It just would not be summer without thunder and lightning

Storm rolling in at Caloundra. Flinty McCullough. Photo Contributed
Storm rolling in at Caloundra. Flinty McCullough. Photo Contributed Contributed

THEY are as much a part of Queensland's summer as Fourex, backyard barbecues and Christmas beetles banging on the lights at night.

But why do thunderstorms come along on some days to break the heat and not others?

The answer is that thunderstorms need more than just hot weather to form.

They are a "perfect storm” of three ingredients, a brutal concerto of instability, humidity and what the experts call a lifting or initiating mechanism.

Bureau of Meteorology forecaster Matthew Marshall said instability occurred when the air temperature in the upper atmosphere was relatively cool and the ground temperature was relatively warm.

Mr Marshall said warm air tended to rise, creating an uplift of air and movement in the upper atmosphere between the air masses of differing temperature.

"The warm air, once it's headed up, it's surrounded by cool air it wants to keep moving up and up and the cool air has to move,” he said.

"If the air in the upper atmosphere is also warm, you don't have that instability.”

He said humidity contributed towards the instability by allowing the air to stay warmer as it rose.

In Queensland, two main lifting mechanisms work up the instability into something worse.

The ground is hot, having been baked by the sun all day, causing the air to rise. And surface troughs, long masses of low air pressure, move through, prompting a process of pressure equalisation.

Mr Marshall likened the process to the opening of a beer bottle, which foamed out as the air bubbles stored at high pressure rushed into the lower pressure atmosphere.

"All the air around the trough wants to fill up the low pressure gap. All the air is moving it the trough and so it goes up,” he said.

"When it goes up, it condenses, and and it keeps moving up, it condenses more and more. If it keeps moving up, then hail starts to form, and all these different precipitations keep forming.”

But what makes some storms worse than others?

The Bureau of Meteorology's New South Wales website says the buoyancy of the rising air and the wind speed and direction - called wind shear - influence the severity of a storm.

Low to moderate instability in the upper atmosphere with minimal wind shear will likely mean non-severe thunderstorms.

Instability with low wind speed and sheer is likely to see a storm which rises quickly and produces large hail but collapses within half an hour with little widespread damage.

On the other hand, increases in wind sheer can see cells form within a thunderstorm, increasing the longevity and spread, with the possibility of hail and flooding.

The worst of the worst storms occur when buoyancy and wind sheer reach their optimum, creating "supercell” storms which are the most damaging of them all, responsible for hail, damaging wind gusts, tornadoes and heavy rain.


Hail is formed by a strong uplift, pulling water droplets higher and higher until they freeze. When they become to heavy, the hail starts to fall. The stronger the uplift, the bigger the hail, as small hail will be pulled higher and get wetter and grow bigger.


Lightning is formed when iced raindrops bump into each other in the clouds, creating electrical charge. The top of a cloud becomes positively charged and the bottom of a cloud becomes negatively charged. A positive charge builds up on the ground below the cloud until the attraction between positive and negative becomes too great and they reach out to connect, creating what is termed a lightning strike.


Thunder is caused by lightning. A lightning bolt opens up a gap in the air and the thunder is the sound of the gap closing. Because light travels faster than sound, you see lightning before you hear thunder.

Topics:  lightning storm summer thunder weather

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