Cause of thunder

Thunder is the sound associated with lightning flashes. A lightning discharge creates an ionized path (plasma) in the air, and the high current that flows during the return stroke heats up the plasma to tens of thousands of degrees (C). The rapid expansion of the channel creates a shock wave that relaxes into a sound wave after some distance, which we hear as thunder.

Thunder is basically just the sound of an explosion, but from an extended line-shaped sound source. A piece of fireworks will give a bang because it explodes at a point; but because sound waves travel relatively slowly, a long spark or lightning will give a continuous explosive sound, which we hear as thunder.

Types of thunder

Those who have experienced many thunderstorms, may have noticed that not all thunder sounds similar. Most thunder in a single storm may sound similar, but from storm to storm thunder may sound differently.

Thunder may consist of claps, peals and crackle. A deep booming sound, similar to a sonic boom, is called a peal; a typical rumble is a clap and the high-pitched crackling sound sometimes heard before the main peals or claps are crackles.

Cloud to ground lightning usually creates louder thunder than intracloud lightning. With flashes such as this one, you hear thunder from the closer branches (soft crackle) before the thunder from the main channel (loud peals).

The loudest thunder we hear is mostly produced by cloud to ground discharges, since these can be very close and also have return strokes that generate a lot of current.

Since sound travels about 1 km every 3 seconds (or 1 mile every 5 seconds), and the extend of lightning is of the order of several km, thunder usually lasts several seconds. You hear different parts of the lightning channel(s) as the thunder continues.

Thunder of IC lightning

IC lightning, or intracloud lightning, usually occurs many km above ground level and is therefore far away. Thunder from this lightning mostly sounds as a continuous rumble. Thunder from IC lightning very high up in the anvil of a storm is usually not heard very well, especially if the storm is somewhat distant. This is due to the fact that sound travels faster in air at higher temperature (closer to ground) and the fact that the temperature of the air decreases with increasing altitude makes the sound refract upward. Thus, because IC lightning high up in a cloud is usually not as loud as CG lightning and is more distant, it will be heard poorly or not at all, while thunder from cloud to ground lightning may still be heard.

Thunder of CG lightning

With thunder of cloud to ground (CG) lightning, what you usually hear first is a high-pitched crackle, followed by louder crackle, then a few claps or peals, and a decaying rumble.

The initial crackle is usually caused by branches of the lightning channel that were closer to you than the main channel, and therefore the sound of those arrives earlier.

The main lightning channel that contains the main return stroke current produces the loudest thunder. If the channel is very tortuous on the scale of the wavelength of the sound, the thunder may not be very loud. It won't be loud either if the discharge current is not very high.

The rumble that continues for many seconds after the main part of thunder, is caused by more distant parts of the lightning, maybe higher up in the cloud. If you listen carefully, you can sometimes even tell what parts of the lightning channel you are hearing in the cloud, since you can tell directions if you listen with both ears.

Unusually loud thunder

Some storms produce CG lightning that sounds quite loud. Lightning can produce very loud booming thunder if the return stroke current is large or longer-lasting than usual. Two types of lightning that may produce loud thunder are:

• Cloud to ground lightning originating from the top of a thunderstorm. The lightning channel can be very long, much longer than cloud to ground lightning that originates low in the cloud. As a result, there is more charge deposited along the channel and the discharge current will heat up the channel more.

• Ground to cloud lightning, which sometimes initiates from tall objects such as transmitter towers or skyscrapers. In this case the current is very low (there is no return stroke) but lasts very long, and the channel is heated up more.

In both cases, the thunder may sound like a series of sharp booms similar to sonic booms. Every peal of thunder is associated with one part of the channel, the later peals from parts further away from you (i.e. higher up).

The overall shape and tortuosity of the channel also determines how thunder will sound. If many sound waves from different parts of the channel arrive at your ears in phase with eachother, the sound will be louder than if they arrive out of phase. If a lightning channel is very tortuous, there may be many parts of the channel that emit thunder soundwaves at different phases.

The spectacular bolt-from-the-blue lightning often creates loud thunder.

Sounds heard during a close return stroke

Sometimes, if cloud to ground lightning is very close, you may hear a click or snap at the instant of the flash, before the thunder.

Snaps are occasionally heard from contact streamers connected to ground, that were close to you but didn't connect to the main leader propagating to ground, before another streamer further away successfully contacted the leader. If you are in a field far away from any houses and powerlines, any click heard during a lightning flash is most likely a failed contact streamer close to you.

A clicking sound at the instant of a close flash can sometimes be heard in or near houses. This click is electromagnetic in nature. Internal wiring in houses may move suddenly when inductive currents are generated, or powerlines nearby may spark due to an induced high voltage.

Thunder from close and distant lightning

Thunder from close CG lightning sounds noticeably different (apart from being louder) in that the sound is generally higher pitched than more distant lightning. The higher pitch is the actual thunder, however; more distant thunder sounds deeper because higher frequencies of sound don't propagate as well through the air as lower frequencies.