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'Like a living creature': Winter fog is quite a sight, but what causes it and when is it just mist?

Georgie Burgess, Saturday June 20, 2020 - 18:58 EST
ABC licensed image
Fog surrounds the Tasman Bridge in Hobart. - ABC licensed

On her farm in Molesworth, Sally Wise watches the morning fog and mist weave its way around her property.

"It just forms the most beautiful moving, living picture on the hills beside us," she said.

"It moves up around the house and then retreats back down to the bottom paddock and we're left bathed in beautiful sunshine.

"We still watch this beautiful mist which is like a living thing with a multitude of moods, that comes up and waxes and wanes - it's just wonderful."

Ms Wise said some days the fog doesn't clear until late morning, but she doesn't mind.

"It hangs in the air, and surrounds you and surrounds the house," she said.

"You can walk 100 steps from the house and get the tendrils of mist swirling around you and the temperature drops as you go.

"It's one of the most beautiful features of the property. It's just like a living creature."

What causes fog?

Bureau of Meteorology forecaster Simon Louis said fog was all about how much moisture could be held in the air.

"The sun goes down in the evening and the temperature starts to cool down over night and it can cool down enough so that it gets to a point where there's more moisture in the air than the air can hold," he said.

He said the relative humidity gets to 100 per cent.

"Instead of it being able to cool down any further you start to get these cloud droplets condensing out and it turns into mist and fog."

He said most fogs formed over night because of the cooling down period, and the surface losing heat.

The result is small water droplets suspended in the air.

Valleys are fog hotspots

Mr Louis said fog was most common in valleys and formed when skies were clear and there was no wind.

In Tasmania, the Derwent Valley, Huon Valley and Tamar Valley are most prone to fog.

"One of the reasons for that is there tends to be more moisture down in those valleys than there would be further up in the mountains," he said.

"There's a process that happens over night where it starts to cool off, and it gets cold faster up in the mountains.

"Because cold air is denser than warm air, it tends to sink down into those river valleys."

He said, as the air sinks and encounters higher humidity, it gets to the point where it is too cold to hold the moisture and it produces fog in the valley.

Difference between fog and mist

The difference between fog and mist was visibility, Mr Louis said.

He said one kilometre was the dividing line.

"If you can see further than a kilometre horizontally, we call that mist," he said.

"If it's less than that, it's fog.

"Fog is just a thicker version of mist; there's more droplets in the air so you can't see as far."

Mr Louis said predicting the extent of fog was sometimes challenging, because it was localised.

"The clearance time of fog can have a really big impact on the temperature forecast," he said.

"If we are expecting fog to clear late morning, we might be expecting the temperature for somewhere like Ouse to get up to 12.

"But if the fog doesn't clear, which does happen from time to time, you can wind up with a top temperature of 4 or 5."


© ABC 2020

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