Darwin residents have reported seeing rare green lightning in a record-breaking storm that created the strongest wind gust since Cyclone Tracy.
Tony Wood saw the green lightning above his Brinkin unit.
"The clouds glowed very momentarily green in two big flashes," he said.
Bureau of Meteorology (BOM) senior forecaster Ian Shepherd said green lightning was "rare but not unusual".
"There have been quite a few reports of green flashes of lightning from storms which have a lot of lightning in them," he said.
One theory is that sunset colours may interfere with the blue lightning.
Mr Shepherd pointed to another theory from Rice University professor emeritus Arthur Few.
"He suggests that green lightning occurs quite often inside of clouds, but we don't see it," he said.
"It's what we call a lightning streamer.
"If you can see the streamer as it's developing, then sometimes it can be green in colour.
"And they suggest the green colour is caused by oxygen in the atmosphere being activated by the electric charge along the lightning bolt."
Together in electric dreams
University of Tasmania Physics head of discipline Andrew Cole saw green lightning strike metres from his car once.
"Lightning goes through a definite lifecycle over the milliseconds that it exists as a flash," he said.
"Depending on how you're looking at it, and what the atmospheric conditions are, you can see different colours emphasised.
Professor Cole explains lightning as "superheating" nitrogen and oxygen.
"You see some characteristic colours, then the air can be heated up to 30,000 degrees [Celsius] and that will glow extremely brightly, like white hot," he said.
"And then if it heats up the air around that, as it cools down, you'll see it go through red-hot yellow.
"So depending on where it happens to be, and what atmospheric conditions are like, that can change the colours that you perceive."
Professor Cole said green lightning may also be caused by electrical equipment such as transformers or power lines.
"They will arc greenish or violet many times," he said.
BOM forecaster Jessica Lingard said Saturday's storm was severe, with 33 millimetres of rain in Darwin's gauge and a top wind gust at 9.20pm of 137 kilometres an hour.
"Not only a record gust for October but also the strongest wind gust at the airport since [Cyclone] Tracy moved through in 1974," she said.
Mark Dickson from Humpty Doo said he remembered the lighting flashing green during Cyclone Tracy.
"I looked out the windows, which didn't have any curtains, just glass louvres, and I could see just green flashes. It really blew us away," he said.
Did you get any photos or video of the green lightning? Email conor.byrne@abc.net.au
