Thunder is a natural phenomenon that we hear during a storm immediately after lightning. From the time delay between the occurrence of lightning and the recording of thunder, we can determine how far the storm is from us. But why do we hear thunder at all?
Under the action of heat, water evaporates into the air. This warm air rises and clouds form. In higher colder areas, water vapor cools rapidly, forming small crystals and larger hail. The crystals rise and the hail falls. This leads to collisions in which the crystals are charged positively and the hail is negatively charged. The cloud is thus negatively charged in the lower parts and positively charged in the upper parts. Differences in charge between different parts of the cloud, different clouds, or between the cloud and the ground can cause a discharge that we see as lightning.
During lightning, the ambient air heats up to 30,000 °C. The subsequent change in pressure by heating causes the ambient air to expand rapidly, which is accompanied by a loud shock wave called thunder.