms: when air flows together, lifting occurs. This sometimes takes the form of an organized convergence line in front of cold fronts (squall line). Cold air flowing out (outflow) from already active thunderstorms can also lead to lifting and thus to new thunderstorms.

Figure 2: Thunderstorm triggered by a cold front. Graphic: Karl Heinz Hack

Even though thunderstorms often occur in the second half of the day, there are sometimes thunderstorms at night or in the morning. Here, trough axes play a decisive role as triggers. These CBs have a high base at around 10,000 ft. Even a small dynamic trough (a dent in the altitude pressure field) or a thermal trough (some cold air at high altitude) is sufficient to activate the air mass. On a summer day with unstable and humid air, several triggering mechanisms are usually responsible for the development of thunderstorms.

Wind is another factor that must not be overlooked on the thunderstorm checklist. Without sufficient airflow, only isolated and short-lived thunderstorms develop, in which the resulting downdrafts immediately suppress the updrafts. With sufficient wind shear, updrafts and downdrafts are spatially separated from each other, allowing long-lasting and stronger thunderstorms to form. If there is also rotation in the updraft area, a supercell can develop. Thunderstorms can also organize themselves into long-lasting multicell complexes that cover a large area and produce significant precipitation.

Where are CBs most common in Switzerland?

In Ticino and the Pre-Alps, there are thunderstorms on around 20 to almost 40 days per year. There are also more thunderstorms along the Jura than in the Swiss Plateau. The fewest thunderstorms occur in the inner Alps, as can be seen in the following figure on thunderstorm and lightning frequency.

Figure 3a: Average number of thunderstorm days per year from 2000 to 2024, graph: MeteoSwiss

Figure 3b: Average number of lightning strikes per year and square kilometer from 2000 to 2024, graph: MeteoSwiss

What are the dangers for VFR aviation?

Every TCU and CB causes turbulence, which can range in severity from moderate to extreme. This means that they can pose varying degrees of danger to aircraft. Turbulence does not only occur within the thundercloud. The cold air flowing out of the CB (outflow) causes extreme wind shear near the ground, which can sometimes even be felt at some distance from the thunderstorm. In the mountains, abrupt channeling effects of the outflows occur. It is therefore important to closely monitor the wind at an airport during takeoff and landing, even at some distance from thunderstorms. Downbursts are particularly dangerous during the phases of flight close to the ground, as the following graphic clearly shows. Downbursts are extreme downdrafts associated with CBs, which can occur with or without precipitation. Another danger is heavy rain, which can very quickly reduce visibility.

Figure 4: Wind change for an aircraft on final approach due to a downburst. Graphic: Karl Heinz Hack

Hail is very dangerous for aviation. Strong updrafts are necessary for it to form. Only then can a heavy hailstone grow for a long time in the CB. This requires a moving thunderstorm with corresponding wind shear. Pilots must be aware that hailstorms can occur not only within or below the thundercloud. Strong winds often throw hailstones into the cloud-free space outside the CB. It is therefore advisable to fly at a safe distance from the storm cloud – and certainly not under the anvil cloud.

Figure 5: Structure of a moving storm with a hail tower. Graphic: Karl Heinz Hack

Flight preparation in stormy weather conditions

Thunderstorms are one of the most dangerous weather phenomena for aviation. Therefore, the following rule applies: They must be avoided and flown around at a safe distance. The following rule of thumb can be used to calculate a safe minimum distance:

• At least 20 NM behind the storm. You are behind a storm when it has already passed.
• At the front of the storm, maintain a distance of 1 NM per knot of wind speed (30 NM at 30 KT).

If thunderstorms cannot be ruled out before the start of the flight, possible detours should be planned, alternative airports should be identified, and additional fuel reserves should be taken into account.

The following aviation weather products provide information about the weather situation and any likelihood of thunderstorms:

• SIGMET: Warning of phenomena that pose a serious threat to flight, such as TS or TSGR
• Low-level SWC: Information on TCU and CB, including tops
• Flight weather forecast: Information on CB and special “Hazards” section
• TAF: Mention of TS, GR, GS, TCU, CB, and gusts, possibly with probability
• GAFOR: Information on possible thunderstorms on specific routes

It is also important to keep a close eye on the current weather, using radar, satellite, and webcam images. Thunderstorms do not arise suddenly. They “announce” themselves in advance. If you correctly interpret the first signs, such as increasingly powerful cumulus clouds, you can avoid dangerous situations.