Australia Records Highest Ever Air Pressure in Tasmania

Preliminary data shows the highest ever air pressure in Australia was recorded in Tasmania this morning, and not from expats watching the World Cup.

At 9:50am, the weather station in the small town of Ouse, 67 kilometres north-west of Hobart, recorded an air pressure of 1044.5 hectopascals — exceeding the previous national record of 1,044.3hPa at Launceston on June 7, 1967.

The record is due to a very intense high-pressure system centred directly over the state, a system which is currently controlling the weather over most of the country.

Highs form when air is travelling in a downward motion towards the ground, which causes compression of the atmosphere at the surface and an increase in pressure.

What stands out with the current high, is its slow movement and the time of year.

After tracking through the Bight on Saturday, the high slowed down as it approached Tasmania on Sunday, and has now completely stalled.

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The lack of movement is allowing the air to keep piling up over the one location, a major reason the pressure at the surface has risen so abnormally high.

The timing in the middle of winter has also contributed, with temperatures dropping well below freezing on Monday morning over inland Tasmania, and since cold air is dense, this automatically would have added a hectopascal or two to the readings.

High to bring icy nights and low sea level

The most obvious change in the weather under such an intense high is bitterly cold nights.

High pressure systems are associated with clear skies and light winds, which allows heat to radiate away from the surface very efficiently during the night.

The highland station at Liawenee in central Tasmania recorded the lowest minimum on Monday of -8.1 degrees Celsius, while Ouse dipped to -5.5C.

Hobart's official station is too close to the Derwent River to regularly drop below freezing, but the nearby inland stations of Bushy Park and Grove dropped to -3.0C and -2.3C.

The high is also leading to widespread frost over Victoria and southern New South Wales.

Coldstream on the outskirts of Melbourne woke to an icy -2.7C while the Tullamarine Airport dipped to 0.1C.

With the high remaining stalled during the first half of the week, even colder minimums are possible during the next few nights.

A less noticeable change is the higher pressure literally pushes the sea level down, with an increase of 1hPa responsible for around a 1 centimetre drop in the water level.

This is the reverse of what occurs during a tropical cyclone when exceptionally low pressure causes a storm surge and coastal flooding.

With the pressure around 20 to 30hPa above normal, the water level across south-east Australia is currently around 20 to 30cm below levels during a regular weather pattern.

Pressure to keep on rising in a warmer world

The current system should finally head off the east coast of Tasmania on Friday, however there is clear longer-term trend towards high pressure systems dominating our weather charts.

The shift is a well-established impact from climate change and is partly due to an expansion of the tropics.

Air rises near the equator due to solar heating, then spreads laterally north and south until it cools and sinks over the subtropics or mid latitudes — which is what causes highs to regularly form and drift over our region.

But with this circulation from the tropics to subtropics, called the Hadley Cell, now supercharged and expanding in a warmer world, the sinking column of air is both stronger, and also occurring further south.

A comparable event brought similarly high readings near 1044hPa just two years ago and data from the BOM since 1950 shows a clear upward trend in both the number and intensity of highs in our region.