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What causes the spectacular bolts of crawler lightning to flash across the sky?

Kate Doyle, Monday January 7, 2019 - 19:25 EDT
ABC image
The lightning crawled across the East Kimberley sky on Friday. - ABC

Tentacles of light flashed across the night sky over the weekend but what causes the spectacular crawler lightning?

Jackson Browne, meteorologist at the Bureau of Meteorology, Darwin has shed some light on the issue.

He said crawler lightning appeared in the upper parts of thunderstorms — in the anvil — the flat expanse of cloud which spreads out at the top of a storm.

"Crawler lightning normally starts as what we call intracloud lightning — lightning that travels wholly within the cloud.

"Once [the lightning] gets into the upper reaches of the anvil it can spread out and become quite visually pleasing to look at."

Mr Browne is an expert in both weather and understatement.

"If you look at the structure of a storm, you can imagine in the bulk of the storm is the trunk and then the anvil is more like the canopy spreading out.

"The lightning tends to take on that sort of forked appearance of a tree with that lightning spreading up in the upper canopy of the storm."



What causes crawler lightning?

For a thunderstorm to be called a thunderstorm it needs to have electrical discharge, or lightning within the storm.

Thunderstorms have quite a bit of energy in them, according to Mr Browne.

"There's a lot of electrical imbalances due to the way the water and ice sort of rubbing up against each other.



"Lightning is just the equalisation of those electrical imbalances in the cloud and that happens at all different levels," Mr Brown said.



He said that crawler lightning was a constant feature in thunderstorms but often our view of it was obscured by clouds or rain.

"You need a fairly good line of sight to view crawler lightning. Thunderstorms are cloud machines, they extend right from the very surface up to the very top reaches of the weather part of the atmosphere.

"It really does help if you've got a thunderstorm that isn't raining on you, isn't projecting cloud on you and has a very large anvil that crawler lightning can present itself on."

But not all storms are created equal for crawler lightning.

"There are particular storms which favour them and they are called meso-scale convective systems and that's the sort of storm that went through Darwin early on Sunday morning.

"It favours what they call the trailing stratiform region of the storm. It's got quite a large anvil on the back end and it allows these storms to really project up this crawler lightning on the back end."

Types of lightning

Crawler lightning is a subset of a intracloud lightning.

According to Mr Browne intracloud lightning is by far the most common type lightning and it occurs wholly within the thunderstorm.

Sheet lightning is another form of intracloud lightning.



"Say if there was a little bit of cloud hindering that view then that would become what we call sheet lightning — where you can just see the cloud light up, but you don't see any details about the individual lightning strikes."



Then there are the types of lightning that go out of the cloud.

"You've got a cloud to cloud lightning which goes from one active thunderstorm cell to another, neighbouring thunderstorm cell."

Then there is the classic cloud to ground.

"This is where you get lightning from any depth of the storm down to the surface. They tend to be less common than the first two," according to Mr Browne.

"Then lightning can take on different polarities. Most lightning is negatively charged, but you can get some positively charged lightning which will come up from the ground, but that represents only about 5 per cent of all strikes."



Impacts

So what impacts do we need to be wary of with these spectacular bolts shooting across the sky?

"Next to none, to be honest. It's up in the upper reaches of the thunderstorms. So apart from looking visually pleasing there is none," Mr Browne said.

So an impressive natural phenomena that we don't have to feel bad about enjoying and marvelling over, how refreshing.

But of course it would be remiss not to mention that there are plenty of impacts from cloud to ground lightning to be wary of when caught in a thunderstorm.


- ABC

© ABC 2019

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