Climate and Weather
The climate of South Australia has frequently been described as a 'Mediterranean' type with mild wet winters and hot dry summers. In fact, this description can be applied only to the southern settled districts which are subject to seasonal shifts in weather influences deriving from the continental interior to the north and the Southern Ocean to the south.
The dominant weather features are the eastward moving high and low pressure systems and the seasonally varying northerly continental and tropical influences. Although there is popular recognition of four seasons, the principal seasonal contrasts are warm-to-hot, dry summers and cool-to-mild winters. The highest rainfall occurs along the southern coasts and the Mount Lofty Ranges (with an average annual rainfall of 1200 mm in the vicinity of Mount Lofty); the lowest rainfall occurs in the region of Lake Eyre where the average annual totals are less than 150mm.
Most of the rain in the southern districts of the State falls during the winter months when the sub-tropical high-pressure belt is displaced to the north over the Australian continent. Migratory lower-pressure systems are thus able to extend further to the north, allowing strong cold-frontal activity to penetrate across the southern settle areas. A two-to-three-day cycle of easterly moving high and low pressure systems is a familiar weather pattern in southern Australia.
The two examples of surface pressure distributions over Australia during winter are for 2 July 1984 and 8 June 1982. On the synoptic map for 2 July 1984, a strong cold front allied with a deep cyclonic depression south of Tasmania had passed through South Australia. On this day, rain had fallen throughout southern coastal regions with strong winds, low temperatures and 'stream' shower activity in southern areas behind the front. This particularly strong southwesterly airstream from sub-Antarctic regions penetrated well inland and was responsible for falls of snow in the Mount Lofty and Flinders ranges. To the west, an anti-cyclone moving into the Great Australian Bight slowly influenced South Australia and moderated conditions.
This type of pattern usually occurs when an upper-level long-wave trough is dominating weather conditions over southeastern Australia and the movement of cyclonic depressions and fronts from the south-south-west into South Australia can persist and extend a spell of wet and cold winter conditions. Cold days, with a maximum temperature of less than 12°C, may follow, particularly in the ranges (map showing average number of cold days per year). Low temperatures can also develop if a persistent southeasterly flow and cloudy but dry conditions dominate.
The synoptic situation of 8 June 1982 was some what different as the anticyclonic cell appears to have blocked the westerly movement of systems in the region of Tasmania. Cyclonic activity and a strong frontal passage were evident over Western Australia and produced heavy rainfall, but in South Australia, a northeasterly airstream off the continent produced clear skies and warm mid-winter days. Nevertheless, such conditions at night had obviously led to below freezing temperatures (minimum of -0.4°C at Adelaide) and frosts. If the high continued to block, any depressions and fronts approaching from the west would be likely to be deflected to the south-east away from the State. The persistence of such conditions produced the drought that existed throughout the winter and spring of 1982.
During the summer months of December to March, the subtropical high-pressure belt is displaced to the south of the continent, and frontal activity results in northerly to south to south-east wind changes, or summer 'cool changes', with little or no rain. Summer rainfall is generally low over the State. Occasionally, however, unstable 'moist infeed' at middle to lower levels of the atmosphere from the north-east or north-west, associated with tropical weather systems over northern Australia, can produce thunderstorms and heavy rainfall in the north of the State. Very heavy summer falls in the north are often caused by incursions of the summer monsoon trough and/or rain depressions which are generally the remnants of tropical cyclones. More locally, the orographic influence of the Flinders Ranges can be decisive in some of these situations.
The synoptic chart for 9 February 1984 illustrates the type of surface pressure distribution and rainfall pattern associated with warm-to-hot summer conditions over the State. A high situated to the south of the Great Australian Bight was directing southeasterly flow over South Australia. On that day, Adelaide temperatures ranged from a minimum of 15.4°C to a maximum of 28.0°C. Inland areas in the north experienced higher daily temperature because of the warming of the air mass on its longer passage over land.
In the southern parts of the state, periods of high temperatures occur during summer when the high pressure cell is displaced further to the east and airflow is from a northerly quarter. If such a high pressure cell is very slow-moving or becomes quasi-stationary over eastern Australia, its 'blocking' action can interrupt the eastwards movement of weather systems or deflect them on tracks further to the south. The persistence of hot, dry northerly winds then promotes hot spells which can lead to very high bushfire danger. The table of heatwaves in Adelaide illustrates the number of consecutive days of 'hot spells'. There have been only a few runs of days above 40°C in the ninety-seven years of recorded weather.
The map showing the average number of hot days with daily maximum temperatures above 30°C indicates the varying extent of continental influence. In the north, long overland trajectories in every direction ensure a predominance of hot days with clear skies and high evaporation rates. The average annual evaporation rate shown on the evaporation map for a Class A pan exceeds the average annual rainfall rate by about 3650 mm in the far north-east of the State and 200 mm in the south.
Because of Adelaide's coastal location, it has a predominantly temperate climate. During summer, the moderating influence of land-breezes and sea-breezes is an important factor. Such conditions are reflected in the average daily maximum and minimum temperatures and the bar graph of average monthly rainfall for the city. Average daily maximum temperatures range from a peak of 30°C in January to a low of 15°C in July; average daily minimum temperatures range from a peak of 17°C in February to a low of 7°C in July.
Most of Adelaide's rainfall occurs from April to October. This is the period when rainfall is vital for most crop and pasture development in the southern regions. The map of average annual rainfall in the vicinity of Adelaide shows that, despite its relatively low elevation, the barrier of the Mount Lofty Ranges produces a marked west to east rainfall gradient of 450-650 mm between the Adelaide Plains and the Mount Lofty Ranges.
Adelaide has one of the longest continuous rainfall records of any capital city in Australia. The bar graph of each year's April-to-October rainfall from 1839-1984 depicts marked fluctuations during the agricultural growing season over those 145 years. No distinct long-term trends are evident. Debate continues in scientific circles about the analysis and interpretation of trends in long-term climatic records.