Australia Weather News

Don't like the weather in Melbourne? Wait five minutes, so the saying goes.

It is a bit of a cliche, but it sometimes really does feel like Melburnians routinely experience four seasons in one day.

Perhaps most famous are the afternoons where the oppressive heat suddenly gives way to a swift drop in temperature, followed by rain, thunderstorms and a comparatively chilly night.

Adelaide also experiences these so-called "change days", Bureau of Meteorology senior forecaster Tom Delamotte said.

But Melbourne has a few characteristics that make it more prone to them, particularly this time of year.

"And they're often our busiest days," Mr Delamotte said, of his work at the BOM.

"We have to deal with things like heat and fires and thunderstorms, all in a couple of hours."

Not just a load of hot air

As Australia emerges from winter, the land north of Melbourne starts to warm up.

Northerly and north-westerly winds sweep hot air down from the red centre, bringing warm weather with them.

But the ocean off Australia's south coast, which brings cool breezes to Victoria from the west, doesn't heat up as quickly as the mainland.

This lag means, around this time of year, there's a bigger temperature difference between hot air from the north and cold from the ocean.

And that provides excellent thunderstorm-forming conditions.

"To make a thunderstorm you need three things: lift, instability and moisture," Mr Delamotte said.

When a cold air mass blows off the ocean into western Victoria, it wedges under the warm air layer already there, forcing it into the cooler layers above.

When warm air cools, water vapour condenses out to become clouds.

With enough moisture, big, dark cumulus clouds laden with rain form along and behind the cold front (which is the leading boundary of a cold air mass).

Bingo! Now we have a band of storms in the state's west. Next, it needs to travel to Melbourne.

Speedy travel from west to east

Another feature of change days is how quickly the temperature drops.

A cold front can pick up quite a bit of speed as it travels eastwards, triggering thunderstorms as it goes.

The mass of cold air behind a cold front is denser, so has higher pressure than the warmer air in its path. This pressure difference pushes the cold front along.

It's the same process that gives rise to sea breezes, Mr Delamotte said.

Warm inland temperatures, and associated low air pressure, means cool air travels from the relatively higher air pressure over water.

Without mountain ranges in western Victoria to stop or slow them down, the cold front and thunderstorms blast across the state, usually striking in the afternoon around the hottest part of the day.

It's sometimes hot enough for thunderstorms to develop ahead of the change, but more often than not, they accompany the cold front and subsequent rapid temperature drop.

The shape of the Victorian coast, too, influences Melbourne's storms, Mr Delamotte said.

After rounding Cape Otway, cold fronts can push up towards Melbourne, gathering speed and giving thunderstorm development an extra boost.

Timing is everything

Unsurprisingly, timing when these sudden cold fronts will hit Melbourne is no easy feat.

So the next time you find yourself cursing the forecast while running for cover as a storm appears out of the blue, maybe cut meteorologists a bit of slack.

To help predict the weather, the Bureau of Meteorology relies on a few different computer models, but they don't always come up with the same forecast.

Change days' unpredictable nature can also have a considerable effect on the maximum temperature of the day.

"If it arrives a couple of hours later, that could mean the maximum temperature is a lot higher," Mr Delamotte said.

"If it arrives early, it could mean the maximum temperature is a lot lower."