The NSW Rural Fire Service said it was the smoke that kept bushfires from flaring up earlier this week — but how can smoke possibly be helpful?

We are not making this up. On Tuesday, RFS spokesman Ben Shepherd said:

"We were fortunate that we didn't see the conditions deteriorate as much as initially forecast, and that was, unbelievably, actually due to that incredible amount of smoke that was settled over the Sydney area.

"It didn't allow the ground to warm up enough and it didn't allow the winds to mix down and held back the worst of the fire danger."

It most certainly seems unbelievable, so how does it work?

When the inversion is in place the smoke gathers in the light winds but when the inversion breaks down strong hot winds can make their way to the surface and the fire. ( Supplied: Bureau of Meteorology )

Why so smoky?

How much smoke there is hanging around is dependent on how the atmosphere is acting ... and how much smoke is being produced, of course.

Usually, the atmosphere gets cooler with height, allowing the smoke to rise and vent away from the surface.

But if there is a warm layer of air above, called an inversion, it can act to trap the cooler air below and prevent it from mixing.

Nick McCarthy, a PhD student studying bushfire-atmosphere interactions at the University of Queensland, said a good way to think about it was in the context of a domestic fireplace.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 37 seconds 37 s Sydney Harbour barely visible through smoke

"If you open up the flue, that's going to increase the vertical ventilation and get the smoke out. But if you leave the flue on, it's going to keep the smoke capped," he said.

Inversions are often associated with cooler air near the surface overnight or sinking air above high pressure systems.

Dr Mika Peace, fire research meteorologist with the Bureau of Meteorology, said inversions had led to the smoke over Sydney this week.

"If we've got a ridge of high pressure, so a high pressure system, we typically see an inversion above the surface which can act as a lid to the smoke," she said.

"That's what's been happening in Sydney over past couple of days, where we've had a lid or a layer above the surface where you've got warmer air above it and that acts as a cap to the smoke going up."

Below the inversion, the cool, still air sits and the smoke builds more strongly than if it was allowed to mix with the dry, windy air above.

Loading

How does smoke reduce fire danger weather?

Once the smoke is trapped, it can start affecting the air around it.

Dr Peace said a dense layer of smoke near the surface, like there was near Sydney on Tuesday morning, meant that when the sun came up and tried to warm the ground, it didn't get through the smoke layer.

"So you actually get much slower daytime heating than what you'd normally expect," she said.

Lower temperatures are obviously good for keeping those fires down.

Usually the sun would burn through the inversion but the smoke helps to delay this, acting as a feedback loop, further prolonging the reduced fire danger.

Mr McCarthy said one of the main factors for predicting fire behaviour was forecasting when that inversion would lift.

"Once that inversion lifts, the wind comes down to the surface and you start to get those increased fire behaviour activities," he said.

"But if you have an inversion that is trapping a lot of smoke, what that can essentially do is prevent the time that the inversion ends up lifting, or 'mixing out' is what we call it.

"Once the atmosphere is mixed down, and once that solar radiation can hit the surface, that's when we start to see those fire behaviours kick in."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 35 seconds 35 s Sydney's air quality plummets from surrounding bushfires.

But before you get carried away setting off smoke bombs, the smoke does have its downsides — reduced air quality and reduced visibility, just for starters.

Mr McCarthy said while smoke could influence the fire danger indexes, the overall impact was really not dramatic.

"Obviously the smoke is constantly being produced, so it's not really a significant help to firefighting operations; it's not going to make the situation a whole bunch easier by having smoke.

"But there is this factor where it can just be that extra few hours without the winds mixing down into the valleys, and potentially the direct radiation onto the plant, that can ease conditions just a little bit throughout some of these really bad days."

What's it going to take to clear it?

There will be smoke as long as there are fires, and there will be fires until there is widespread rain — unfortunately there's none currently on the forecast for NSW.

Rain would be a great way to wash the smoke out of the atmosphere too, but a wind change is another way to remove it.

Unfortunately, it would take the whole air mass moving out to remove all the smoke, and while there are wind changes every week or so, and while there are still fires producing smoke, any relief will only be temporary.

Rain may not be on the forecast, but at least there are more favourable conditions for the NSW east coast over the next few days, giving authorities a chance to perform more backburning (where they deliberately light fires in the path of fires), starving the bushfires of fuel.

But this does mean the smoke is likely to continue.