The northern and southern parts of Western Australia have entirely contrasting climates; the north is tropical, with summer rainfall, while the south has a Mediterranean climate. The major determinant of the weather is the movement of an anticyclone that produces winds in an east-west direction across the continent for about half the year. In winter this system moves to the north and is responsible for clear skies, sunny days, and easterly winds in the tropics. To the south of the anticyclonic system, westerly winds and a procession of cold fronts associated with the “roaring forties” (windy zone between latitudes 40° and 50° S) bring cool, cloudy weather and rain and westerly gales along the southern coast. The anticyclonic belt has moved so far south by the summer that its axis is off the southern coast. Easterly winds prevail over most of the state, but in the far north a depression develops, bringing westerly monsoon (wet-dry) wind patterns to the coastal districts northeast of Onslow and to parts of the Kimberley.

Several tropical cyclones (known elsewhere as hurricanes or typhoons) develop offshore during the northern wet season, which lasts from about December to March. They frequently move inland between Broome and Onslow, although occasionally they have traveled south of Perth before curving inland. Tropical cyclones can be highly destructive, but they are also beneficial, bringing widespread rain to otherwise parched inland areas.

The highest annual precipitation occurs in the extreme north, on the Mitchell Plateau in the Kimberley, and in the extreme southwest, between Pemberton and Walpole in the karri (Eucalyptus diversicolor) country. In both locations mean annual rainfall is more than 55 inches (1,400 mm). Precipitation decreases south and north from both locations and with increasing distance inland from the coast. The driest areas receive less than 8 inches (200 mm) annually, and possibly less than 6 inches (150 mm).

The hottest months are November in the Kimberley, December a little farther south, and January/February in the rest of the state. July is the coldest month. Wyndham is the most consistently hot place, with a mean maximum monthly temperature of about 96 °F (35.6 °C) throughout the year. Marble Bar has the highest seasonal mean maximum monthly temperatures in Australia, registering near 100 °F (38 °C) from November through March. In the southwest, sea breezes (known in Perth as the “Fremantle Doctor”) blow most afternoons during the hot months and bring relief from the high temperatures near the populated coast.

In winter, temperatures may fall below freezing over most of the inland part of the state south of the Tropic of Capricorn; on occasion, the temperature has dropped to the low 20s F (about −6 °C) in the southwestern part of the state. Frosts may be widespread over the southern part of the state and occasionally extend into the tropical zone but are not generally troublesome. They are most frequent in July and August. Snow is rare, and only in southern areas, especially in the Stirling Range, does it occasionally lie on the ground for a few hours.

Plant and animal life

Western Australia supports a wealth of flora and fauna, especially in the southwestern region—which is considered a particularly fertile “hot spot” of biodiversity. More than 10,000 species of vascular plants have been documented in the state, and some one-third of these, including many carnivorous varieties, are endemic to the area.

The Kimberley region is sparsely wooded, primarily with eucalypts, but also with the distinctive, moisture-storing boab (which has close affinities to the Indian and African baobabs). Spinifex grass is ubiquitous, as is generally the case throughout the portion of the state that lies north of the Tropic of Capricorn. The deserts of the state’s central and eastern regions are partly vegetated by spinifex and various eucalypts; some mulga trees grow in the swales between the dunes. In the Pilbara area mulga and acacia shrublands are interspersed with eucalypt woodlands and spinifex grasslands. In the southeast the Nullarbor Plain is textured with bluebush and saltbush shrubs, acacias, eucalypts, and mallee scrubland.

The state’s only true forests are found in the Yilgarn block. These consist of eucalypts, and there is an extremely rich understory. The dominant trees are jarrah (E. marginata), marri (E. calophylla), and the spectacular, tall (up to 275 feet [85 metres]) karri. These forests are protected in vast state reserves and in surface-water catchments.

Western Australia is host to some 150 species of mammals and several hundred species of birds and reptiles. Common marsupials include kangaroos, wallabies, wombats, possums, and bandicoots. Some three dozen species each of bats and small rodents also inhabit the state’s diverse landscape. Dugongs, dolphins, and whales are found in coastal waters. Many of the offshore islands have seal and penguin populations. Waterbirds, including ducks, plovers, terns, egrets, herons, and others, are abundant in the wetlands of the southwest. Eagles and other raptors are prominent inland, as are cuckoos, nearly two dozen types of parrots, and a spectrum of smaller birds such as finches, wrens, honeyeaters, and flycatchers. Skinks and snakes, each represented by well in excess of 100 species, are the most plentiful of Western Australia’s reptiles. Dozens of species of geckoes, other lizards, and amphibians have also been recorded. Freshwater and marine turtles live in coastal and inland waters.

Clearing (mostly for agriculture), the introduction of predators such as foxes and feral cats, and competition for food and degradation of habitat by domestic sheep, goats and cattle, camels, donkeys, horses, pigs, and feral rabbits have had severe effects on the state’s fauna and flora. Hundreds of species of plants are rare or threatened, and many have become extinct. More than half of the medium-sized mammals have disappeared from the Wheatbelt since European settlement, and many of their populations have also disappeared or declined drastically from the arid pastoral lands and from the central, sandy deserts. The Department of Environment and Conservation, established in 2006, has been charged with implementing government policies to aid in preserving the state’s biodiversity.