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The island continent of Australia features a wide range of climatic zones, from the tropical regions of the north, through the arid expanses of the interior, to the temperate regions of the south. Widely known as 'The Dry Continent', the landmass is relatively arid, with 80% having a median rainfall less than 600 millimetres (mm) per year and 50% less than 300 mm (the average is 450 mm). Seasonal fluctuations can be large, with temperatures ranging from above 50 degrees Celsius to well below zero. However, extreme minimum temperatures are not as low as those recorded in other continents, due to Australia's relatively low latitude, the lack of high mountains to induce orographic cooling (which is in the order of -0.6 degrees Celsius per 100 metres increase in elevation) and because of the large expanse of relatively warm surrounding oceans.
1.5 AVERAGE ANNUAL RAINFALL
As outlined earlier, the rainfall pattern of Australia is strongly seasonal in character, with a winter rainfall regime in the south and a summer regime in the north.
The dominance of rainfall over other climatic elements in determining the growth of specific plants in Australia has led to the development of a climatic classification based on two main parameters, median annual rainfall and the incidence of seasonal rainfall.
Evaporation and the concept of rainfall effectiveness are taken into account to some extent in this classification, by assigning higher median annual rainfall limits to the summer zones than to the corresponding uniform and winter zones. The main features of the seasonal rainfall are:
Table 1.6 shows the monthly rainfall for all capital cities, as well as for Alice Springs and Davis Base in Antarctica.
Darwin shows the rainfall distribution pattern typical of the wet summer and dry winter seen in far northern Australia, and Brisbane the wet summer/relatively dry winter typical of south-eastern Queensland. By contrast, Adelaide and Perth show the wet winter/dry summer pattern whereas Sydney, Melbourne, Canberra and Hobart show a relatively uniform pattern of rainfall throughout the year. Alice Springs shows a low rainfall pattern throughout the year typical of arid inland areas.
Precipitation at Davis Base is mainly as snow, but is measured as water after melting. The pattern reflects the very low precipitation levels on the Antarctic continent.
A rainday occurs when more than 0.2 mm of rain falls in 24 hours, usually from 9 am to 9 am the next day. The frequency of raindays exceeds 150 per year in much of Tasmania (with a maximum of over 250 in western Tasmania), southern Victoria, parts of the north Queensland coast and in the extreme south-west of Western Australia. Over most of the continent the frequency is less than 50 raindays per year. The area of low rainfall with high variability, extending from the north-west coast of Western Australia through the interior of the continent, has less than 25 raindays per year. In the high rainfall areas of northern Australia, the number of raindays is about 80 per year, but heavier falls occur in this region than in southern regions.
The highest daily rainfalls for each state/territory are listed in table 1.7, and the highest annual rainfalls are listed in table 1.8.
Thunderstorms and hail
A thunderday at a given location is a calendar day on which thunder is heard at least once. The average annual number of thunderdays varies from 88 per year near Darwin to less than 10 per year over parts of the southern regions. Convectional processes during the summer wet season cause high thunderstorm incidence in northern Australia. The generally high incidence of thunderdays (40-60 annually) over the eastern upland areas is caused mainly by orographic uplift of moist air streams.
Hail, mostly of small size (less than 10 mm diameter), occurs with winter-spring cold frontal activity in southern Australia. Summer thunderstorms, particularly over the uplands of eastern Australia, sometimes produce large hail (greater than 10 mm diameter). Large hail capable of piercing light-gauge galvanised iron occurs at irregular intervals and sometimes causes widespread damage.
Generally, snow covers much of the Australian Alps above 1,500 metres for varying periods from late autumn to early spring. Similarly, in Tasmania the mountains are covered fairly frequently above 1,000 metres in these seasons. The area, depth and duration are highly variable. Light snowfalls can occur in these areas at any time of year. In some years, snow falls in the altitude range of 500-1,000 metres. Snowfalls at levels below 500 metres are occasionally experienced in southern Australia, particularly in the foothill areas of Tasmania and Victoria, but falls are usually light and short lived. In some seasons, parts of the eastern uplands above 1,000 metres from Victoria to south-eastern Queensland have been covered with snow for several weeks. On sheltered slopes around Mt Kosciuszko (2,228 metres) small areas of snow may persist through summer, but there are no permanent snowfields.
