Canberra is usually a pristine city. Its skies are so clear that many of Australia's top astronomy sites are either in the ACT or nearby.

Key points: Canberra's mountain surrounds and prevailing winds make it a perfect smoke trap

Canberra's mountain surrounds and prevailing winds make it a perfect smoke trap Warmer winds from the west will break down the pollution, but worsen the fire risk

Warmer winds from the west will break down the pollution, but worsen the fire risk Meteorologists say the city will likely continue to be smoke-blighted 'until the fires are out'

Yet for two weeks over the holiday period, the national capital had the world's worst air quality among all cities that publish records.

As the pollution from bushfires in surrounding New South Wales peaked, people who spent time outside were effectively smoking two-and-a-half cigarettes an hour.

The smoke seeped into most buildings, too. Offices, shops, museums and restaurants closed. Hospitals had to abandon some procedures.

After a brief respite over Monday and Tuesday, the choking haze returned on Wednesday.

And while the smoke's comings and goings cannot be predicted precisely, meteorologists say it is likely to stay for some time.

There are three main reasons: Canberra's hills, its prevailing winds — and, of course, the fact that the city is surrounded by blazes of unprecedented scale and ferocity.

The Brindabella trap

The bush capital is surrounded by mountains — a perfect smoke trap. ( Flickr: Simon Yeo )

Canberra's topography helps trap thick fogs in winter. In the same way, it has turned the city into a smoke pit during this summer's bushfire crisis.

The capital lies in the hollow between two mountain ranges: the Brindabellas to the west and the main spine of the Great Dividing Range to the east.

As Bureau of Meteorology forecaster Grahame Reader put it: "it's a bit of a bowl surrounded by hills".

"So long as the winds aren't strong enough to disperse the smoke, there's nowhere for it to go," he said.

This also explains why some of the worst pollution has been in Tuggeranong, in Canberra's south, which lies in a deeper valley.

Mr Reader said the local landscape created an effective trap for the smoke.

"Once you have a temperature inversion combining with those hills, it's like a lid on top, which stops the smoke from escaping upwards," he said.

The cool evening breeze

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 30 seconds 30 s Marie Donnell's time-lapse video shows smoke pour into Woden in Canberra's south

A temperature inversion is the lower atmosphere turning upside down, in a way.

Australian National University honorary lecturer Clem Davis, a former meteorologist, said that, at most times, the air at ground level was hotter than the air above it.

But in certain conditions, the ground-level air became cooler.

"The hotter air above acts like a lid and traps everything below the inversion … all the pollution is stuck," Dr Davis said.

These inversions are a normal part of Canberra summer: the prevailing evening winds are east or south-easterly breezes, which bring sea-cooled air into the city.

This summer, however, that cool air is also dumping ash and fine particulate pollution from fires to the east, and the hot air above is holding it down in the Canberra "bowl".

When will it clear?

The main problem is neither the hills nor the winds — it's that the nearby bushfires are so massive and have the potential to keep burning.

Dr Davis was in the bureau's forecasting office in Canberra during the 2003 firestorm that destroyed 500 homes.

While that was "horrendous", he said it did not bring the prolonged pollution that has blighted Canberra this summer.

"And I can't really see that we're going to get any real clearance from the smoke until the fires go out or until we actually get some westerly winds," he said.

Warmer winds from the west or north-west break down temperature inversions, clearing trapped smoke.

But Mr Reeder said they brought with them a new risk: bigger fires.

"The downside is it will be much hotter. And if there are fires to the west or the north-west, those winds will bring it over Canberra," he said.

"Either way, while the fires burn, Canberra will probably continue to be smoked out."