What is the cause of rolling thunder

Why does it thunder during a thunderstorm?

The thunder is a result of lightning. The lightning creates a so-called lightning channel in which extremely high temperatures arise within a few millionths of a second. This causes the air to expand like an explosion - and it thunders.

A lightning bolt is extremely fast. It only takes a few thousandths of a second to hit a tree or roof from a cloud. One can have the impression that the thunder that follows is generated directly by the lightning itself.

Lightning over Berlin

When lightning strikes the earth from a cloud, a so-called pre-discharge creates a lightning channel. Shortly before it reaches the earth, a catch charge is generated there, which runs counter to the summons. This closes the lightning channel and the full lightning discharge begins. In the center of the lightning duct, the air is heated to around 27,000 to 30,000 degrees Celsius within a few millionths of a second. This is so short that the heated air does not have time to expand during the flash. It is therefore in a state of very high pressure, about 10 to 100 times higher than in the normal state. This compressed air then explodes at supersonic speed in the form of a shock wave outwards, out of the lightning channel, in all directions. The associated sound waves generate the well-known thunder.

After the main discharge, up to 40 partial discharges can follow within thousandths to hundredths of a second.

The same effect of the explosive shock waves in the air also takes place when lightning strikes between clouds. This type of lightning is even clearly in the majority at around 80 percent.

The thunder spreads at the speed of sound of 330 meters per second. So it is much slower than the flash that becomes visible at the speed of light. That is why it can usually only be heard afterwards, unless the lightning strikes in the immediate vicinity of the observer.

The cause of the lightning discharges are electrical voltages between clouds and earth / clouds of a few 100 million volts. They generate currents of up to several 100,000 amperes in the lightning channels, which causes the lightning channel to heat up enormously.

The voltages are generated by the separation of positive and negative electrical charges within thunderclouds. Here, moist, warm air masses, ice particles and water droplets play the main role. As a result, positively charged ice particles accumulate at the top of the thundercloud, compared to a negative charge center of water droplets at the bottom. The positive counter-charge then arises on earth through electrical induction.

Lightning strike the Eiffel Tower

Since the electrical discharge of the clouds prefers the shortest route to the ground, the lightning bolts can usually be seen coming straight from above. Since we are usually on the ground, we hear the shock waves from the end of the lightning channel first, followed by the crash of subsequent shock waves from further up. The situation is different when lightning strikes at a constant distance from the observer, because then the shock waves can arrive at the observer at about the same time and thus generate the well-known thunder-bang.

Lightning does not always follow a single channel. Very often it splits into several channels. Then the thunder noise changes, as several lightning channels send out shock waves that can also reflect off each other and thus generate a series of thunder with a lower volume in the form of a rumble. Similar effects are also caused by lightning bolts moving towards or away from the observer, because then the shock waves arrive only gradually.

Creation of shock waves

In compressible, i.e. compressible media, changes in pressure propagate in the form of waves that distribute the pressure in the medium and thereby reduce it. Depending on whether the pressure rises or falls, one speaks of compression or thinning waves. The speed of propagation of these waves is equal to the speed of sound for small pressure differences.
But if, for example, a pressure increase occurs faster than it can be reduced at the speed of sound, then the propagation process changes. The change can then no longer be passed on quickly enough and the compression waves rise, because the medium adapts to the disturbance almost immediately with a strong increase in pressure and density followed by a sudden increase in temperature. This leads to what is known as a shock wave, which spreads further as a shock wave. As the energy of the shock wave is absorbed in the medium, it decays back to a normal pressure wave.