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9.37 Climatology

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What does the word "climate" mean? Many people think of it as "what the weather is usually like". But climate is more than a summary of average conditions: it also includes information about the natural variability of the atmosphere and the likelihood of particular events. Our definition and example will make it clearer.

Like all countries in the southern hemisphere, Australia's four seasons follow the sequence:

  1. Summer: December to February
  2. Autumn: March to May
  3. Winter: June to August
  4. Spring: September to November

Even though the four 'official' calendar seasons have the same names as the northern hemisphere seasons, the weather during these seasons is very different to northern hemisphere weather patterns. Australia is generally a very dry place, so summers can get much hotter. The pattern of rainfall is also distinct - some places have abundant rain at one time of the year and almost none at other times.

All different kinds of weather

Because Australia is such a large country, its weather varies significantly in different parts of the continent. Living in Australia can involve everything from sunbathing on the beach in scorching summers to knocking snow off your boots after a day of skiing and from sweating out the humidity during the build-up at the beginning of the wet season, to wrapping up snug and waiting at the bus stop in the pouring rain.

In the north there are tropical regions with high temperatures and high humidity and distinct wet and dry seasons. In the centre of the country are dry, desert regions with high daytime temperatures and low amounts of rain. In the south are the temperate regions with their moderate rainfall and temperatures ranging from hot to cold.

The temperature in Australia changes with the seasons, but in general it ranges between highs of 50 degrees Celsius to lows of sub-zero temperatures. The lowest temperatures reached in Australia, however, are not comparable to the extreme lows experienced in other continents. This is partly because Australia lacks very high mountains and enjoys the presence of warming oceans around its coastal regions.

Australia's tropical regions

The tropical regions of Australia are in the north of the country. They include the central and northern parts of the Northern Territory and Queensland, and the northern parts of Western Australia. The weather in the Australian tropics has two very different seasons: the wet season and the dry season.

The wet season lasts about six months in summer and spring, between December and March. It is hotter than the dry season, with temperatures between 30 and 50 degrees Celsius. This is because of the high humidity during the wet, which is caused by large amounts of water in the air. During the wet there is a lot of rain, which frequently causes flooding.

The dry season lasts about six months in autumn and winter, usually between May and October. Temperatures are lower and the skies are generally clearer during the dry. The average temperature is around 20 degrees Celsius.

The 'build up' is the humid time of year between the wet and dry seasons. It usually lasts for three or four months. Things become quite tense during the 'build up' as people sit and swelter in the humidity while waiting and hoping for the first rains to come. The humidity continues day and night with no respite, so when the rains finally do come everyone enjoys their cooling relief.

Australia's dry regions

The driest regions of Australia are found mostly in central Australia, stretching from most of central and southern Western Australia, through the southern parts of the Northern Territory and most of South Australia, to the far west regions of Queensland and New South Wales, and the north-western parts of Victoria.

The dry and desert regions of Australia are characterised by intense heat during the day and intense cold at night. Temperatures range from around 40 degrees Celsius in the summer to between 16 and 24 degrees Celsius in the winter. At night the temperature can vary from 19 degrees Celsius to zero degrees Celsius. These areas receive little rainfall. Most of central Australia is normally in a state of drought.

Australia's temperate regions

The temperate areas of Australia are found on the south-eastern coast, reaching south from Tasmania through most of Victoria and New South Wales into the southern parts of Queensland. Temperate regions are also found in the southern most parts of South Australia and the south-western tip of Western Australia.

The weather in temperate Australia is quite changeable throughout the year, with an average temperature of around 30 degrees Celsius in the summer, and cool to cold winters with an average temperature of around 15 degrees Celsius. The summer frequently extends into periods of heat wave and drought, while the winters, while usually cold, wet and windy, are quite mild in comparison to winters in many European countries.

Snow is uncommon in temperate Australia, and unheard of in the dry and tropical regions, but along the Great Dividing Range, the mountain range that passes through New South Wales and Victoria, there are regular winter snowfalls.

