For Sydney residents, the smell and sight was eerily reminiscent of late 2019 when Black Summer bushfires encircled the city.

Fortunately, the thick smoke haze which descended upon the city overnight was from hazard reduction burns in pockets of bushland in the city's north and west, not bushfires, even though it was thick enough to trigger smoke alarms.

The smoke was so dense on Sunday evening that views of the city skyline were impeded even from relatively close range.

Image: There one night, gone the next. Source: @Dannews on Twitter.

As we write this story before lunchtime on Monday, both the smell and the sight of the smoke have mostly cleared, leaving blue skies on an otherwise very pleasant early spring day.

Smoke from hazard reduction burns conducted yesterday has settled across much of the Sydney overnight. The smoke will start to clear this morning however further burns are scheduled for today so residents of Sydney & Central Coast may continue to see & smell smoke until mid-week pic.twitter.com/svzZoG52tK

— NSW RFS (@NSWRFS) September 10, 2023

For the first time in two years, air quality was poor across the Sydney basin. By mid-morning it had returned to a good rating in all parts of the city except for Penrith, where it was fair. You can check the hourly updated air quality here.

The two-hour loop below shows a pretty interesting phenomenon, with smoke on Sunday afternoon appearing to move in both a westerly and easterly direction at the same time, just before dark.

How was this possible?

If you look at our Sydney observations for Sunday, you'll notice that winds over the city were westerly in the morning, easterly for a few hours in the afternoon as the sea breeze kicked in, then westerly again later in the evening.

Smoke was pushed west on the easterly, but as it rose, the upper-level westerlies above the level of the sea breeze pushed it right back out to sea.

Meanwhile the RFS warns that more smoke can be expected in coming nights and mornings this week over the Sydney basin.

Elsewhere in Australia, hazard reduction burns are also taking place. There are numerous prescribed burns currently underway (as they're called in the NT), where there is much more grass cover than usual in many areas due to unseasonable midyear rain. You can check the map for the latest locations here.