Every bushfire is different and every bushfire is dangerous, but there is a classic set-up that signals disastrous fire danger in Australia.

Sure, fire needs a spark, but then drought, high temperatures, low humidity, heavy fuel loads and the lie of the land all play their part.

The most devastating fires in Australia's history — Black Saturday (2009), Ash Wednesday (1983) and Black Friday (1939) — all occurred in hauntingly similar conditions, conditions repeated several times over in the summer of 2019-2020.

What makes some days disastrous?

Fire is a part of life in Australia, but some conditions can signal catastrophic danger. ( ABC Weather: Kate Doyle )

"Our worst fire days, no matter where you are, is when you've got hot, dry winds blowing off the centre of the continent," said Dr Mika Peace, a research meteorologist and fire expert at the Bureau of Meteorology (BOM).

"So what you're looking for is a really windy air mass which is going to blow the air from the centre of the continent towards the population bases."

Adding a feature like a trough or front that changes the wind direction increases the danger.

These are the conditions that have led to the worst fires in Australian history including Black Saturday. ( ABC Weather: Kate Doyle )

"For south-eastern Australia, we're really looking at frontal systems where you've got a really strong high pressure system and approaching front," Dr Peace said.

Before the front comes through, strong winds bring hot, dry air from the north, but after the front the winds come from the south-west.

"What happens is you've got an ignition point and the fire will burn from north to south in the northerly winds.

"Whatever distance the fire has burned from north to south, when the wind change comes through, that becomes your new fire line and it opens up the whole of the eastern flank of the fire.

"So you get really rapid fire spread out to the east."

When northerly winds are replaced by a cold front bringing strong south-westerlies, anyone on the eastern side of the fire is in danger. ( ABC Weather: Kate Doyle )

It was this rapid change to the east that burned through towns like Marysville on Black Saturday, the Pinery (2015) and Wangary (2005) fires in South Australia, and was involved with a near miss for Victorian fire crews early 2019.

Then there are firestorms

Dr Peace said the other dangerous factor was the instability of the air mass.

"An unstable atmosphere can lead to really deep plume development," she said.

A deep mixing atmosphere means you get an energetic fire plume and can lead to pyrocumulonimbus developing — that's when the fire develops its own thunderstorm.

"Fire-generated thunderstorms or pyrocumulonimbus can ignite new fires downwind through lightning strikes," Dr Peace said.

Pyrocumulonimbus storm clouds form over intense fires aided by an unstable atmosphere. ( ABC Weather: Kate Doyle )

"You also get really gusty, turbulent winds which, if they blow down over the actual active fire front, can completely change the direction and the speed of fire spread on the surface.

"This has a really big impact for any ground crews or anyone who's near the fire front because you get these really rapid changes in direction."

An unstable atmosphere can also increase spot fires, when burning vegetation is lifted up and carried through the air to start more fires downwind. On Black Saturday, spot fires were recorded 35 kilometres ahead of the inferno.

Eucalypts make Australia the most dangerous place in the world for spot fires; the trees have a high oil content so the vegetation can stay alight longer as it travels through the air.

So the key message is fires can spread rapidly — waiting until you see smoke before deciding if you should leave is not good enough.

You will not always get an official warning.

Countdown to disaster

Claire Yeo, the BOM forecaster embedded with the Victorian state control centre in the lead up to Black Saturday, said officials knew conditions were going to be bad that day.

"When you hear that anecdotal evidence coming from old-time land managers, making comparisons to Ash Wednesday, you stand up and listen," she said.

"Particularly when you start to see a pattern like what we were seeing for Black Saturday, which was the classic set-up: hot north-to-north-west winds and late wind change."

The difference was that it was looking worse.

Loading...

In 2009, parts of Victoria were coming off the lowest 12-year rainfall deciles on record. It was also a record hot week, with temperatures around 45 degrees for three days straight.

There was a drought leading into Ash Wednesday, but not that bad.

Ms Yeo said she could see as early as the preceding Monday that Saturday was going to be horrific.

"Every single day leading up to Black Saturday I was able to say the same thing: the models are doing the same thing, it's still painting this dire picture."

She said when the figures were put into the forest and grassland fire danger indices (which incorporate information on fuel load, temperature, relative humidity and wind), the result was off the scale.

Black Saturday triggered the creation of a new category of fire danger: "code red" or "catastrophic".

How do you communicate that much risk?

These are the conditions that have led to some of the worst fires in south-west WA. ( ABC Weather: Kate Doyle )

"Even though I could see that it was going to be a disastrous day, particularly as there was already fire in the landscape, I didn't comprehend that you could possibly have that many people perish," Ms Yeo said.

She gave briefings to the fire chiefs and then premier John Brumby to explain the threat and how confident BOM was in the forecast. They in turn held media conferences to warn the public, which were uncommon at the time.

"I had to sort of come to terms with it, that many people could die even though I felt the warnings were well and truly out there.

"I went through the whole feelings of: 'Didn't I say enough? Was my communication not effective enough?'

"But over time, and after a bit of counselling and reflecting on what I had done leading up to those days, I thought I couldn't have expressed how bad it was going to be any more than what I did, which was talking about the absolute extreme.

"It's just that as humans, sometimes we find it hard to come to terms, to fit a warning with what the actual reality is."

These are the conditions that led to catastrophic fires in Queensland in December. ( ABC Weather: Kate Doyle )

'You know that people are dying'

Alen Slijepcevic, now deputy chief officer with the Country Fire Authority, was working in the emergency coordination centre as Black Saturday unfolded.

"I had never experienced anything like it before and I'm hoping I'll never experience it again ... but I have my doubts," he said.

Mr Slijepcevic said he first thought of the day as something out of the ordinary when he received the forecast and discussed it with Ms Yeo.

"Claire was the first forecaster that gave us the glimpse. She said, 'I have never seen this before. It will be really, really nasty'."

When a catastrophic fire takes hold, little can be done to fight it. ( ABC Weather: Kate Doyle )

Nasty it was.

"On that day the wind change was incredibly strong. For the whole day we had a strong north-westerly blowing the fires into the south-east."

They knew the change was coming and that the conditions would be bad, but that wasn't enough to stop the disaster from unfolding.

"It was very stressful," Mr Slijepcevic recalled.

"We had parts of the information but not the full picture; you know that something bad is happening, you don't know how bad, but you know that people are dying in their houses and burning in their cars.

"We had lots of issues with the communications. We did not have good systems for the communities."

Some fires are unfightable … and it's getting worse

As the landscape becomes fire-prone, the risk to communities increases. ( ABC Weather: Kate Doyle )

Mr Slijepcevic said crews had been reasonably successful on Black Saturday in attacking some fires and putting them out, but once the flames were in the forest and took hold they were impossible to extinguish, even with aircraft.

"I think people need to understand that there are limits to what fire services or land management agencies can do in those circumstances."

The maximum intensity of a fire that aircraft can extinguish is 6,000 kilowatts, Mr Slijepcevic said. On Black Saturday he said the intensity reached 150,000 kilowatts.

"[Using aircraft] is a complete waste of time at that phase of the operation."

He said bushfires had become a social problem as much as a physical problem.

"We know that with climate change and more people moving into flammable landscapes, we are creating more risk.

"People will have to accept and learn to live with fires because that will be our norm into the future. People will have to be better prepared."