It was a beautiful mild spring Thursday morning with the temperature topping 20°C by 9 am, then all of a sudden, Wollongong's day started looking very different.

A sea fog rolled into town, catching locals in the city of approximately 300,000 residents by surprise.

Image: Poolside sun-bathing officially ruined. Source: André Brett.

"About an hour ago it was a bright sunny morning, low 20s, calm, barely a hint of a breeze, bit humid. None of that has changed except now it looks like this,” Wollongong-based historian André Brett wrote on Twitter.

Image: That beach umbrella tells you that the sun was out not too long ago. Source: André Brett.

Others also shared the unexpected incursion of sea fog.

Fog in Wollongong NSW, warm 23degrees Celsius 10.30am. Weird. pic.twitter.com/BA0QZcyCi1

— Ledge Hermit (@HermitLedge) October 27, 2021

So what caused the fog?

"Sea fog occurs when warm, moist air passes over cooler water, causing the moisture to condensate into fog as the warm air rapidly cools," Weatherzone meteorologist Felix Levesque explains.

And that would normally explain things neatly. Except that in this case, the water off the Sydney and Illawarra region (where Wollongong is located) right now is not particularly cool. According to the Manly Hydraulics Laboratory offshore buoys:

• The water temp off Port Kembla (at the southern tip of Wollongong) at 9 am on Thursday was 20.2°C.
• The water temp off Sydney at 9 am on Thursday was 19.8°C.
• This reporter can verify that Sydney’s water is lovely and warm right now, having taken a delightful dip in Botany Bay (less than 70 km north of Wollongong) just yesterday afternoon.

So if the water over the whole region is warm, and if sea fog requires warm, moist air passing over cooler water, what happened?

"It's likely that recent northeasterly winds may have caused some near-shore upwelling (deep cold water coming to the surface near the coast)," Felix Levesque explains.

"Sea surface temperature analysis maps do show a bit of cooler water off that region."

Image: The green, yellow and blue patches offshore are the cooler patches. Source: IMOS.

So there's an answer which seems quite reasonable. Most of the water off the Illawarra coast is not cold, but small patches of cold water that upwelled due to recent northeasterly winds may have been enough to cause the fog, which was still enshrouding the 'Gong as we published this story, about two hours after it arrived.