Australia Weather News

It thrills photographers and frustrates air travellers — the city of Launceston, in Tasmania's north, is no stranger to a bout of thick fog.

The fog that descended upon Tasmania's northern capital on the weekend threw hundreds of people's travel plans off-course as many flights were cancelled, delayed or diverted to other airports.

But why is Launceston so prone to foggy conditions, and why aren't other parts of Tasmania so badly affected? The ABC spoke to a meteorologist at the Bureau of Meteorology to find out exactly what causes it.

Why is Launceston so foggy?

The BOM's Luke Johnston said it was a variety of factors, including topography and the way people heat their homes.

"Launceston is particularly susceptible to fog,"

"In general in Tasmania we have a series of cold fronts which come across the state with predominantly westerly winds and Launceston gets a lot of shelter from the Central Plateau, so that often means it's quite sheltered," he said.

Mr Johnston said radiation fog — the kind of fog that blankets cities during the winter — needs clear skies and light winds in order to form, and Launceston's sheltered climate provided the best conditions for it to develop.

"[Clear skies and light winds] are two of the key ingredients you need for radiation fog to develop and as the ground cools rapidly, cloud begins to form on the ground, and that's essentially what this type of fog is," he said.

Launceston is also in the Tamar Valley, and fog tends to form in lower-lying areas.

Mr Johnston also said the smoke from wood heaters "makes it much easier for fog to form".

Is it normal for fog to take so long to clear?

While Launcestonians are well-accustomed to fog, they might have been surprised about how long it took to clear.

The literal cloud over the city persisted for the better part of two days and Mr Johnston said that's uncommon.

"Normally, it would clear well before midday," he said.

It was a perfect storm of weather that kept holiday-makers stranded.

Mr Johnston said earlier rainfall had cleared some fog, but as the sun came up, it returned.

"Once the sun came up, the showers had cleared away but that left Launceston in a situation where it had pretty clear skies, it had very light winds and high levels of moisture on the ground and the sun mixed with the wetness of the ground to create a fog that thickened."

Wind the key

So why does Hobart mostly miss out? Essentially, because it's too windy.

"On a perfectly still night in Launceston there's often winds of 15-20 kilometres per hour in Hobart, which makes it hard for the cloud to form that low to the ground," Mr Johnston said.

When Hobart does get fog, it usually rolls in from the north — as in the Bridgewater Jerry phenomenon — rather than forming on the spot as it does in Launceston.

While Hobart might miss out, other parts of the state do become shrouded during the winter.

Mr Johnston said the Fingal and Huon valleys get foggy due to their low-lying geography, but the Upper Derwent Valley tended to get the most significant fog.

"It's pretty uncommon to get fog that lasts all day around Launceston, but it actually happens quite a few times a year in the Upper Derwent Valley, so Ouse, Bushy Park and even sometimes New Norfolk have fog for most of, if not all of the day," he said.

That stubborn fog plays havoc with meteorologists, with temperature forecasts being out by as much as eight degrees because the sun can't get through the clouds to heat up the ground.

It's also hard to pick when it might clear.

"In Tasmania in particular it's easy to say when it will or won't fog but it's hard to pick when it's going to clear because you have to take into account how deep it is, how thick it is, where it is," Mr Johnston said.