In the annual mean pressure distribution, a high pressure zone (above 772 mm.) With the direct axis from Cape Leeuwin to Brisbane covers the southern part of Australia. On the equatorial side of the zone the pressure decreases regularly towards the north-west up to 754 mm.; in most of this northern region the southeast trade winds blow. To the south of the high pressure zone, this lowers rapidly until, near the Antarctic lands, the lowest terrestrial pressures are reached: from 30 ° to 70 ° S. there is, therefore, the steepest known pressure gradient.
During the year, the high pressure center moves north and south (with the sun) for a distance of 10 ° latitude; so in July the anticyclone moved east from Perth to Brisbane; in January it is pushed back far to the south and reaches above Melbourne. Since the high pressure areas are dry regions, and the low pressure ones are humid, it follows that the humid and dry periods are related to this path of the anticyclonal area. In the summer, the northern region is subjected to the cyclones (low pressure) of the monsoon, which bring storm rains to the coasts and inland; in winter they go much further north and no longer touch the continent, but this is reached by the low pressure Antarctic zone which, moving from the polar regions, brings rain to the southern coasts. The most notable phenomenon in the distribution of pressure, however, is given by the semi-permanent nature of a part of the summer cyclones which seem to be “anchored” on north-western Australia, and whose thermal effects have already been noted. Migrant cyclones form from this low-pressure area that push east or south-east over Central Australia, but don’t always bring rain there. For Australia geography, please check franciscogardening.com.
The northern half of Australia is subject to trade winds which are also subject to seasonal sun shifts. The northern edge of the trade wind zone in fact moves from the 5th S. (April-October) to the 12 °, along the east coast, and up to the 22 ° on the west (November-March), due to the presence, on this last, of the mentioned semi-permanent low pressure area. The southern margin moves, in the same periods, from 22 ° to 28 ° S., but reaches the latitude of Perth (32 °) to the west in the middle of summer; the trade winds blow from the south-east and during the summer it seems that they are simply pushed back up by the monsoon, because in Java they dominate at an altitude of 1500 meters, even when the monsoon blows over the lowlands.
In winter the southern seas are invaded (with the low pressure Antarctic zone) by strong westerly winds (Roaring Forties) much less constant than the trade winds; these same winds over part of Tasmania are persistent. Although Australia is only a small continent, a monsoon-like phenomenon clearly shows up in the southeastern highlands. Thus in Melbourne, in winter, a cold wind from the north prevails which blows from the frozen continent and appears in Sydney as a western wind. In the summer, in Melbourne the most frequent wind blows from midday towards the heated highlands, and in Sydney, for the same reason, a north-east wind blows.
The whirlwinds are not very frequent in Australia, but there are several examples for Eastern Australia, where they occur mostly during thunderstorms following heavy rains and on hot days. Hurricanes rage along the tropical coasts between 10 ° and 30 ° S. On the western side they are numerous, especially in January, but frequent until the end of April; on the eastern side, the worst months are February and March: hurricanes are limited to the hottest months and always associated with exceptionally strong tropical cyclones. Their route is almost always parabolic, going from the north-east to the south and then to the south-east. Among the most disastrous, which caused considerable victims and damage, are the following: Wallal (Western Australia), April 1887 and 1888; Onslow (Western Australia), December 1897; La Grange (Western Australia), April 1908; Cossack (Western Australia), April 1898 and March 1918. On the east coast of Queensland, particularly strong hurricanes occurred in January 1896, March 1899, March 1911, December 1916, January-March 1918.
