As Cyclone Nivar bears down towards the TN coast, we were treated to some spectacular rolling thunder overnight. The kind that starts off as a low rumble and builds up like a dubstep drop into a ear-shattering final crack.

And since it woke all of us up, it was an opportunity to do an #ELI5 on thunder with the son. But to explain thunder, one has to understand lightning, because a thunder is essentially the sonic boom that accompanies lightning

Lightning happens when a massive difference in electrical charge happens between clouds or between cloud and ground. When this difference in voltage becomes too high, things are settled by electrons moving en masse from one point to another to equalise the situation.

And when that happens, the air around the lightning flash heats up to 30,000 Celsius, which causes the air to expand rapidly. And this expansion is faster than the speed of sound

Which then causes sound waves to bunch together and when the hit the ground, we hear a boom, which is thunder. This is quite similar to what happens when the supersonic aircraft travel over land. They generate a similar boom as they compress air in front faster than sound.

This is why the Concorde could only fly from London/Paris to NYC but not to LA/Chicago or anywhere in Asia. Supersonic flights over land will cause sonic booms that are quite disruptive to daily life.

This video from several decades ago is one of the clearest and simplest explanations of sonic booms I've ever seen

Back to thunder, the scale of cumulonimbus clouds that produce thunderstorms is often underappreciated. The base of the cloud could be a kilometre above you but the top of the cloud could be 15-20km high!