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
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
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
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.