Nicola Anstice, of Corran School, asks :-

Why does thunder roll?

Erick Brenstrum, a physicist with Meteorological Service of New Zealand Ltd, responded.

Thunder is caused by the explosive expansion of air heated by a bolt of lightning to around 30,000 Celsius, which is five times the temperature of the sun's surface. Although the lightning lasts only a fraction of a second, the sound of the thunder is often drawn out over many seconds. This is because it takes longer for the sound from the top of the lightning stroke to reach an observer than it does for sound from where the lightning meets the ground.

Sound takes about three seconds to travel one kilometre through air at sea level. So, if an observer is 4 kilometres away from where a bolt of lightning reaches the ground, the sound from that part of the lightning takes 12 seconds to reach them. The sound from part of the lightning 3 kilometres above the Earth's surface has to travel 5 kilometres to reach the observer, and so arrives after 15 seconds.

In hilly country, the sound of the thunder is likely to be further drawn out by echoes.

Also when there are layers of air of different density, there can be partial reflection of sound at the boundaries between the layers, which lengthens the path the sound travels to reach an observer.