Regions of Australia

Victoria Weather

The climate of Victoria is characterised by a range of different climate zones, from the hot, dry Mallee region of the northwest to the alpine snowfields in the northeast of Victoria. Median annual rainfall ranges from less than 250 mm in parts of the Mallee to in excess of 1800 mm over some of the mountainous regions.

The mountains of the Great Divide in Victoria attain a maximum height of 1986 metres at Mt Bogong near the town of Mt Beauty. There are several peaks in excess of 1500 metres in the northeast of Victoria. The Great Divide extends westwards almost to the South Australian border, with most peaks below 600 metres except in the mountainous area called the Grampians or Gariwerd, near Stawell, where Mt William's summit is 1167 metres.

To the west and north of the Great Divide the land flattens out to the dry inland plains. It is in the Mallee where the hottest temperatures in the State most commonly occur during summer, and where the annual median rainfall drops below 250 mm.

The coastal strip, south of the ranges, is generally wetter except in the far east where the Strezlecki Ranges shelter the East Gippsland District from the moisture-laden westerly winds. The climate changes across the State are reflected by marked changes in vegetation that ranges from mallee scrub in the northwest, through irrigated plains in the north and the wetter grazing lands of the south to the forested mountainous country of north-eastern Victoria.

New South Wales Weather

New South Wales is entirely in the temperate zone. The climate is generally mild, equable and mostly free from extremes of heat and cold, but very high temperatures occur in the northwest and very cold temperatures on the Southern Tablelands. The Great Dividing Range, running approximately north to south in the east of New South Wales, has a large impact on the climate, creating four distinct climate zones; the coastal strip, the highlands, the Western Slopes and the flatter country to the west.

The climate of the coastal strip is influenced by the warm waters of the Tasman Sea, which in general keep the region free from extremes of temperature and provide moisture to increase rainfall, the annual median of which ranges from about 750 mm in the south to 2000 mm in the north.

The mountains of the Great Divide attain a maximum height of 2228 metres at Mt. Kosciusko, and there are several peaks in excess of 1500 metres, extending up to northern NSW. Travelling from east to west across the range, the elevation abruptly increases away from the coastal plain, and then west of the divide it gradually descends onto the Western Plains. Consequently winter snowfalls are experienced over what are aptly called the Tableland regions.

On the Western Slopes the rainfall gradually decreases, together with the frequency of winter snowfalls. Average maximum temperatures gradually increase as height above sea level decreases.

Further to the west the land slowly flattens out to the dry inland plains, notable for cold nights. It is in the far northwest where the hottest temperatures in the State most commonly occur during summer, and where the annual mean rainfall drops below 200 mm.

The way in which the climate changes across the State is reflected by marked changes in vegetation, which ranges from the subtropical rainforests of the northeast to the fragile alpine heathlands in the Southern Alps, through the dry forests and undulating pasturelands of the midwest to the dry plains of the northwest.

Outdoor activities also vary significantly across the State. Mild winters along the North Coast favour beach activities whilst at the other extreme the snow fields of the southern Alps become a winter playground for skiers.

Perhaps the most basic need of human settlement is water; early Australian history has many examples of new settlements that foundered due to lack of adequate water supplies. Settlement to the west of the Great Dividing Range was made more difficult by a lack of a reliable water supply.

Settlement onto the open plains that flourished during years of good rainfall foundered during drought periods, In more recent times, irrigation schemes have been developed to harness some of the heavy rainfall and snowmelt from the Tablelands and control its release westwards.

Northern Territory Weather

The Northern Territory's climate is distinctly different from that of southern Australia, and varies greatly between the Territory's northern part, known as the 'Top End' and the southern extremities.

Physical features: Four-fifths of the Territory lies north of the Tropic of Capricorn. The country within about 150 km of the coast is mainly flat or undulating up to about 200 m elevation, with extensive coastal swamps or wetlands in some parts. The interior of the 'Top End' is dominated by the rocky Arnhem Land plateau.

