During winter, in May, June, July, and August, there is a well-marked area of high pressure centreing in the Western Plains of New South Wales, with the axis running approximately from Brisbane to Perth. The low-pressure isobars of 29.90 run parallel to this direction, through Port Darwin, in the north, and through Tasmania, in the south.
With the spring the monsoonal bulge from the north-west begins to move in and split the high pressure region into two—one area lying over Perth and the other over Brisbane.
In November and December the cyclonic area dominates north and west Australia, gradually concentrating over the intensely hot region around Pilbarra, Western Australia.
Local Variation in Pressure.
Full data for the march of the isobars are avaliable for 1910 only, so that the following brief remarks refer to that year. They amplify the foregoing account, for very many more stations, giving very characteristic features to the isobars, are here used.
There is considerable resemblance between the direction of the isotherms and isobars, both being chiefly controlled by latitude. In January, 1910, there was a low-pressure area (29.6) situated just south of the Gulf of Carpentaria (within the Queensland hot loop), and another just off the Pearling Coast (29.6), The remaining isobars ran east and west with a definite tendency to follow the double curve of the south coast, and the maximum (30.0) appears in the extreme south of Tasmania.
In February the isobars were moving north as the continent cooled. The two northern loops were well marked over Nullagine (Western Australia), and Camooweal (Queensland) respectively, but the 30.0″ isobars formed a closed area over the Kosciusko cold loop. In March the isobar 29.7 followed the whole north coast, while 30.05 defined the south coast, bending north, however, over the Kosciusko Alpine area.
As the winter approached the high pressures tended to centre themselves around Kosciusko, the lower isobars forming around that elevated area. This was especially true in May and June. Then the high-pressure area spreads westward right across the continent, in July (midwinter) forming a broad belt between the tropic and the Bight with an average over 30.0 inches.
In August, as the centre of the continent warmed the high pressure retreated on Kosciusko from the west and north, and in September the low pressures (29.85, &c.) invaded Australia by the two hinterland loops of Pilbara and Carpentaria. In October the isobars had much the same distribution as in April, though the gradient was steeper in the south-east, over Tasmania.
In November the summer conditions began to recur, a low-pressure area (29.7) forming over the Pilbara area, and the Carpentaria loop running down into Queensland. The highest pressures were now on the New South Wales Coast, being pushed seawards as the highlands warmed under the southern sun. The monsoonal condition of Australia was very strikingly shown n the isobars of December, 1910, when the hot region (from the Pearling Coast to Camooweal, Queensland) was occupied by a belt of low pressure (29.65) around which isobars were arranged, the highest pressures being along the south-west coast from Leeuwin to Eucla.
