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.