Giant hail threat increases for Australian cities as climate warms

The frequency of "giant" hail and the number of hail days in a season could increase substantially for multiple Australian cities as the climate warms, a new study has found.

The University of New South Wales study modelled the severity of hailstorms for the current and future climate across Australia's most hail-prone major cities, including Brisbane, the Sydney/Canberra area, Melbourne, Adelaide and Perth.

It is one of the first studies to simulate how hailstorm frequency and severity — one of the most costly natural perils in Australia — may change in the future.

It also looked at the WA Goldfields town of Kalgoorlie, which the researchers said was a hotspot for hail.

Giant hail more frequent

The study found that while the overall frequency of hailstorms only increased for a few cities, the chances of "giant hail" would become more frequent by the end of the century for Melbourne, Perth, Kalgoorlie, Sydney and Canberra.

The "future" scenario assumes a warming of about 2.8 degrees Celsius above pre-industrial levels in 2080–2100, which the researchers say is within the range of projections for our current emissions pathway.

In Melbourne, the likelihood of 10cm hailstones hitting the city would increase from once every 20 years to once every three years, according to the study's findings.

"In Melbourne, we saw that in the historical simulations, you might expect a 10cm hailstone, so that's like a pretty big hailstone, to occur once every 20 years in those simulations," the author of the research paper, Dr Tim Raupach, said.

"But in the future scenario, that time reduces to every three years.

Kalgoorlie saw similarly big increases in the frequency of giant hail — with 10cm hail recurring every six years in the future, compared to 18 years now.

In Sydney and Canberra, giant hail already has a risk of happening once every three years, on average, but this goes to once every two years in the future scenario.

In Perth, the chance of seeing giant hail with a diameter of 5cm increased from 14 per cent in any given year to 21 per cent, but the trends for 10cm hail were less clear.

Giant hail has been shown to cause substantial damage to cities in the past, with some of Australia's most expensive insurance disasters coming from hailstorms.

The Bureau of Meteorology issues warnings when hail is over 2cm in size.

More hailstorms for Sydney, Canberra and Brisbane

The simulations also predict the overall frequency of hailstorms will increase by nearly 30 per cent in Sydney and Canberra and 15 per cent in Brisbane — two areas already very prone to hail.

"So those east coast cities — we saw increases in the frequency in the future simulations but in the other places we looked at, the changes were not significant, so no big changes there," Dr Raupach said.

In Melbourne, Brisbane and Sydney and Canberra, it's not just how often the hailstorms and large hail come around that is increasing.

The stones are also predicted to get bigger in general, with the biggest stones in any given storm in Melbourne predicted to increase by nearly a centimetre in diameter on average, according to the study.

Big hail, big costs

Hail is one of the most costly natural disasters, responsible for more than 20 per cent of insurance losses in Australia from 1967 to 2023.

This includes the 1999 Sydney Hailstorm — Australia's most expensive natural disaster in insurance history — which saw an "avalanche" of cricket ball-sized hailstones blasting through roofs, windows and cars.

The freak storm caused $1.7 billion in insured losses at the time, estimated at $8.85 billion if it were to happen today.

Perth residents also remember the 2010 hailstorm, which left the city in tatters and led to over $1 billion in insurance claims. There are still cars on the road today bearing golf-ball-sized dents.

Dr Raupach said Australia's booming rooftop solar installations could make our cities and towns even more vulnerable to cost blow-outs now.

"Hail can damage solar panels. We've seen it happen — in Brisbane in 2020 and in the US as well," he said.

Given the hefty price tag of these events, Dr Raupach said it was something Australia needed to consider, noting hail was not in the building regulations.

"This was a study looking at the end of the century," he said.

"If climate change is making hailstones larger, then first of all, we should reduce emissions so that we kind of tamp down the effects of climate change.

Despite the study's findings, Dr Raupach described the impact of climate change on hail as still being "uncertain", noting this was only one study.

He said further studies were also needed on wind speeds, which could act to make the damage of hailstorms far worse by blowing the stones sideways into windows.

Why hail is getting bigger

One of the main reasons for hail getting larger is the warming of the atmosphere, according to Dr Raupach.

With every degree the atmosphere warms, it can hold 7 per cent more moisture.

This extra moisture acts as "fuel" for a storm when the right weather system comes along, meaning it has the potential to be more powerful and create larger hail.

But Dr Raupach said other "dynamical" parts of weather made the situation more complicated from place to place.

"The ingredients you need for a hailstorm [are] instability in the atmosphere and you need the wind to be changing with height, that's called wind shear, and you need plenty of moisture," he said.

"Let's take Brisbane as an example. There, we see an increase in the instability but we see a decrease in the wind shear, and that seems to have an offsetting effect … that might explain why we're not seeing these changes in hailstone size in Brisbane.

"But in Melbourne, for example, we're seeing increases in both of those ingredients, and so it makes sense that we're seeing changes in the hail size there."

Adelaide did not show any significant trends in either frequency or hail size.

Hobart and Darwin were not included in the study because they "don't get hail as often", according Dr Raupach.