To the southwest of the plateau lie the rugged hills of the southern Katherine region, while in the east the land generally rises more gently through the hilly country of the southern Roper-McArthur District to the grassy plains of the Barkly Tableland. These systems of hills divide the coastal river drainage systems from the broad but shallow inland basin, where streams are usually dry for most of the year. South again the land rises very gradually; western areas are dominated by sandy desert.

Toward central Australia, the land rises more steeply into a higher plateau and rocky ranges, where a number of peaks exceed 1500 m elevation. The plateau declines steeply toward the sand dunes of the Simpson Desert in the southeast whilst the Lake Amadeus trough separates it from the lower ranges of the far southwest.

Major circulation systems: Two major atmospheric pressure systems affect Territory latitudes: the subtropical ridge of high pressure cells (highs or anticyclones), and a broad tropical low pressure region called the monsoon trough. The subtropical highs move from west to east across southern Australia in winter, and further south in summer, usually separated by low pressure troughs or cold fronts.

The highs provide the driving force behind the southeast trade winds which dominate the Territory's weather in the winter months. The monsoon trough is a broad area of low atmospheric pressure running east-west through the tropics in the summer months. It follows the sun, shifting north and south between the hemispheres with the seasons. In the southern hemisphere it is the meeting place of the dry east to southeast winds generated by the subtropical highs, and the moisture-laden north-westerly monsoon winds.

During the summer it lies for lengthy periods over north Australia, and is the source of much rainfall. The northern and, to a lesser extent, central parts of the Territory experience two distinct seasons: the 'wet' (October to April) and the 'dry' (May to September).

The change between seasons is usually gradual, with transition months of October and November (often called the 'build-up') at the start of the wet, and April at the end. In central parts the contrast between wet and dry is not generally as marked as in the north.

The Alice Springs district is dry for much of the year, and has an erratic rainfall pattern, with a slight summer maximum. While zero rainfall can be experienced in all calendar months, significant totals are also possible in all months, but are more likely in summer.

Winters (June to August) are cool and summers (December to February) hot; the terms 'spring' and 'autumn' are not usually applied to the transition seasons in between. During the wet season, weather in the north is largely determined by the position of the monsoon trough, which can be in either an 'active' or an 'inactive' phase.

Typical wind systems at different times of the year:

The active phase is usually associated with broad areas of cloud and rain, with sustained moderate to fresh north-westerly winds on the north side of the trough. Widespread heavy rainfall can result if the trough is close to, or over land.

An inactive or 'break' period occurs when the monsoon trough temporarily weakens or retreats north of Australia; it is characterised by light winds, isolated shower and thunderstorm activity, sometimes with gusty squall lines.

A typical wet season consists of a prolonged inactive period during the build-up, followed by two or three active/inactive cycles, each full cycle lasting from about four to eight weeks. Inactive periods are usually longer than active ones.

Tropical cyclones can develop off the coast in the wet season, usually forming within an active monsoon trough. Heavy rain and high winds, sometimes of destructive strength, can be experienced along the coast within several hundred kilometres of the centre of a cyclone.

In the southern and central parts, weather is more variable from October to April than in the north. Sometimes decaying tropical cyclones or the monsoon trough move well south into the central regions, bringing widespread rain and thunderstorms. In general, though, east to south-easterly winds and fine conditions predominate. Temperatures can be scorching, and dust devils, whirling dust pillars raised by columns of rising hot air, are frequently seen.

During October to November bushfires are fairly common - usually ignited by lightning from dry, gusty thunderstorms. From May to September the prevailing south-easterlies bring predominantly fine conditions throughout the Territory.

Rainfall in the north is low to nonexistent in most areas, although light showers are common about the northeast coast and occasionally develop elsewhere over the northern Top End. Controlled burn-off and uncontrolled human initiated bushfires are widespread in the north during this season, particularly in later months, fuelled by the abundant wet season growth that has been dried by the prevailing south-easterlies.