Average annual air temperatures range from 28 degrees Celsius along the Kimberley coast in the extreme north of Western Australia to 4 degrees Celsius in the alpine areas of south-eastern Australia. Although annual temperatures may be used for broad comparisons, monthly temperatures are required for detailed analyses.
July is the month with the lowest average temperature in all parts of the continent. The months with the highest average temperature are January or February in the south and December in the north (except in the extreme north and north-west where it is November). The slightly lower temperatures of mid-summer in the north are due to the increase in cloud during the wet season.
Average monthly maximum and minimum temperatures for all capital cities, and also for Alice Springs and Davis Base in Antarctica, are shown in table 1.6.
Temperatures in Darwin in tropical northern Australia are relatively constant throughout the year. In other cities, there is a greater seasonal variation between summer and winter months. The seasonal variation in temperature, as well as the difference between maximum and minimum value in any month, is greater for the inland cities of Canberra and Alice Springs than it is for the coastal cites, where proximity to the ocean moderates temperature extremes.
Average monthly maxima
In January, average maximum temperatures exceed 35 degrees Celsius over a vast area of the interior and exceed 40 degrees Celsius over appreciable areas of the north-west. The consistently hottest part of Australia in terms of summer maxima is around Marble Bar in Western Australia (150 km south-east of Port Hedland) where the average is 41 degrees Celsius and daily maxima during summer may exceed 40 degrees Celsius consecutively for several weeks at a time.
In July, a more regular latitudinal distribution of average maxima is evident. Maxima range from 30 degrees Celsius near the north coast to 5 degrees Celsius in the alpine areas of the south-east.
Average monthly minima
In January, average minima range from 27 degrees Celsius on the north-west coast to 5 degrees Celsius in the alpine areas of the south-east. In July, average minima fall below 5 degrees Celsius in areas south of the tropics (away from the coasts). Alpine areas record the lowest temperatures; the July average low is -5 degrees Celsius.
The highest extreme maxima in Australia are recorded in two regions: the Pilbara and Gascoyne regions of north-western Western Australia; and a broad belt extending from south-western Queensland across South Australia into south-eastern Western Australia. Many stations in this region have exceeded 48 degrees Celsius. Extreme temperatures in this southern belt are higher than those further north, due to the long trajectory over land of hot north-west winds from northern Australia, and the lower moisture levels in summer compared with northern Australia.
Most other stations in mainland Australia, except those near parts of the Queensland or Northern Territory coasts or above 500 metres elevation, have extreme maxima between 43 and 48 degrees Celsius. Most Tasmanian stations away from the north coast have extreme maxima between 35 and 40 degrees Celsius. The lowest extreme maxima are found in northern Tasmania (e.g. 29.5 degrees Celsius at Low Head) and at high elevations (e.g. 27.0 degrees Celsius at Thredbo in New South Wales).
While high temperatures are more common inland than they are near the coast, the highest temperatures recorded differ little between the two, except in Queensland, the Northern Territory and northern Tasmania. Notable extreme maxima observed near the coast include 50.5 degrees Celsius at Mardie and 49.1 degrees Celsius at Roebourne in Western Australia, and 49.4 degrees Celsius at Whyalla and 47.9 degrees Celsius at Ceduna in South Australia.
Extreme maximum temperatures recorded at selected stations, including the highest recorded in each state/territory, are shown in table 1.9.
