How far away a thunderstorm is with the 3 second rule

The Met Office has issued a yellow and amber weather warnings for thunderstorms for many parts of the UK for Monday, June 12 and Tuesday, 13.

The warning is in place for Northern and South East Scotland, North and South West England as well as parts of Wales and Northern Ireland.

Brits have been warned to prepare for some disruptions including difficult driving conditions, possible flooded roads as well as a slight chance of power cuts.

The warning is in place for Northern and South East Scotland, North and South West England as well as parts of Wales and Northern Ireland. ( Getty Images) (Image: Getty)

How can you tell how far away a thunderstorm is from you?

Thunder will always be heard after the lightning strike is seen, the Met Office has said. 

This is due to the fact that light travels significantly faster than sound.

The weather service went on to explain that lightning and thunder are generated simultaneously but with the speed of light at 299,792,458 m/s and the speed of sound at 340.29 m/s, there will always be a gap between the two.

We can measure how far we are from a thunderstorm by measuring the time between seeing the flash of lightning and then the start of the thunder.

The length of this period (in seconds) can be divided by three to give an approximate distance (in kilometres).

Can there be lightning without thunder?

The Met Office has explained that there are occasions where sometimes lightning is seen but there is no thunder heard.

The weather service has explained that this is either because thunder is rarely heard more than 20 km away or because the atmospheric conditions lead to sound bending upwards and away from the surface.

Have you heard of the three second thunderstorm rule? ( Getty Images) (Image: Getty Images/iStockphoto)

What is the 30-30 lightning rule?

The Met Office said it is estimated a lightning strike hits somewhere on the Earth's surface approximately 44 times every second, a total of nearly 1.4 billion lightning strikes every year.

In the UK, this works out to be about 300,000 times a year according to the Royal Society for the Prevention of Accidents (ROSPA).

It estimates that around 30-60 people are struck by lightning each year in Britain, and on average, 3 (5-10%) of these strikes are fatal.

Although our chances of being struck by lightning are around one in 10 million, according to the British Medical Journal, there are measures we can take to help us stay safe. 

One of these measures is known as the 30/30 which ROSPA describes as a "good way of ensuring one is sheltering during the most risky parts of the storm".

The rule advises that if the time between the flash to bang is 30 seconds in length or less you should seek shelter.

You should then stay inside this shelter until 30 minutes past the last clap of thunder.

It explains: "This ensures that any distant strikes at the beginning of the storm (lightning can travel up to 10 miles), or trailing storm clouds at the back of the storm do not take anyone by surprise".