Perth and Western Australia's southern regions have been greeted by a thick blanket of smoke haze in recent days, as planned burn-offs continue around the region.
Data from the Department of Water and Environmental Regulation (DWER) showed South Lake and Collie registered 117 and 112 micrograms of very fine particulate matter in the air during Monday morning, meaning the air quality was "very poor".
It makes the concentrations of pollutants in South Lake five times Beijing's 2021 average.
The lingering smoke haze is a common fixture of autumn, with light winds and mild temperatures ideal for planned burns in southern WA.
But the conditions are also the perfect set-up for smoke haze to stick around, trapped under a phenomenon known as an inversion.
How do inversions work?
Typically, air temperature decreases with height, allowing the warm air at the surface to freely rise through the atmosphere and disperse.
But as the name suggests, an inversion is the other way around.
Bureau of Meteorology (BoM) forecaster Jessica Lingard said during an inversion there was cooler, denser air at ground level and warmer air aloft, which acts as a "lid" and stops the air from rising and dissipating.
"As you can imagine, if you've got constant smoke coming out of a fire and nowhere for it to go, it all sort of just spreads out underneath this lid."
It is not until the heat of the day warms the air up at the surface that the two layers of air can begin to mix through, and the inversion is broken.
What's needed for an inversion?
Ms Lingard said there were two key conditions that created a temperature inversion.
Those were a clear and still night — the same recipe for a cold night.
The same conditions also create a foggy morning.
On clear nights, there are no clouds in the sky to trap the heat and as a result, heat is able to escape the earth's atmosphere.
"So what happens overnight, obviously, is the temperature cools down," she said.
"So we end up with this layer of cool air right by the surface and above that is a layer of warmer air, and that means that those two layers can't mix together."
On a still night, there is less wind to mix the air up, allowing the cold air next to the ground to separate from the air higher up.
When does it happen?
These conditions are most common in the cooler months of the year, from April until September.
But Ms Lingard said inversions occur regularly, all around the country.
"But we really notice it here in the metro area, because this time of year there is an awful lot of burning going," she said.
"So we notice it where the smoke has been trapped, but in other areas, where there isn't the smoke, you wouldn't even know what's happened."
During period of "poor" or "very poor" air quality, DWER recommends people reduce or avoid outdoor physical activity if they develop symptoms like cough or shortness of breath.
It also recommends staying indoors and closing windows and doors until outdoor air quality is better.
Smoke haze is forecast to remain on Tuesday morning in Perth and parts of the South West, before a cold front on Wednesday brings rain to southern parts of the state.