The lowest temperatures in Australia have been recorded in the Snowy Mountains, New South Wales, where Charlotte Pass (elevation 1,760 metres) recorded -23.0 degrees Celsius on 18 June 1994 (table 1.10). Outside the Snowy Mountains, the lowest extreme minima on the Australian mainland are found above 500 metres elevation in the tablelands and ranges of New South Wales, eastern Victoria and southern Queensland. Many stations in this region have recorded -10 degrees Celsius or lower, including -14.6 degrees Celsius at Gudgenby, Australian Capital Territory and -14.5 degrees Celsius at Woolbrook, New South Wales. Temperatures below -10 degrees Celsius have also been recorded in central Tasmania. At lower elevations, most inland places south of the tropics have extreme minima between -3 and -7 degrees Celsius, and such low temperatures have also occurred in favoured locations within a few kilometres of southern and eastern coasts, such as Sale, Victoria (-5.6 degrees Celsius), Bega, New South Wales (-8.1 degrees Celsius), Grove, Tasmania (-7.5 degrees Celsius) and Taree, New South Wales (-5.0 degrees Celsius).
In the tropics, extreme minima below 0 degrees Celsius have been recorded at many places away from the coast, as far north as Herberton, Queensland (-5.0 degrees Celsius). Some locations near tropical coasts, such as Mackay (-0.8 degrees Celsius), Townsville, Queensland (0.1 degrees Celsius) and Kalumburu Station, Northern Territory (0.3 degrees Celsius) have also recorded temperatures near 0 degrees Celsius. In contrast, some exposed near-coastal locations, such as Darwin, have never fallen below 10 degrees Celsius, and Thursday Island, in the Torres Strait, has an extreme minimum of 16.1 degrees Celsius.
Periods with a number of successive days having a temperature higher than 40 degrees Celsius are relatively common in summer over parts of Australia. With the exception of the north-west coast of Western Australia, however, most coastal areas rarely experience more than three successive days of such conditions. The frequency increases inland, and periods of up to 10 successive days have been recorded at many inland stations. This figure increases to more than 20 days in parts of western Queensland and north-west Western Australia. The central part of the Northern Territory and the Marble Bar-Nullagine area of Western Australia have recorded the most prolonged heat waves. Marble Bar is the only known station in the world where temperatures of more than 37.8 degrees Celsius (100 degrees Fahrenheit) have been recorded on as many as 161 consecutive days (30 October 1923 to 7 April 1924).
Heat waves are experienced in the coastal areas from time to time. During 11-14 January 1939, for example, a severe heat wave affected south-eastern Australia: Melbourne had a record of 45.6 degrees Celsius on the 13th and Sydney a record of 45.3 degrees Celsius on the 14th. This heatwave also set record high temperatures in many other centres in New South Wales, Victoria and South Australia.
The Kimberley district of Western Australia is the consistently hottest part of Australia in terms of annual average maximum temperature. Wyndham, for example, has an annual average maximum of 35.6 degrees Celsius.
Other aspects of climate
The frequency of frost, which can cause serious losses of agricultural crops, depends on a number of factors. In coastal areas the relatively warm ocean temperatures ameliorate those on land, while distance from the Equator and elevation above sea level are major cooling influences. In addition, variations in topography can lead to local effects such as the accumulation of cold air in frost hollows. Hence frost hazard is greatest in areas which are away from the coast, are at relatively high elevations and have complex terrain which allows cold air drainage down slopes.
Parts of Australia most subject to frost are the eastern uplands from north-eastern Victoria to the western Darling Downs in southern Queensland where there may be more than 10 nights a month with readings of zero degrees Celsius (or under) for three to five months of the year. On Tasmania's Central Plateau similar conditions occur for three to six months of the year. Frosts may occur within a few kilometres of the coasts except in the Northern Territory and most of the north Queensland coasts.
Frosts may occur at any time of the year over most of Tasmania, large areas of the tablelands of New South Wales and much of inland Victoria, particularly the north-east. Frosts start in April and end in October over most of the interior of the continent, and on the highlands of Queensland as far north as the Atherton Plateau. Minimum temperatures below zero degrees Celsius can be experienced in most of the subtropical interior in June and July.
