If you're from, or have been to Australia, you've probably seen these signs.
Whether in country towns, or along stretches of country roads, these signs are a feature
of the Australian countryside.
But what do these categories mean, how are they calculated, and what on earth is catastrophic?
This is a Fire Danger Rating sign.
It doesn't predict the weather, or even the likelihood of fires, but it lets the community
know how much of a threat a fire would be, if there were one.
Before we find out how this danger level is calculated, let’s find out what each of
the ratings mean.
The Low-Moderate, High, and Very High Danger ratings are used to show that if a fire were
to start, that it should be able to be controlled.
The potential fire’s danger obviously increasing from Low-Moderate through to Very High.
Warnings and weather conditions should still be continued to be monitored.
So we're already past Very High on the danger rating scale – that brings us to severe.
Fires on a severe danger rating day may be uncontrollable.
Well-prepared buildings that are actively being defended could provide safety.
This is one step below our next tier, extreme.
On an extreme fire danger days, if fires were to start, they are expected to be uncontrollable
and unpredictable.
only buildings that are constructed to withstand bushfire and are actively defended could provide
safety.
On severe and extreme fire danger days, anyone who is not of the utmost preparedness is usually
recommended by local authorities to leave the area (as long as it is safe to do so).
The catastrophic tier, or Code Red in Victoria, was introduced after the Black Saturday fires.
The commission reviewing those fires felt the community needed a level above extreme
to fully grasp the danger of a fire with a very high fire danger index score – which
of course we will talk about in a moment.
Catastrophic fire danger days are the most dangerous.
No houses are expected to withstand a bushfire in these conditions.
Local fire authorities will normally recommend people evacuate the area the day before this
rating comes into effect, as long as it is safe to do so.
So now we've talked about what each of the ratings means – let's find out how they
are calculated.
As mentioned before, one of the factors that goes into setting the danger rating in Victoria,
is the Fire Danger Index.
The fire danger index for a particular area is calculated by the Bureau of Meteorology.
There are actually 2 index scores, one for forested areas, and one for grasslands.
We're going to focus on the Forest Fire Danger Index.
The Fire Danger Index is calculated from several parameters, from a fixed fuel load per hectare
– which is one of the ways the calculation could be improved.
Temperature (things burn easier in high heat), relative humidity (things burn easier in dry
air), wind speed (high winds fan flames), a fixed value for how much fuel there is per
hectare – which is one of the ways the calculation could be improved - and something called the
Drought Factor – which is a number to represent how dry the ground is, including ground vegetation.
Once we have our Fire Danger Index score, we can correlate this with a Fire Danger Rating
level.
However other factors can also influence the Fire Danger Rating, as well as the Fire Danger
Index score.
For example, the Fire Danger Index doesn't take into account how changeable the wind
is, which can significantly increase the danger posed by fires.
Nor does it adjust for current or recent fires in the area.
And of course, Australia has more than 2 different biomes.
There are different forests, different grasslands, and everything in between.
So, once the Fire Danger Index is calculated, and local considerations are taken into account,
then the fire danger rating is set – to advise communities of the danger posed by
potential fires on that day.
Neither system is perfect – and one might particularly argue that the different Fire
Danger Ratings aren't actually that clear about what they mean, particularly around
the middle of the scale.
But until either system is updated, this is what Fire Danger Ratings mean, and this is
how they're calculated.
Australia is a country where bushfires shape and reshape the landscape, and Australian
communities are normally well-prepared for this ever-present risk.
However sometimes, despite best preparations, fires are uncontrollable.
This season's fires have been particularly extreme and damaging.
And fire seasons are expected to become longer and even more severe as a result of climate
change.
As I'm writing this video, many people, communities and wildlife have already been affected by
this fire season – and it’s not over yet.
I'll leave links in the description of a couple of charities and volunteer organisations that
you can donate to if you'd like to support those affected by bushfires.
Thanks for watching, and see you in the next video.