Cold fronts between subtropical high cells frequently move across the Alice Springs district. This is particularly in the winter months, when they may also occasionally reach the Top End as a wind shift, separating the moist coastal air mass from the dry inland air mass. Winds before a front tend to be warm to hot and, in summer, humid.

The front's passage may be marked by thunderstorms or, if rainfall has been very low for a prolonged period, a wall of dust. The cool south-easterlies which follow the front generally clear the sky of cloud and are often very dry.

Large areas of cloud, known as 'northwest cloud bands' occasionally blow across Australia from the tropical Indian Ocean. These can bring widespread rain, and sometimes storms, to southern parts, especially if a cold front moves into the area and enhances the cloud band.

Queensland Weather

The variation in climate across an area the size of Queensland is considerable. Low rainfall and hot summers in the inland west, a monsoon season in the north, and warm temperate conditions along the coastal strip contrast with low minimum temperatures that can be experienced inland and about the southern ranges.

The climate of the coastal strip is influenced by the warm waters of the Coral and Tasman Seas, which in general, keep the region free from extremes of temperature and provide moisture for rainfall. The annual median rainfall along the coastal strip is generally within the range of 1000 to 1600 mm. increasing to over 3200 mm along parts of the north Queensland coast near Innisfail.

The mountains of the Great Dividing Range in Queensland attain a maximum height of 1622 metres at Mt Bartle Frere near Innisfail, and there are several peaks in excess of 1000 metres, mainly in the north and southeast. Along sections of the Great Divide, the elevation abruptly increases away from the coastal plain, and then west of the Divide it gradually descends onto the western plains.

On the western side of the Great Divide, the rainfall drops quickly to an annual median of about 700 mm. and then gradually decreases further. At the same time, average maximum temperatures gradually increase with increasing distance from the coast.

Further to the west the land slowly flattens out to the dry inland plains, marked by cold nights. It is here that the hottest temperatures in the State most commonly occur during summer, and where the annual median rainfall drops below 200 mm.

The way in which the climate changes across the State is reflected by marked changes in vegetation, which ranges from the tropical rainforests of the coastal zone of north Queensland to the cooler forests of the southern highlands, through the pastoral belt of areas such as the Darling Downs to the dry saltpans of the western inland.

Agriculture is one major source of income, and tourism, particularly along the coastal strip, is another. Outdoor activities also vary significantly across the State. Tropical cyclones are a natural hazard from about November through to May in coastal regions. They bring with them devastating winds, heavy rain and the threat of coastal inundation from tidal surges.

Whilst tropical cyclones are a threat to coastal communities, they are a major source of rain for the dry inland regions. Settlement to the west of the Great Dividing Range was made more difficult by the lack of a reliable water supply. Settlement onto the open plains that flourished during years of good rainfall foundered during drought periods.

Western Australia Weather

Western Australia covers an area of some 2.5 million square kilometres and extends over a latitude band from about 14 degrees south to 35 degrees south. Consequently there are quite a number of climatic zones, ranging from the north Kimberley, where heavy rains are experienced in the summer 'wet' season, through the mostly dry interior with its excessive summer heat, to the southwest with its distinctively Mediterranean climate. Snow occasionally falls in the far south during winter, particularly on the Stirling Range.

The size of the land mass is a major influence on the State's climate. There is a general decrease in rainfall and an increase in the range of temperatures experienced as one moves away from the coast.

Owing to a history of geological stability, much of Western Australia consists of a broad, relatively featureless plateau between 300 and 600 metres above mean sea level, with only the Pilbara and the Kimberley having any major areas of rugged country. The highest peak is Mount Meharry at 1251 metres, in the Hamersley Range.

The highest land in the south is to be found in the Stirling Range, where Bluff Knoll reaches 1096 metres. Though less pronounced than in most other States, topographic features do exert a significant influence in some areas.

