As bushfires continue to burn through the Blue Mountains in New South Wales, ACT Rural Fire Service chief officer Andrew Stark answers a few common questions about how firefighters battle the blazes.

How do firefighters fight a massive bushfire?

A fire is made up of three principle elements: fuel that burns, oxygen that allows the combustion to occur and heat.

In a normal fire situation you can pour a bit of water on it or you could put a blanket over a fire to smother it. But with a bushfire you can’t get rid of the oxygen because it’s out in the open, it’s also pretty hard to … take the heat away …[as] we’re talking about tens of thousands of kilowatts of energy in a bushfire.

So the main role in fighting a bushfire is to take away the fuel. That’s when we create a fire break.

Often that’s done with dozers or graders to actually put a break in the line so the fire burns through the bush and comes to an area with no fuel, which could be a road or a break that’s built.

How wide do fire containment lines need to be?

It depends on the type of forest that’s burning.

With grass fires, the flames can be quite high. But if you think about grass that might be a couple of feet high and tends not to be throwing many embers out, you can use quite a narrow break and it comes to that and stops.

But with our tall eucalypt forests the fires are burning up in the canopy often faster than the fire that’s burning on the ground. That’s when we talk about crown fires.

And obviously one of the things about our eucalypt forests is all the leaves come off trees and all the embers, and that’s what we call spotting. Spotting can go hundreds of metres, it can go kilometres.

So the break needs to be wide enough to slow the main front of the fire, but it also needs to be wide enough to stop those embers and sparks jumping and going straight across.

In the current Blue Mountain fires they would be looking for their back-burns to be in hundreds of metres if not kilometres before the main front hit it.

How do firefighters use the water on their trucks?

All the water that firefighters carry is all about cooling the fire.

So in bushfire fighting if the fire is small enough or if there is a safe place that firefighters can use the water on their trucks and hoses [then it is used] to cool the fire as it comes.

But in those great big fires, obviously they’re coming with such ferocity and energy, then it’s about removing the fuel.

How much rain is needed to extinguish a massive bushfire?

You would want more than 15 to 20mm at once to actually halt a fire like they’ve got in the Blue Mountains.

If that 20mm came over a few hours then that rain would help, and if you had a smaller amount but it rained and rained and rained for a couple of days then obviously that would have a cooling effect and that slows the fire and also wets the fuel ahead of the fire.

But if they have a rain event of anything less than 10mm it really won’t have any kind of effect in slowing these fires and will in fact just extinguish back-burns that obviously aren’t burning with the same ferocity because they tend to be burning downhill and back into the main fire and not with the same intensity that the main fire is running.

What effect can different terrain have on bushfires?

For every 10 degrees you increase the uphill slope you double the speed of the fire. And obviously downhill you have the reverse effect.

How is the speed of a fire determined?

The speed of the fire is related to the fire danger index, which is made up of the drought factor, which is how dry the bush is over the longer term. And then the temperature at the time, the humidity, and the wind speed has a large effect on the speed of the fire’s rate of spread. The temperature and humidity combined contribute to the intensity of the fire.

For how long do firefighters need to monitor burnt-out bushland after the bushfire has been contained?

For at least a month or at least until significant rainfall happens in the interim.

There would be very active patrols on the perimeter and probably patrols by aircraft. But as the weeks move on it would retreat to just perimeter patrols.

It becomes quite obvious where smoking trees are or little unburnt bits of the bush might be. The patrols are active, but it’s anticipated where the problems might be.