The median frost period over the continent varies from over 200 days per year in the south-eastern uplands areas south of the Hunter Valley, New South Wales, to none in northern Australia. The annual frost period generally decreases from about 100 days inland to below 50 days towards the coast in the southern regions of the continent, but there is widespread local variation. In Tasmania the frost period exceeds 300 days on the uplands and decreases to 100 days near the coast.
Australia is a dry continent in terms of the water vapour content or humidity of the air, and this element may be compared with evaporation to which it is related. Moisture content can be expressed by a number of parameters, of which the most commonly known is relative humidity. This can be thought of as the relative evaporating power of the air; when the humidity is low, a wet surface, like our skin, can evaporate freely. When it is high, evaporation is retarded. People can feel this as discomfort or even stress as the body's ability to perspire (and hence cool) decreases with increasing relative humidity. The combination of high temperature and high humidity is potentially dangerous for people who are active in such conditions.
The main features of the relative humidity pattern are:
Global (short wave) radiation includes that radiation energy reaching the ground directly from the sun and that received indirectly from the sky, scattered downwards by clouds, dust particles, etc.
A high correlation exists between daily global radiation and daily hours of sunshine. On the north-west coast around Port Hedland, Western Australia, where average daily global radiation is the highest for Australia (640 milliwatt hours), average daily sunshine is also highest, being approximately 10 hours. Sunshine is more dependent on variations in cloud coverage than is global radiation, since the latter includes diffuse radiation from the sky as well as direct radiation from the sun. An example is Darwin where, in the dry month of July, sunshine approaches twice that of the wet (cloudy) month of January, but global radiation amounts for the two months are comparable.
Sunshine here refers to bright or direct sunshine. Australia receives relatively large amounts of sunshine although seasonal cloud formations have a notable effect on its spatial and temporal distribution. Cloud cover reduces both incoming solar radiation and outgoing long wave radiation, and thus affects sunshine, air temperature and other climatic elements on the Earth's surface.
Most of the continent receives more than 3,000 hours of sunshine a year, or nearly 70% of the total possible. In central Australia and the mid-west coast of Western Australia, totals slightly in excess of 3,500 hours occur. Totals of less than 1,750 hours occur on the west coast and highlands of Tasmania; this amount is only 40% of the total possible per year (about 4,380 hours).
In southern Australia, the duration of sunshine is greatest about December when the sun is at its highest elevation, and lowest in June when the sun is lowest. In northern Australia, sunshine is generally greatest over the period August to October prior to the wet season, and least over the period January to March during the wet season.
Seasonal changes in cloudiness vary with the distribution of rainfall. In the southern parts of the continent, particularly in the coastal and low-lying areas, the winter months are generally more cloudy than the summer months. This is due to the formation of extensive areas of stratiform cloud and fog during the colder months, when the structure of the lower layers of the atmosphere favours the physical processes resulting in this type of cloud. Particularly strong seasonal variability of cloud cover exists in northern Australia where skies are clouded during the summer wet season and mainly cloudless during the winter dry season. Cloud coverage is greater near coasts and on the windward slopes of the eastern uplands of Australia and less over the dry interior.
The formation of fog depends on the occurrence of favourable meteorological elements - mainly temperature, humidity, wind and cloud cover. The nature of the local terrain is important for the development of fog and there is a tendency for this phenomenon to persist in valleys and hollows. The incidence of fog may vary significantly over distances as short as one kilometre.
Fog in Australia tends to be more common in the south than the north, although parts of the east coastal areas are relatively fog-prone even in the tropics. Incidence is much greater in the colder months, particularly in the eastern uplands. Fog may persist during the day, but rarely until the afternoon over the interior. The highest fog incidence at a capital city is at Canberra which has an average of 47 days per year on which fog occurs, 29 of which are in the period May to August. Brisbane averages 20 days of fog per year. Darwin averages only two days per year, in the months of July and August.
