Kay King, of Portobello, asks :-

Some people think that with sheet lightening there is no thunder. I have always thought that any lightening caused noise, but I don't completely understand what causes thunder!

Rebekah LaBar, a meteorologist with MetService, responded.

All lightning causes thunder, even if we can’t hear it. Lightning is a discharge of electricity through the atmosphere, after a large electric field builds up. A lightning bolt is only 2 to 5 cm in diameter, but is so powerful it can rapidly heat the air up to 30,000 degrees Celsius, or five times hotter than the surface of the sun!

The ideal gas law states that pressure is proportional to density and temperature, which means if you increase the air temperature while the air density remains the same, then the air pressure has to rise as well.

Because it is so hot, lightning literally splits the air when it strikes. The density does not change much but the pressure inside the lightning channel rapidly increases as the temperature increases. Now think about a balloon for a moment; if you blow it up too much (thus increasing the pressure against the sides of the balloon), it will pop. Something similar happens with lightning; the pressure within the lightning bolt becomes too great and it sends out a shock wave in an effort to balance things out. This shock wave is what we hear as thunder.

Since light travels faster than sound, we see lightning before we hear thunder. For every three seconds between the lightning flash and the first sound of thunder, the lightning is about one kilometre away.

If the lightning is over 25 km away, you will usually not hear any thunder because the sound waves will spread out too much before they reach you. This is partly why you don’t always hear thunder with sheet lightning, because it is usually too far away. If you do hear thunder though, you may be close enough to be within range of the next strike. Remember, when thunder roars, go indoors!