What causes thunder and lightning?

THUNDERSTORMS can be quite exciting if you're tucked up in a duvet and watching one from the comfort of your own home - but not so much if you're caught in it outside.

So what actually is thunder and lightning, and how can you work out how far away it is?

What is thunder and lightning?

Keeping track of the UK's unpredictable weather is a full-time job.

If you've ever wondered what's actually happening when a thunderstorm strikes, you're not alone.

A lightning strike is when an electrical discharge from the atmosphere is sent down into an Earth-bound object.

You will see a bolt of light - which is often jagged - and hear the sound of thunder, which is the noise caused by the extreme heat of the lightning causing the air to expand rapidly.

Lightning can happen between clouds, from cloud to earth or from cloud to air.

Most lightning strikes do not touch humans and the odds of being struck are very low.

According to the , lightning hits the ground about 300,000 times a year in the UK.

Thirty to sixty people are struck by lightning in Britain annually, and three - or 5-10% - of these strikes result in death.

People who spend a lot of time outdoors, such a golfers, climbers, hill walkers, and agricultural workers are thought to be at the highest risk.

What causes thunder and lightning?

, “thunderstorms develop when the atmosphere is unstable” which happens when “warm air exists underneath much colder air”.

Lightning is formed when cold air and warm air meet, with the latter rising and forming clouds.

With the right conditions, this cloud builds to form a cumulonimbus cloud which produces thunder and lightning.

The warm air has water droplets which combine and freeze to form ice crystals.

These droplets then become too heavy to be supported by the updraught and fall as hail.

The hail within the cloud bumps with smaller crystals creating a negative charge, which forms at the base of the cloud.

The Earth and objects, such as steeples, become positively charged resulting in current being passed between the two charges,

Thunder accompanies lightning as it is the noise produced by the rapid expansion and heating of air.

If you see lightning but don't hear thunder, it doesn't mean that it wasn't there, it just means it was too far away to hear.

This is often called heat lightning (or silent lightning) as it often happens in summer.

How to work out how far away thunder and lightning are

You can use the time between the flash of lightning and the rumble of thunder to work out how far away the storm is.

When you see the flash of light count the number of seconds until you hear the thunder - Mississippi counting is recommended, only the very confident should freestyle.

For every five seconds you counted the lightning is one mile away.

This is because light travels faster than sound, so although they originate in the same place the sound takes longer to arrive where you are.

The general rule is: if the gap between the lightning and thunder less than 30 seconds, it is close enough to pose a threat.

It is recommended that you wait for 30 minutes after the storm has passed before you resume you normal outdoor activities.

Can you have thunder without lightning?

Though they're referred to separately because of the way we perceive them, they always occur together.

Thunder is actually the sound caused by the lightning so by definition you can't have one without the other.

Depending on the distance, the sound can be a short violent crack or a slow, low grumble.