73,700 lightning strikes: How the storm that stunned Sydney exploded

A stewing Sydney summer evening broke in spectacular fashion on Wednesday with a storm emblematic of the hotter, wetter, more humid summer predicted for much of Australia by the Bureau of Meteorology.

At 8.25pm, the dew point temperature – a direct measure of water in the air – surpassed 22 degrees, well past the threshold that feels horribly muggy.

A line of storms track east over Homebush.Credit: Wolter Peeters

But then the humidity snapped with a rainstorm that belted the city with 24 millimetres of rain in 19 minutes.

Lightning flashes lit up the deluge – 73,700 strikes flared within 100 kilometres of the CBD, according to Weatherzone, and 8777 struck the ground.

Two million lightning strikes hit Australia as a continent-spanning line of storms roared over battered towns from Tasmania to Queensland.

“It’s one of the most lightning-intense storms that I’ve ever seen,” said Sydney climate scientist Dr Tim Raupach, who studies the complicated role of climate change in this generation of severe storms.

Lightning over NSW during the storms that thrashed the east coast on Wednesday night.Credit: Weatherzone.com.au

The basic ingredients for thunderstorms are moisture and rapidly rising air.

“Air near the surface is warm, and up higher in the atmosphere, it’s cooler. If a little bit of that warm air gets into the cold air, then it rises like a hot air balloon, and it causes updraughts to form,” said Raupach, from the University of NSW.

Storm clouds condense if there’s enough moisture in the air for the updraught to draw on. Add twists and turns in wind speed and direction – called wind shear – and a chaotic supercell storm can form.

The more violently wind shear destabilises a storm system, the more lightning is unleashed, as differences between positively and negatively charged regions of a cloud grow more dramatic.

73,700 lightning strikes were recorded within 100km of Sydney between midday and midnight on Wednesday.Credit: Weatherzone.com.au

The Sydney Weather Nerds Facebook page had a field day, with Sydneysiders reporting flying trampolines, branches smashing cars and a tornado-like squall whipping through Moorebank.

“The air pressure in Sydney got down to 992, which is incredibly low,” said the owner of the page, longtime weather observer and storm-chaser Paul. “Those are the sort of pressures you’d find associated with a developing tropical cyclone, for example. And that was over land.

“Troughs are the Earth’s ability to manage its heat budget. It’s trying to drag down hot air from the tropics to the poles, and in return, it shunts up cold air from the poles, from Antarctica.

Lightning seen from Anzac Memorial Bridge in Newcastle.Credit: Getty Images

“What we saw yesterday was a dynamic, dramatic attempt by the atmosphere to try and balance that budget.”

The storm was so severe because a phenomenal amount of warm air rocketed high into a cold region of the atmosphere, generating storm clouds which unleashed the atmospheric light show.

“That sort of ‘strobing’ effect of the lightning is what you normally get in the tropics. What I observed yesterday was that a lot of the cloud heights were above 16 kilometres – that’s significant,” Paul said. “In Sydney, it was definitely unique and rare.”

The one-two punch of a cold front combined with a thunderstorm is the most frequent cause of extreme rain in NSW, according to the National Environmental Science Programme.

A section of a roof from townhouses in Carlingford lays on a garden area after Wednesday night’s storm.Credit: Kate Geraghty

The effect of climate change on storms is a matter of ongoing scientific investigation. But the average intensity of rain unleashed by thunderstorms in Australia has increased by about 27 per cent in the past four decades, according to a paper by Melbourne climatologist Associate Professor Andrew Dowdy.

The atmosphere’s spongelike ability to soak up moisture is increasing by 7 per cent for every degree of warming, meaning that when it rains, it’s far more likely to pour.

“When there’s more water there, there’s more energy that can be released as that water condenses, and you can get a more vigorous storm. And stronger updraughts mean larger hail,” Raupach said.

“A hailstone that spends a long time growing in the storm, because it can be held up by an updraught for a really long time, is going to end up a lot bigger.”

Raupach’s previous research revealed the number of hail-prone days in Sydney has spiked by 40 per cent over the last two decades.

Days prone to hail have increased by 40 per cent in Sydney.Credit: Edwina Pickles

However, warmer temperatures mean hail also melts faster. So we may see less frequent hailstorms all up, but when they do strike, it’s likely to be the more damaging stones large enough to make it to the ground.

The bureau predicted a hot, wet, humid summer in October. Last year was the hottest on record, weather agencies confirmed last week, and surpassed 1.5 degrees of warming.

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