Thunder is the sound produced by the rapid heating of air by a lightning strike.
The word thunder is derived from the 'Thor', the Norse god of thunder, known for his ability to forge thunderbolts.
Essentially, thunder is the sound produced by lightning. When lightning strikes, the narrow channel of air through which it travels reaches temperatures of up to 30,000 °C almost instantly. This intense heating causes the air to rapidly expand outward into the cooler air surrounding it creating a rippling shockwave which we hear as a rumbling thunder clap.
Depending on its formation and location, this thunder clap can be heard as either a sudden, loud crack or a low, long rumble. Thunder lasts longer than lightning due to the time it takes for the sound to travel from different parts of the lightning channel.
The intensity and type of sound heard by the listener depends upon the conditions on the atmosphere and how close the listener is to the lightning - the louder the thunder, the closer the lightning.
When a lightning strike is close, the thunder is heard as a loud clap or snapping sound. When the sound of thunder has a rumbling quality, it is the sound waves reaching the listener at different times owing to the shape of the lightning strike. In rare cases, the sound of thunder at very close range has been known to cause injury to humans and damage to property.
How far away is a thunderstorm?
Thunder will always be heard after the lightning strike is seen owing to the fact that light travels significantly faster than sound. Both the lightning and thunder clap are generated simultaneously but with the speed of light at 299,792,458 m/s and the speed of sound at 340.29 m/s, there will be always be a gap between the two to the observer.
Your distance from a thunderstorm can be estimated by measuring the time between seeing the lightning flash and the hearing the start of thunder. The length of this interval in seconds can be divided by three to give an approximate distance in kilometres.
Sometimes lightning may be seen but there is no thunder heard. This is either because thunder is rarely heard more than 20 km away or because the atmospheric conditions lead to sound bending upwards and away from the surface.