The mid-latitude anticyclones are the chief determinants of Australia's two main prevailing wind streams. In relation to the west-east axes of the anticyclones these streams are easterly to the north and westerly to the south. The cycles of development, motion and decay of low-pressure systems to the north and south of the anticyclones result in diversity of wind-flow patterns. Wind variations are greatest around the coasts where diurnal land and sea-breeze effects are important.
Orography affects the prevailing wind pattern in various ways, such as the channelling of winds through valleys, deflection by mountains and cold air drainage from highland areas. An example of this channelling is the high frequency of north-west winds at Hobart caused by the north-west to south-east orientation of the Derwent River Valley.
Perth is the windiest capital with an average wind speed of 15.6 km/h; Canberra is the least windy with an average wind speed of 5.4 km/h.
The highest wind speeds and wind gusts recorded in Australia have been associated with tropical cyclones. The highest recorded gust was 267 km/h at Learmonth, Western Australia on 22 March 1999 (occurring with Tropical Cyclone Vance); gusts reaching 200 km/h have been recorded on several occasions in northern Australia with cyclone visitations. The highest gusts recorded at Australian capitals were 217 km/h at Darwin and 156 km/h at Perth.
Drought, in general terms, refers to an acute deficit of water supply to meet a specified demand. The best single measure of water availability in Australia is rainfall, although parameters such as evaporation and soil moisture are significant, even dominant in some situations. Demands for water are very diverse, hence the actual declaration of drought conditions for an area will generally also depend on the effects of a naturally occurring water deficit on the principal local industries.
Since the 1860s there have been 10 major Australian droughts. Some of these major droughts could be described as periods consisting of a series of dry spells of various lengths, overlapping in time and space, and totalling up to about a decade. The drought periods of 1895-1903 (the so-called 'Federation drought'), 1958-68, 1982-83 and 1991-95 were the most devastating in terms of their extent and effects on primary production. The latter drought resulted in a possible $5b cost to Australia's economy, and $590m in drought relief by the Commonwealth Government. The remaining major droughts occurred in 1864-66 (and 1868), 1880-86, 1888, 1911-16, 1918-20 and 1939-45.
In this same period, several droughts of lesser severity caused significant losses over large areas of some states. They occurred in 1922-23 and 1926-29, 1933-38, 1946-49, 1951-52, 1970-72, 1976 and 1997-2000.
South-eastern Australia (New South Wales, southern Queensland, Victoria, Tasmania and the settled parts of South Australia) contains about 75% of the nation's population, and droughts affecting this region have a markedly adverse impact on the economy. There have been nine severe droughts in south-eastern Australia since 1888, and these were encompassed within the major Australian droughts specified above, except for the severe drought in 1972. Drought definitions, and the area of coverage and length of droughts, together with related information, may be obtained from the article Drought in Australia in Year Book Australia 1988.
Widespread flood rainfall may occur anywhere in Australia, but it has a higher incidence in the north and in the eastern coastal areas. It is most economically damaging along the shorter streams flowing from the eastern uplands eastward to the seaboard of Queensland and New South Wales. These flood rains are notably destructive in the more densely populated coastal river valleys of New South Wales - the Tweed, Richmond, Clarence, Macleay, Hunter and Nepean-Hawkesbury - all of which experience relatively frequent flooding. Although chiefly caused by summer rains, they may occur in any season.
The great Fitzroy and Burdekin river basins of Queensland receive flood rains during the summer wet seasons. Much of the run-off due to heavy rain in north Queensland west of the eastern uplands flows southward through the normally dry channels of the network of rivers draining the interior lowlands into Lake Eyre. This widespread rain may cause floods over an extensive area, but it soon seeps away or evaporates, occasionally reaching the lake in quantity. The Condamine and other northern tributaries of the Darling also carry large volumes of water from flood rains south through western New South Wales to the Murray, and flooding occurs along their courses at times.
Flood rains occur at irregular intervals in the Murray-Murrumbidgee system of New South Wales and Victoria, the coastal streams of southern Victoria and the north coast streams of Tasmania.