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Average minimum temperatures in all seasons are highest in northern Australia and near the coast, and are lowest in the mountainous areas of the south-east. The highest average January minimum temperatures (near 27°C) are found near the north-west coast, while in winter they exceed 20°C at some coastal locations in northern Australia and on the Torres Strait and Tiwi Islands.
Low minimum temperatures are highly sensitive to local topography, with the lowest minimum temperatures occurring in high-elevation valleys, as cold air drains from hills to valleys overnight, making hilltops and ridges warmer overnight, even in areas with local relief of only a few tens of metres. In the most favoured locations in the mountains of New South Wales average minimum temperatures are below 5°C in January and -5°C in July, while most inland areas south of the tropics have average July minima between 0° and 6°C.
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 locations in this region have recorded temperatures exceeding 48°C. 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, the lower moisture levels in summer compared with northern Australia, and the generally lower elevation (when compared with areas such as the southern Northern Territory and east-central Western Australia, both of which are largely more than 500 metres above sea level).
Most other locations in mainland Australia, except those near parts of the Queensland and Northern Territory coasts or above 500 metres elevation, have extreme maxima between 43° and 48°C. Most Tasmanian sites away from the north coast have extreme maxima between 35° and 40°C. The lowest extreme maxima are found along the north coast of Tasmania (e.g. 29.5°C at Low Head) and at high elevations (e.g. 27.0°C at Thredbo (Top Station)).
While extreme 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°C at Mardie and 49.1°C at Roebourne in Western Australia, and 49.4°C at Whyalla and 47.9°C at Ceduna in South Australia.
Extreme maximum temperatures recorded at selected locations, including the highest recorded in each state/territory, are shown in table 1.16.
Prolonged heat waves, with a number of successive days over 40°C, are relatively common in summer over much of inland Australia, as well as parts of the north-west coast. Many inland locations have recorded ten or more successive days of such conditions, increasing to 20 or more days in parts of western Queensland and northern South Australia, and 50 or more days in north-western Western Australia. These heat waves can be accompanied by oppressively warm nights, with Oodnadatta (South Australia) recording an Australian record nine successive nights above 30°C in February 2004.
The coastal areas, though, can be affected by extreme heat over a period of one or two days. The most extreme heatwave in the recorded history of south-eastern Australia occurred in January 1939. Adelaide (46.1°C on the 12th), Melbourne (45.6°C on the 13th) and Sydney (45.3°C on the 14th) all set record high temperatures during this period, as did many other centres in New South Wales, Victoria and South Australia. This extreme heat contributed to the 'Black Friday' bushfires, in which almost two million hectares were burnt and 71 lives lost (see the Bushfires section in the Environment chapter in Year Book Australia 2004).
The lowest recorded temperatures in Australia have been in the Snowy Mountains of New South Wales, where Charlotte Pass recorded -23.0°C on 28 June 1994 (table 1.17), with a number of other locations recording temperatures below -15°C. It is likely that comparably low temperatures occur in similarly sheltered locations in the Victorian highlands, but no observing stations away from the exposed peaks exist in this area.
Away from the Snowy Mountains, the lowest extreme minima in Australia are found above 500 metres elevation on the tablelands and ranges of New South Wales, eastern Victoria and southern Queensland, as well as in central Tasmania. Many locations in this region have recorded -10°C or lower, including Gudgenby, Australian Capital Territory (-14.6°C) and Woolbrook, New South Wales (-14.5°C). At lower elevations, most inland places south of the tropics have extreme minima between -3°C and -7°C, 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°C), Bega, New South Wales (-8.1°C), Grove, Tasmania (-7.5°C) and Taree, New South Wales (-5.0°C).
