It's not quite December 2019 all over again, but Sydney skies are extremely smoky this Tuesday, with a strong smell of smoke in the air, due to numerous hazard reduction burns on the fringes of the city.

In fact, the layer of smoke is so bad that parts of Sydney are currently recording air pollution levels considerably worse than Beijing and not much better than Delhi. When your air quality is similar to the sprawling car-choked capital cities of the world's two most populous nations, you know it's not good.

Another smoky morning in #Sydney as the RFS carries out hazard reduction burns across NSW.

But why so much smoke in Sydney? A temperature inversion is preventing it from mixing up into the atmosphere.

🎥: @sophie_walsh9

pic.twitter.com/Q7TWXtywLH

— Ben Domensino (@Ben_Domensino) April 26, 2021

Measuring air pollution is complex because you're actually measuring numerous things.

• The NSW Department of Planning, Industry & Environment has a page detailing levels of ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide, airborne particles (of stuff like ash, dust and even sea salt).
• It also publishes an overall AQC (Air Quality Categories) Index, which takes into account all those contributing factors.
• Like the NSW fire danger ratings that break the situation into one of six simple categories ranging from Catastrophic to Low-Moderate, the ACQ is an easily digestible five-category range which gives the public a clear indication of what's what.
• The scale ranges from Good to Fair to Poor to Very Poor to Extremely Poor.

Large parts of southwest Sydney had a rating of Extremely Poor on Tuesday morning, with the rest of the city divided into the Poor and Very Poor categories. You can keep in touch with the latest readings here. They’re updated hourly.

Back to those international comparisons for a moment. It's obviously difficult to compare cities overseas because each government agency tends to use a different method to measure air pollution.

One way to get an idea is through a site called the World Air Quality Index, a Beijing-based site that compares real-time air quality in 130 countries and counting.

The Air Quality Index (AQI) measures a range of factors, similar to the NSW AQC Index. One of those indices is airborne particles, and on that front, Sydney measures up poorly this morning.

For example, at 10 am, numerous sites in Sydney had readings well in excess of 200 PM2.5 – which means airborne fine particles less than 2.5 µm (micrometres, or a millionth of a metre) in diameter – whereas most sites in greater Beijing were in the 100 range.

Image: Hazard reduction burn over the Blue Mts, west of Sydney this week. Source: @drone_pics_n_art on Instagram.

As Weatherzone meteorologist Ben Domensino points out in the video a little higher up in this story, smoky conditions in Sydney are being exacerbated by a temperature inversion, which is when the temperature increases with height and prevents the smoke from mixing up into the atmosphere.

That situation should dissipate as the week progresses. Meantime, it may pay to break out one of the surgical masks we've all got close at hand due to Covid.

And to find out where the burns are taking place, check out the NSW Rural Fire Service page here.