Near the lower west coast, for example, a rapid increase in rainfall can be measured from the coastal plain to the top of the Darling Range, followed by a marked decrease to the east. Inland temperatures are modified to some degree by the elevation of the land, but the effect is not large.

Tasmania Weather

Since Tasmania lies between 40°S and 43.5°S and is an island with no place more than 115 kilometres from the sea, its climate is classified as temperate maritime. On the coast the range of daily temperature is about 7°C, but inland, the range is almost double, indicating a slight continental effect.

Mountainous regions cover a large proportion of Tasmania and attain a maximum height of 1617 metres at Mt Ossa, which rises from a central plateau. The central plateau includes several peaks in excess of 1500 metres. A smaller mountainous region in the northeast of Tasmania culminates with Ben Lomond, at an elevation of 1573 metres.

Prevailing westerly winds produce a marked west to east variation of cloudiness and rainfall, but the variation of temperature is more governed by the elevation and distance from the coast.

Summers are mild and are characterised by greatly lengthened days. The sun reaches a maximum elevation of 70 to 73 degrees in the midsummer, giving about 15 hours of daylight. In midwinter, the sun's elevation does not exceed 20 to 23 degrees, and the shortest day consists of about nine hours of daylight.

In winter and early spring, westerly winds reach their greatest strength and persistence (they are also known as equinoctial gales) causing a distinct maximum in the rainfall distribution in the west and northwest. In the east and southeast, rainfall is more evenly distributed throughout the year.

South Australia Weather

The climate of South Australia varies, from hot and dry in the interior to the milder, wetter climates of the southern Mount Lofty Ranges and the southeast coast of South Australia. Median annual rainfall ranges from about 100 mm in the area east of Lake Eyre to more than 1000 mm on the higher parts of the Mount Lofty Ranges.

Physical features: The State of South Australia occupies approximately the central third of the southern half of the continent of Australia with the ocean to the south. From the head of the Great Australian Bight, near the State's western border, the coastline tends south-eastwards to the Victorian border. This trend is interrupted by two major indentations, Spencer Gulf and Gulf St Vincent.

The waters of these two gulfs have a moderating influence on temperatures along their coasts. Kangaroo Island, to the west of Victor Harbour, is the predominant island off the South Australian coast. Large parts of South Australia are relatively flat or slightly undulating and approximately half the State is less than 150 metres above sea level.

The most significant mountain ranges are the Flinders and the Mount Lofty Ranges that extend from Cape Jervis in the south to the northern end of Lake Torrens. North of Peterborough there is a divergence from the main range that stretches via the Glary Ridges to the Barrier Range and Broken Hill in New South Wales. These ranges influence the climate by enhancing the rainfall in their immediate vicinity.

Temperature also decreases with increasing altitude. The western half of the State is largely occupied by a low plateau, over which an intermittent series of low ranges stretches from the Flinders to the somewhat higher Musgrave Ranges in the far northwest.

Climate controls: The seasonal variation of weather in South Australia is controlled by the position of the subtropical ridge of high pressure: During the warmer half of the year (November to April), this ridge is located along latitudes to the south of the Australian continent. High pressure systems (anticyclones) generally move eastwards along the ridge but have a favoured position south of the Great Australian Bight.

Consequently, the most frequent air stream across most of South Australia during this period is from the southeast to east. Although cold fronts associated with southern low pressure systems penetrate the ridge from time to time during summer, they generally fail to produce much useful rain.

Warm moist air of tropical origin occasionally moves into the State from the north during summer and thunderstorms may then develop.

In autumn the subtropical ridge moves north and remains over the Australian continent for most of the colder half of the year (May to October). During this period favoured locations for centres of anticyclones are the Great Victoria Desert and central New South Wales. The most frequent winds are from the northwest to southwest. Frontal systems associated with depressions travelling eastwards across the ocean have a significant influence on the weather in southern South Australia during this season.