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AUS: Cold snap intensifies across the continent

Exquisite needles of hoar frost decorate a barbed-wire fence (top), while frost and hoar frost coat trees to their tops along a roadside near Wangaratta VIC (above). A normal frost is made up of frozen dew or deposits of white ice crystals which form provided the air is saturated and is at or below zero degrees. Ground frosts are common, even when the air temperature a metre or so above ground level is above freezing point, because in the calm conditions that favour frost formation there is usually a steep temperature drop -- as much as 5 to 7C -- in the metre above the ground. Hoar frosts differ from normal frosts in that they are formed by droplets of supercooled water vapour that freeze on contact with frozen surfaces, such as the barbed wire above or ice crystals already attached to objects. This produces delicate needles, fans and other crystal structures. In the roadside photo above, the vapour is very evident as a supercooled fog, and sub-zero temperatures and frost extend to the tops of the trees. Frost formed by supercooled droplets of fog coming into contact with frozen objects is often referred to as rime; the main difference is that rime formation assumes that wind is blowing constantly past the objects, leading to a build-up of rough ice crystals on the windward side of the objects. In this case, while fog has enhanced the frost build up, there has been no wind. Photos: Rob Alexander

The strong 1035hPa high over SE AUS intensified its frosty grip this morning. The temperature anomaly map below shows that most of the continent other than NE QLD and southern TAS experienced below to well-below normal overnight lows, but the actual minimum temperature map shows the extraordinary spread of sub-zero temperatures.

Particularly cold pockets developed in NE VIC, in NSW from the Riverina to the Central Tablelands, and in far-flung parts of SA and WA. Records toppled in these areas (full list below), some notable ones being:

• In NE VIC, Benalla (49 year history), Rutherglen (42 years) and Wangaratta (20 years) all registered their coldest temperatures on record for any month. Rutherglen's -7.5 knocked 0.9 off the previous all-time record.
• Just over the border in NSW, Corowa's -5.0 was also its coldest temperature in 34 years of observations.
• Minus 3.0 at Parkes was its coldest June reading in half a century
• In SA, though it was exceptionally cold in the Murray Valley and Central North (Yongala recorded -7.2), records fell at coastal locations. Elliston, on the western coast of the Eyre Peninsula, dropped to -0.8, its first sub-zero June temperature in 45 years, while Kingscote's -2.0 on Kangaroo Island was a new all-time low in 12 years of record. Adelaide's low of 1.3 was the cities coldest temperature since 1989.
• Six centres in the SE quarter of WA set new records. Eyre, Forrest and Carnegie, all with relatively short histories, set new all-time records, with Eyre's screen minimum air temperature a very low -5.9 and the grass minimum -7.0. Balladonia and Salmon Gums, with 40 and 36 year histories, had their lowest June minimum on record.
• The cold night extended into the tropics in WA, with Karatha and Paraburdoo reporting their lowest temperatures in over 10 years of operation.
• Fog continued through all or most of the day in parts of NE VIC and the NSW Southern Tablelands, producing some unusually low maximum temperatures. At Corryong, the mercury only made it to 2.0C, 1.1 below the town's previous coldest day on 9 July 1995 in 33 years of record.

The Goulburn-Murray area of NE VIC is no stranger to winter frost, but 23 of the past 25 mornings have produced sub-zero temperatures at Wangaratta. This morning's low of -7.2 followed minima of -5.6 yesterday and -6.6 the morning before, producing unaccustomed domestic experiences for the area's inhabitants. Household water pipes have frozen and in some cases burst, one resident telling ABC Rural "most people would not think that brass taps would explode with the frost. We've had five brass taps fittings on taps break apart because they can't handle the stress of the frost." Irrigation channels, ponds and dams have frozen with a cap of ice too thick to be penetrated by thrown rocks. One dog owner, whose dog has a regular dip in a water trough on her daily run said "she jumps in every morning no matter how cold it is. A couple of mornings this week she's broken the ice as she's jumped in. This morning she's skated across, hit the lip on the other end and cart wheeled down the paddock. It was so frozen that even a dog jumping in couldn't break the ice." Drivers passing through supercooled fog have reported rime ice buildup on antennas and external mirrors up to 1.5cm thick. Black ice and ice build-up on vehicles due to fog have caused numerous minor accidents in NE VIC, Central Western NSW and around Canberra, with the Great Western Highway between Blackheath and Bathurst and local roads around Lithgow badly hit.

In far northern WA, the unusually cold conditions are feared to have killed 660,000 prawn fingerlings at one aquaculture company. Water temperature has dropped to 14.5C, and marine prawns stop feeding at 20C and die at 13C.

QLD: Strong trade winds on QLD coast. The 1035hPa high has also produced a strong pressure gradient along the QLD coast, the tightly crowded isobars on the surface charts below indicating strong onshore southeasterly trade winds. Weipa recorded its highest wind gust for June in 23 years today (70km/h) while its 24-hour average speed of 25.4km/h to 9am Thursday was also a record. Lockhart River's gust of 67km/h was its strongest for June in 26 years. Cape Flattery in the far north and Double Island Point in the south both reported gusts of 85km/h, while 25 to 50mm of rain was recorded on the Tropical North Coast.

TAS: Strong westerlies in the south. On the other side of the high, strong westerlies have been blowing across southern TAS. Maatsuyker Island, off the South Coast, reported 24-hour average wind speeds of 71.9km/h to 9 this morning and 81.2 to 9am Thursday with gusts to 117km/h late this evening.