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Start counting after you see lightning to tell if you're in harm's way

By Jesse Thompson, Friday January 18, 2019 - 09:53 EDT
ABC licensed image
Darwin is one of the most lightning-prone cities in the world. - ABC licensed

It's a nightly show during the Top End wet season: vivid sunsets and deluges of rain from battering storms.

These spectacular displays also bring plenty of lightning — enough, in fact, .

The explosive crack and rattling buildings in the aftermath of a close strike is a familiar experience to many in the Top End.

But how close is too close?

Paying close attention to that cracking sound can play a role in determining your safety during a thunderstorm, according to meteorologist Caecilia Ewenz.

She said it came down to a simple equation based on the different speeds at which light and sound travel.

"When you see lightning, that is more or less instantaneous because it travels with the speed of light," Dr Ewenz said.



Sound, on the other hand, moves significantly slower, meaning thunder from distant lightning takes longer to reach us.

If thunder and lightning occur close together it means the storm is within close range.

What if they occur at almost exactly the same time?

"The storm is very close. It's probably directly above you," Dr Ewenz said.

The 30/30 rule

Light travels at close to 300,000 kilometres per second, while sound travels much slower at around 300 metres per second.

Jackson Browne from the Bureau of Meteorology said counting the seconds between when you see a strike of lightning and hear the accompanying thunder could help you figure out how far away the lightning is.

"So if it's a count less than three [seconds], [the storm is] a kilometre or less away," Mr Browne told .

"If it's half a second, you can estimate that it's maybe 150 metres away from you."



For her part, Dr Ewenz abides by what's known as the 30/30 rule — if you count to 30 after seeing the lightning and thunder still hasn't sounded, you're outside the storm's 10-kilometre radius.

If it's less than 30 seconds you should consider taking shelter; sporting teams commonly wait 30 minutes until after that happens to resume play.

Where to shelter

The 30/30 theory is a widely used meteorological shorthand.

It fails, however, if the lightning is hidden by mountains or clouds, or if the thunder is in fact the product of a different strike to the one you've seen.

"But usually if you see it in that direction and you hear it from that direction, you should be fine," Dr Ewenz said.



In any event, people within striking range should shelter indoors.

Dr Ewenz said she was told as a child to hide under trees but warned that "it's all rubbish".

"Never go under a tree, because that's usually the highest obstacle in the area," she said.

"The higher the elevation something is, the more likely it is to be struck by lightning."

If you're stuck outdoors, the best advice is to crouch low to the ground and stand with your feet close together, reducing the potential of electricity to cross your body during a strike.

Better yet, if your car is nearby you should shelter inside with the windows closed.

The car will act as a , meaning electricity from a lightning strike will travel down the outside of the car, therefore protecting anyone inside.

So why will lightning rattle my house?

That's the force of "exploded" air after a strike discharges electricity into the atmosphere, Dr Ewenz explained.

"It's because when the lightning strikes, the air around that thunderbolt explodes," she said.

"The lightning goes through the air, the air around it heats up so fast that it sort of explodes, and then it moves through the air fast."


- ABC

© ABC 2019

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