Australia Weather News

With several days of hot weather forecast for Adelaide next week, it might be tempting to call it a "heatwave", but what is a heatwave? Is it as simple as hot weather that hangs about for days on end?

The short answer is no.

In 2014, the Bureau of Meteorology (BoM) switched to a tiered rating system for heatwaves that ranged from low intensity, to severe and extreme.

The Bureau's new definitions were calculated using forecast maximum and minimum temperatures over a three-day period.

Up until 2014 a heatwave definition in Adelaide was defined by different maximum temperature thresholds for a three or five-day period.

But according to BoM's National Heatwave Project director John Nairn, the definition became "more statistical in nature".

"A heatwave now is when it is unusually hot for your location, both in reference to your local climate, and the recent history," he said.

In Adelaide, for example, a maximum of 36 degrees Celsius and minimum of 20C for three days and nights over summer would be considered the threshold low-intensity heatwave, although those figures could be higher or lower because it is their combined average that drives the rating.

"85 per cent of all heatwaves are going to be low intensity," Mr Nairn said.

"You see them often enough that your engineered environment and adapted strategies are pretty safe."

This included utilising air-conditioning, preparing houses for hot weather, and perhaps altering plans to avoid outside activities for peak temperatures.

"But the higher your minimum at night, the sooner temperatures return to a maximum the following day, and you don't get as much relief," Mr Nairn said.

"They're the principles of the system."

Severe and extreme heatwaves risk lives

Severe heatwaves are less common but could be risky for certain groups, particularly the elderly, babies, pregnant women and those on medication.

"There's a lot of vulnerabilities that start to show up once the heatwave becomes severe because there's published evidence we start to see mortality at that threshold," Mr Nairn said.

"Past the severe threshold is the extreme heatwave, which is so rare and so intense that our normally reliable infrastructure becomes more vulnerable."

For example, Mr Nairn said heated petrol could vaporise within filling station pipes and prevent pumping fuel.

"And if it's a big heatwave covering Australia's south-east, and they usually are when they're extreme, there's so much power demand that the power grid starts to fail in locations," Mr Nairn said.

"But the reason why we're resilient to heat is because of our air-conditioning."

The Bureau has forecast a maximum of 34C weather for Adelaide on Monday, 40C on Tuesday, and 37C on Wednesday, but no heatwave warning has yet been issued.

Heatwave definitions more 'scientific'

BoM Media, Engagement and Communication manager Catherine Kennedy said the definitions of heatwaves had to become more scientific.

"What's hot to Adelaide, is not hot to people in Oodnadatta," she said, referring to an outback South Australian town where Australia's highest recorded temperature of 50.7C was registered in 1960.

"We can't stop people having their own definition of heatwaves, but we do have to have a scientific definition for thresholds."

She said the definitions had also better aligned the Bureau with language used by emergency services, such as the Country Fire Service, which adopted a category warning system for bushfires in the 2000s.

"It's like any warning basis where you have rating one, rating two and a red rating three," Ms Kennedy said.

"It's a way for the public and industry to know, if we're going from a low-intensity heatwave to extreme, how much hotter it will be on average, rather than just knowing the maximum temperature."

Despite Adelaide recording a one-day maximum record of 46.6C on January 24, Mr Nairn said the city had not experienced a significant heatwave since 2014 and 2009, both of which "compared with the 1939 heatwave".

That was when the population endured its worst ever heatwave, which lasted 14 days — nine of which remained above 37C — and the city recorded its former maximum temperature of 46.1C.