The smoke haze that choked Sydney earlier in the week has returned, again shrouding the city in a thick fog even though the NSW Rural Fire Service halted hazard reduction burns on Wednesday due to the poor air quality.
Weatherzone meteorologist Brett Dutschke said the combination of very light winds and a temperature inversion caused by a high pressure system was trapping smoke over the city.
"That high pressure is trapping the smoke close to the ground. It's not allowing it to dissipate," he said.
Even without new hazard reduction burns, the drop in wind and stronger pressure overnight on Thursday trapped more smoke following a brief reprieve.
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"Yesterday and the day before, the wind picked up a little bit during the afternoons, and that helped dissipate any of the smoke that was about," Mr Dutschke said.
"But last night we had a slightly stronger temperature inversion, so that's effectively high pressure compressing the air back down to the ground again, allowing that smoke to settle down again."
The NSW Office of Environment and Heritage's air quality index recorded a "hazardous" particle reading in Sydney's north-west on Friday morning, including at Richmond, while visibility is "poor" at North Parramatta, Macquarie Park, and Rozelle.
NSW Health advises residents to cut back significantly on outdoor exercise when air quality is "hazardous", while those with heart and lung conditions should avoid it entirely.
During the day on Friday, winds may pick up slightly – but unlikely enough to lift the smoke, Mr Dutschke said.
"It's going to be a day of really quite gentle winds. So the smell of smoke could be in the air well into the day, mainly in the north-west," he said.
"There could be a bit of smoke about tomorrow as well. Winds look similarly calm for much of the day."
On Friday morning, the RFS tweeted that some of the smoke was rising from the hazard reduction burn carried out in the Blue Mountains on Sunday, Monday and Tuesday. The fire is still burning, and expected to continue for a number of days.
However, no additional burns have been lit in the past 48 hours, and no further burns are planned in the Sydney area for Friday.
RFS Inspector Ben Shepherd said authorities had been meeting twice a day to determine a course of action appropriate to weather conditions, and it was "unlikely" that burns scheduled for this weekend would go ahead.
"We're evaluating everything, even small burns, as small as five to 10 hectares. Anything that could add to this is not going ahead," he said.
"This high pressure system is still sitting over us, so while it's giving us nice warm days it's not letting the smoke clear."
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The wind is not expected to pick up significantly enough to shift the smoke hanging over the city until Sunday, he said.
On Wednesday, following three days of poor air quality in the Sydney area, the RFS tweeted it would suspend hazard reduction burns for at least 24 hours to allow smoke from burns conducted over the weekend to dissipate.
Some of the burns set to be carried out this weekend had already been postponed to avoid clashing with outdoor sporting events including the Sydney Half Marathon last Sunday, Inspector Shepherd said.
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