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How thunderstorms form

Thunderstorms are characterised by towering cloud formations. They form mainly in summer, when water evaporates and rises as a result of strong solar radiation. The rising cloud particles collide with each other and generate an electrical charge. When the resulting voltage is discharged, this appears to us as lightning and thunder.

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Lightning and the ensuing thunder are what characterise a thunderstorm. They usually occur in summer, when solar radiation leads to the formation of powerful clouds.

How lightning occurs

Lightning, the characteristic phenomenon of a thunderstorm, occurs as a result of complex processes. Turbulent currents within the clouds cause collisions of sleet, hail, ice and water particles. During these collisions, both positive and negative charges are generated in the particles. The particles are carried to different parts of the cloud according to their weight, where the positive and negative charge carriers accumulate separately. This creates two or more poles within the cloud, which form an electrical field.

The air between the charged particles initially acts as an efficient insulator that separates the two poles. When the electrical voltage between the poles (potential difference) becomes too great, a kind of short circuit occurs in the form of multiple shock-like movements of the charges, resulting in the voltage being discharged. This discharge appears as lightning.

How thunder occurs

Thunder occurs when the air in the discharge path of a lightning bolt heats up and expands rapidly and explosively. The resulting pressure wave is heard as a loud bang in the immediate vicinity of the thunderstorm. It propagates at the speed of sound (330 metres per second), radiating outwards from the location of the lightning strike. As it does so, the pressure wave progressively lessens in intensity and can then sound like rolling or grinding as it moves further away into the distance. This is thunder.

How thunderclouds form

Three conditions must be met in order for thunderclouds to form:

  • Moisture, as the precondition for cloud particles to form
  • A potentially unstable stratification of air masses to allow sufficiently powerful air movements (an additional trigger may be needed for instability to occur)
  • An uplift mechanism to trigger the formation of the thundercloud.

On days with thunderstorms, large amounts of water vapour will often accumulate in the layers of air near the ground. The cumulus clouds forming through convection are normally prevented by a temperature inversion from growing vertically upwards into the middle and upper levels of the troposphere (above 4 km). An additional mechanism is therefore required to supply the additional energy needed for the cloud to overcome this barrier and expand into the upper air layers. In Switzerland, this is usually provided by upslope winds and valley winds in the mountains.

In simplified form, the life cycle of a thunderstorm cell can be divided into three stages: The cumulus or development stage, the mature stage, and the dissipation stage.

The role of mountains in the development of thunderstorms

In Switzerland, the uplift mechanism (trigger) required for thunderstorm formation is primarily provided by the mountains. The upslope and downslope winds pump the warmed air up over the mountain, causing a greater accumulation of air over the crests, ridges and peaks. The steadily burgeoning airmass dissolves the inversion as time goes on, so that in the course of the afternoon the thunderstorm cloud towers into the sky, causing a high electrical voltage to build in the cloud.

Once precipitation processes are already in progress, the spreading cold air often assumes the role of a trigger in a second stage. The cold air spreads horizontally just above the ground and causes the warm air to lift. Due to this additional push, cumulus clouds that were initially shallow succeed in breaking through the inversion and growing into a second thunderstorm cell (secondary cell).

The challenges of thunderstorm forecasting

On days when thunderstorms form, all these ingredients and processes vary in intensity depending on the location and time of day, and influence each other. Correctly evaluating these factors and accurately forecasting thunderstorm development is challenging. Often, it is only possible to make non-specific statements on a regional scale, and only a few hours to a few days in advance.