Photograph by Raplh Clarke, 1989, entitled 'Heather, Mountains, and Mist' (AOT PH30/1/9229)

The climate of Tasmania has been a major influence on the evolution of its flora and fauna and has played a key role in human migration to the island. The first Tasmanians arrived during the last Glacial Maximum at a time of low global sea level when the Bass land bridge to the Australian mainland was exposed. Discovery by Europeans was achieved by making use of the prevailing winds in the southern hemisphere.

The southern hemisphere is dominated by ocean, and the western and southern shores of Tasmania are steeped in the cold waters adjacent to the Southern Ocean, while the north-east and east coasts experience the warmer waters of the Tasman Sea. Under this oceanic influence Tasmania's climate is classified as temperate maritime, and coastal districts experience the modifying effect of the ocean throughout the seasons. Further inland there is evidence of a limited continental effect and overnight frosts are common from autumn to spring, particularly in elevated localities.

Although the prevailing wind direction is westerly, there is a marked variation in the strength and latitudinal extent of the westerly winds that affect Tasmania throughout the year in response to the annual cycle of mean atmospheric pressure. The pressure distribution is dominated by the migration of the sub-tropical ridge of high pressure northwards over mainland Australia in winter and southwards in summer. In high southern latitudes lower pressures occur in autumn and spring, with the minimum in spring.

The belt of westerly winds is established over Tasmania in winter but the mean wind speeds are strongest in spring. In winter, an increased frequency of blocking anticyclones over the Tasman Sea creates a split in the mean westerly wind pattern near Tasmania. In summer, the sub-tropical ridge reaches its maximum southern extent, the westerlies are not so dominant and coastal sea breezes are common during the afternoons. Nevertheless, cold fronts cross Tasmania intermittently between individual high pressure cells. These fronts are preceded by warm northerly winds which may originate over inland Australia and be followed by cooler south-west or southerly winds of oceanic origin.

The distribution of rainfall is a response to the direction and strength of the wind and its interaction with the topography. Annual means exceed 2000 mm over much of the exposed and elevated western half with a peak of about 3200 mm but range from about 500 mm to 700 mm throughout the midlands and in the drier parts of the south-east and east. There is a secondary maximum of 1200 mm on the north-east highlands. Seasonal peaks in winter and early spring are particularly evident in the western half.

Tasmania experiences large variations in annual and seasonal rainfall from year to year and severe droughts can occur, particularly in the agricultural districts of the north, east and south-east and also on the Bass Strait islands where the influence of the El NiñoľSouthern Oscillation phenomenon is greatest. Summer maximum temperatures generally range between 17° and 23°C, and from about 8° to 14°C in winter. Year-to-year variability in mean temperatures is not great but temperatures near 40°C can occur in the east on individual summer days.

Further reading: Bureau of Meteorology, Climate of Tasmania, Canberra, 1993; J Langford, 'Weather and Climate', in J Davies (ed), Atlas of Tasmania, Hobart, 1965; D Shepherd, 'Some characteristics of Tasmanian rainfall', Australian Meteorological Magazine 44, 1995; A Sturman & N Tapper, The weather and climate of Australia and New Zealand, Melbourne, 1996.

Mike Pook