The Brisbane skyline was largely hidden on Monday morning due to a late September fog. But how rare is fog in southeast Queensland at this time of year, and what caused it?

Many Brisbane locals might have been wondering whether the fog was in their head after a weekend of celebrations fuelled by the Brisbane Lions’ AFL Grand Final victory on Saturday and the Broncos’ Sunday triumph in the NRL Preliminary Final.

But this morning’s fog was the real deal, and it was caused by a combination of atmospheric conditions.

Fog is formed when the air temperature falls to the dew point – the point to which the air must cool in order to produce condensation. The dew point temperature and air temperature were both hovering around 17 to 18°C in Brisbane early on Monday morning, which allowed the fog to form.

Elevated levels of airborne moisture are also required for fog to form, which is why fog is more likely (under the right conditions) over an area which has recently been soaked by rain. Brisbane has had an exceptionally dry September to date, with just 0.8mm of rainfall recorded. So what triggered today’s fog?

As Weatherzone meteorologist Felix Levesque explains, moisture was still present in the atmosphere over the dry southeast Queensland landscape due to the persistent onshore wind flow in recent days.

"Strong northeasterly sea breezes increased the moisture across southeast Queensland east of the ranges,” he said.

Image: Four-hour satellite loop over southeast Qld from 5:30 to 9:30am on Monday, September 29, 2025, showing early morning fog dissipating quickly over Brisbane and nearby areas.

“Some fires were also burning across the area over the weekend. Tiny airborne ash particles act as nuclei for droplets of moisture to attach themselves to, and this helped the fog to form.”

According to Levesque, the third essential factor for today’s fog was clear skies which enabled the relatively cool overnight temperatures.

While today’s Brisbane fog occurred due to a relatively unusual combination of factors for springtime, it was far from unheard of.

Image: Monthly fog frequency for selected Australian airports. Note that Canberra has by the most frequent fogs with around 30 each year (the second graph) while Brisbane has just under one fog on average in September (the blue line in the first graph). Source: BoM.

The table above shows fog frequency for September. While the likelihood of fog in Brisbane drops off significantly towards summer, it is still likely to occur nearly once on average each September.