The Climate and Weather of Australia/Chapter 3
III.—GENERAL VARIATION IN PRESSURE.
At only a few important stations in each State are complete barometric data available for any length of time. These have unfortunately been taken out for differing times of the day.
The twelve monthly charts herewith are therefore only approximations, the data being as follows:—
| State. | Number of Stations. | Year's Record. | Time of Observations. |
|---|---|---|---|
| Western Australia | 34 | All over 12 years, except Balladonia (9), Wiluna (9), Cape Naturaliste (6) and Winning Pool (5) | 9 a.m. and 3 p.m. |
| South Australia | 6 | All over 12 years | 9 a.m. and 3 p.m. |
| Queensland | 10 | All over 10 years, except Boulia (9), Normanton (8), Cunnamulla (7), and Cooktown (9) | 9 a.m. |
| New South Wales | 12 | All over 15 years | 9 a.m. |
| Victoria | 10 | All over 15 years | 9 a.m., 3 p.m., 9 p.m. (reduced to hourly means) |
| Tasmania | 1 | Hobart, 40 years | 9 a.m. and 3 |
The general features of the average monthly isobars as shown on the charts will be briefly discussed; but in these charts the curves are necessarily much smoothed, and many interesting constant local characters can be better investigated from a much more detailed series prepared for one year. (This is done for 1910 in later paragraphs.) Some of the charts for the year are given in figures 29-31.
The annual fluctuations of mean barometric pressure for the State capital cities is given in the annexed graph (Fig. 28).
In January (midsummer) a depression of a monsoonal nature occupies North-west Australia. The Antarctic belt of lows reaches to the South Coast of Tasmania. The warm high land in the south-east appears to give rise to a col of low pressure separating anticyclonic foci over the Bight and over the North Tasman Sea. In February, the conditions remain much the same. In March, the northern low has moved off the continent to the north-west and the anticyclones have strengthened considerably over the two foci. In April a closed high is forming over the south-east of the continent, and elsewhere the isobars run across Australia from west to east.
MEAN MONTHLY ISOBARS.
MEAN MONTHLY ISOBARS.
GRAPHS SHEWING ANNUAL FLUCTUATIONS OF MEAN BAROMETRIC PRESSURE FOR THE STATE CAPITAL CITIES.
Explanation of the Graphs of Barometric Pressure.—On the above graphs the lines representing the yearly fluctuation of barometric pressure at the State capital cities are means for long periods, and are plotted from the Climatological Tables given hereinafter. The pressures are shewn in inches on about 2⅛ times the natural scale, and the corresponding pressures in centimetres are also shewn in the two inner columns, in which each division represents one millimetre.
Interpretation of the Barometric Graphs.—Taking the Brisbane graph for purposes of illustration, it will be seen that the mean pressure on 1st January is about 29.88 inches, and there are maxima in the middle of May and August of about 30.10 and 30.08 respectively. The double maxima appear clearly on each graph.
Fig. 28.
MEAN PRESSURE, APRIL, 1910.
MEAN PRESSURE, JULY, 1910—A TYPE OF MIDWINTER DISTRIBUTION.
MEAN PRESSURE, DECEMBER, 1910—A TYPE OF MIDSUMMER DISTRIBUTION.
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.
The Paths of the Highs and Lows.
Here also data are very incomplete except for 1910, but the main features can be deduced from a study of the tracks of the disturbances in that year.
Australian weather is controlled by three belts of atmospheric eddies. In the north moving (generally) from west to east, along the Tropic of Capricorn, is a procession of low pressure systems which are usually termed monsoonal lows. (The term "Tropical" might be less ambiguous, for in winter, at any rate, there is little akin to monsoonal conditions in Australian low-pressure areas.)
South of latitude 40° is another series of cyclonic eddies probably secondaries strung along the great low-pressure belt of the Southern Ocean. These are called antarctic cyclones. Between the two lies the belt of anticyclones whose path, as we shall see, swings between latitude 30° and 42°, as the sun moves south and back again.
The general tracks of the disturbances are shown on the diagrams, Figs. 32—35 (for 1910), when three facts may be noted in their characteristics. The tropical belt is much more irregular, the paths being often recurved. This series is never well developed in winter, the months of April, May, June, July, and August being generally free from these disturbances.
| Month. | Anticyclones. | Tropicals. | Antarctics. | |||
| No. | Remarks. | No. | Remarks. | No. | Remarks. | |
| January | 5 | 5 | Knot south of Hall's Creek (2 on east coast) | 4 | ||
| February | 4 | 6 | Two tropical storms | 4 | ||
| March | 6 | Of considerable average intensity | 6 | Two of storm intensity in Queensland | 4 | Of slight intensity |
| April | 6 | 3 | Off north coast (none on land) | 8 | Only two noteworthy | |
| May | 6 | Greatest intensity in east, and tendency to slow down there also | 1 | Over north Tasman Sea | 6 | Including two of north-west origin |
| June | 4 | (Two intense. 30.5) | 3 | Slight intensity | 6 | Half of cyclone energy |
| July | 7 | Moderate. Tended to have axis north and south | 2 | Very slight | 5 | Large, slow, stormy off south-west |
| August | 8 | More energy than July (7 normal track) | 1 | Low intensity | 7 | Less intensity; rather quicker |
| September | 5 | (Three, 30.5) | 6 | (Two Pilbarra), (four Carpentaria) | 5 | (Two, vigourous) |
| October | 4 | (Two of considerable intensity) | 5 | Of slight intensity | 4 | Of marked energy |
| November | 6 | Generally slight, diverged to east | 4 | Very persistent. Much thunder | 7 | Quick movement |
| December | 8 | Of feeble intensity, except the last | 8 | Numerous but sluggish | 8 | Numerous. More rapid |
AVERAGE MONTHLY PATHS OF ANTICYCLONES, JANUARY—JUNE, 1910.
JULY—DECEMBER, 1910
TRACKS OF LOWS, JANUARY—JUNE, 1910.
JULY—DECEMBER, 1910.
This table shows (as might be expected) that the anticyclones are most numerous in winter (July and August), and least numerous in midsummer. The tropical depressions are generally unimportant in winter (May, June, July, and August), and increase in number as the sun moves south to a position over the continent. The Antarctic disturbances are somewhat less in evidence at Midsummer. At this period the southern low-pressure belt has moved southward and its accompanying eddies (the Antarctic V's or ows) do not so often influence Australian weather.
Speed of the Disturbances.
Sufficient data are not yet available for very accurate determinations of the speed of the disturbances. But one or two points of interest are obvious in any set of eddy charts.
The tropical lows show a marked tendency to "hang" about certain localities, such as the Pilbara heated area or the region south of the Gulf of Carpentaria. Examples of "hesitation" over Pilbara occur in the months of January, November, and December, and over Carpentaria in September, November, and December.
The velocity of the highs and lows across Southern Australia is also often very irregular. Their rate across the Australian Bight is greater than that they exhibit over South-western Australia, or over the Bass Straits area. Probably the frictional resistance is less over the water which in the Bight is thrust well northward across their path, than over the highlands in South-eastern Australia and Tasmania. But these velocities vary with the months. Thus in June, 1910, the lows off the Leeuwin averaged about 360 miles per day; over the Bight about 550 miles, and over Tasmania about 600 miles per day. In November of the same year the velocity of the lows showed an average of 830 miles across the Bight, and 500 to the south of Tasmania.
It must also be noted that it is difficult to fix the position and velocity of these southern eddies. Their centres lie over water areas often and the positions can only be deduced approximately from the form of their northern isobars.