Question: What causes thunder?

Angela Stokes answered on 16 Nov 2014:

Hi Ian
Thunder is the sound that accompanies lightning during a thunderstorm. Sounds simple enough, but why does lightning make a sound?

Any sound you hear is made up of vibrations. The vibrations travel as a sound wave through the air, until they reach your ear. Lightning is a huge discharge of electricity, and this electricity shoots through the air, causing vibrations to be formed in two ways:

1. The electricity passes through the air and causes air particles to vibrate. The vibrations are heard as sound.

2. The lightning is also very hot and heats up the air around it. Hot air expands, and in this case the air expands very quickly, pushing apart the air particles with force and creating more vibrations.

This is what we hear and call thunder – the rumbling of thunder is simply caused by the vibration or sound of the air affected by lightning. If you’re nearby to a lightning strike, you may have heard thunder as a really loud crack, almost like the sound of a whip being cracked. But, most of the time we hear thunder as a loud, long rumble.

In fact, the crack sound is the direct sound of the lightning near us, reaching our ears. The more common rumbling effect happens when thunder echoes off objects all around us. This happens a lot in towns and cities, where there are lots of buildings for the noise to bounce off. However even in flat areas of land, with no trees or other objects, there is quite often a rumble as the thunder simply bounces off the ground on its way to our ears. All this echoing transforms the original ‘crack’ sound into a longer ‘rumble’!