Thunder is often heard in horror movies.Its sudden sharp crack is enough to get anyone's pulse racing. But thunder is just the harmless noise that accompanies lightning. The air in a thunderstorm cloud, or a 'cumulonimbus' cloud, is continually moving. This causes the cloud to build up enormous differences in electrical distribution. The top of the cloud becomes positively charged and the bottom negatively charged.

Positive and negative charges attract, so eventually an 'electrical discharge' is produced to neutralise the electrical distribution. We see this discharge as lightning. The immense heat of this electrical discharge causes thunder. The discharge heats the air that it travels through to approximately 5 times the temperature of the surface of the sun.

This enormous heat makes the air expand almost instantaneously, causing the pressure to change to about 10 to 100 times that of normal sea level pressure. However, it is the speed at which this happens, not its size, which causes the loud crack.

The rapid expansion of one small segment of the air causes a shock wave to travel through the surrounding air. This shock wave decays into a sound wave that we hear as thunder. Another common type of shock wave is the 'sonic boom', which is heard when Concorde flies overhead. The aeroplane travels through the air faster than its sound waves can, so they pile up on top of each other and create shock waves, which we hear as a loud bang.

So sound has an upper speed limit, and indeed so does light. However, the difference in speeds is so great that light could travel right around the world before sound finished the 100m sprint. This difference provides a handy way to determine the distance away of a thunderstorm. Count the seconds between the lightning and the thunder, each second represents 300 metres.

The volume of thunder is partly determined by the length of the discharge. The longer the lightning strike the more air is affected, so more air will contribute to the noise. Another determination of the volume, as you might expect, is the distance away of the lightning. The further away the thunder, the quieter it gets. This is a very good thing as there are estimated to be 100 thunder claps each second, and we certainly wouldn't want to hear all of them. It may be a harmless accompaniment to lightning, but maybe it should be left to horror films.

Other features in the Weather Basics series:

- Clouds
- Day and Night
- Fog and Mist
- Fronts
- Frost
- High Pressure
- Humidity
- Jet Streams
- Lightning
- Low Pressure
- Mirages
- Northern Lights
- Rain or Showers?
- Thunder
- Watercycle