It is heatwave season — and it has already been a big one for WA.
Perth last week clocked an all-time record, with six consecutive days above 40 degrees Celsius.
The state has now had more individual days over 40C than during any other summer.
Earlier in January, the small town of Onslow, on the Pilbara Coast, equalled the Australian temperature record of 50.7C.
And last month, Marble Bar had 16 days above 45C — the most on record.
Bureau of Meteorology senior forecaster Dean Narramore said most of the temperatures were fairly typical of summer, apart from the 50C days.
But he said the frequency and prolonged nature of the heat events this year had been unusual.
What’s behind the persistent heat?
There are several ways a heatwave can occur, but most of the time they involve hot, dry winds blowing in from the desert for a long period of time.
For WA, they are most commonly driven by a strong and slow-moving high-pressure system positioned in the Great Australian Bight.
Mr Narramore said this set-up had been seen several times in WA this year.
“The whole country's really been stuck in a very stagnant pattern where we've had this constant easterly flow through eastern Australia,” he said.
"That obviously brings cooler, humid, very wet and cloudy conditions [to the eastern states],” he said.
He said when the winds were strong enough, they could also delay or block the sea breeze.
"This year, because we've had stronger easterlies, we've had the trough off the west coast a little bit more regularly, and that's meant the heat has made it to the coast and the cooling sea breeze has sometimes failed," he said.
"That’s allowed for those really high temperatures at Geraldton, all the way down to the South West Capes, to get into the 40s quite regularly this summer."
He said similar weather patterns had driven heat into the Pilbara.
"And the monsoonal weather and the wet phase really hasn't kicked in, so we've had no real cyclones or monsoonal loads across north-western parts of WA," he said.
What counts as a heatwave?
Heatwaves are defined as three or more days of unusually hot weather, relevant to the location and time of year.
They are Australia's deadliest natural hazard, killing more people than bushfires, cyclones or floods.
And there are three levels of heatwave: low-intensity, severe and extreme.
But it is not as simple as the temperature reaching 40C.
Mr Narramore said meteorologists looked at both maximum and minimum temperatures, and often it was the hot nights that made a big difference.
"That's when all of a sudden, it's going to have impacts on people," he said.
He said the vulnerability of people in certain conditions helped guide the classification.
That vulnerability included how acclimatised people were to the heat and how the infrastructure would cope, he said.
Future of heatwaves
According to the latest Intergovernmental Panel on Climate Change's sixth assessment report, it is "virtually certain" hot extremes, including heatwaves, have become more frequent and intense across most land regions since the 1950s.
Mr Narramore said Australia was likely to see more heatwaves in the future.
"We're probably going to see, unfortunately, an increase in heatwaves, including [their] duration, so longer heatwaves, more regular heatwaves and possibly out-of-season heatwaves as well," he said.