In the tropics, extreme minima near or below 0°C have occurred at many places away from the coast, as far north as Herberton, Queensland (-5.0°C). Some locations near tropical coasts, such as Mackay (-0.8°C), Townsville (0.1°C) and Kalumburu, Western Australia (0.3°C) have also recorded temperatures near 0°C. In contrast, some exposed near-coastal locations, such as Darwin, have never fallen below 10°C, and Thursday Island, in the Torres Strait, has an extreme minimum of 16.1°C.
The parts of Australia with the lowest extreme minimum temperatures are also the most subject to frost. The eastern uplands from southern Queensland to eastern Victoria experience ten or more frosts per month in each month from May to September, as do Tasmania's Central Plateau and a few susceptible locations in south-western Western Australia and the Flinders Ranges region of South Australia. At lower elevations frost is less frequent and the season is shorter, although only the immediate coastal margins and the far north can be considered totally frost-free.
Other aspects of climate
In terms of the average water vapour content or humidity of the air, Australia is a dry continent. The amount of moisture in the atmosphere can be expressed in several ways, the most common being relative humidity. This measure can be thought of as the relative evaporating power of the air. When humidity is low, moisture on an exposed wet surface, like our skin, can evaporate freely. When it is high, evaporation is retarded. If the temperature is also high, people will feel discomfort or even stress as the body's ability to cool through the evaporation of perspiration is diminished. The combination of high temperature and high humidity is potentially dangerous for people who are not adapted or acclimatised to such conditions.
The main features of the relative humidity pattern are:
By way of an historical note, it is interesting that, as late as 1927, Griffith Taylor, from the Department of Physical Geography, University of Sydney, was asserting that tropical Australia was an unhealthy place to live, at least for women, because of its climate. However, in recent decades the introduction of air conditioning, more appropriate building design, and improved health measures such as proper sanitation, have greatly increased the comfort levels of those living in the tropics.
Incoming global radiation includes radiant energy reaching the ground directly from the sun and radiation received indirectly from the sky that is reflected and scattered downwards by clouds, dust and other airborne particles.
While there is a high correlation between daily global radiation and daily hours of sunshine, the latter is more dependent on variations in cloud coverage. Daily global radiation is at its strongest, all other things being equal, when the sun is closest to overhead south of the tropics (21-22 December), or directly overhead in the tropics. On the north-west coast around Port Hedland, Western Australia, where average daily global radiation is the highest for Australia (22-24 megajoules per square metre), average daily sunshine is also highest, being approximately ten hours. By way of contrast, in Darwin the global radiation values for the dry month of July and cloudy month of January are comparable, yet the number of sunshine hours for July approaches twice that for January.
Sunshine here refers to bright or direct sunshine. Australia receives relatively large amounts of sunshine although seasonal cloud formations affect spatial and temporal distribution. Cloud cover reduces both incoming solar radiation and outgoing radiation from the Earth's surface, and thus affects sunshine, air temperature and other measures of climate.
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, which is the equivalent of only 40% of the total possible per year.
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.
Average annual pan evaporation exceeds rainfall over most of Australia. It is highest in the north of Western Australia, reaching around 3,400 mm around Wyndham, and exceeds 3,000 mm over most of tropical Western Australia and the central Northern Territory. It is lower in tropical areas with higher rainfall and cloud cover, such as the top end of the Northern Territory and eastern Queensland.
At the other end of the scale, Australia's lowest pan evaporation occurs in Tasmania, ranging from 800 mm per year in the west to 1,200 mm in the east. Over the mainland it is below 1,400 mm over southern Victoria and adjacent parts of South Australia and New South Wales, and around 1,500 mm in the far south of Western Australia.
Over most of Australia evaporation is greatest in summer and least in winter, due to higher temperatures and solar radiation. In the far north, in contrast, the seasonal cycle is dominated by the effect of increased cloud cover during the tropical wet season. In this region evaporation peaks in spring, with a secondary peak in autumn in some places, and is lowest in late summer.
Seasonal distribution of cloudiness varies predominantly in line with seasonal variations in rainfall. In the southern parts of the continent, particularly in the coastal and low-lying areas, the winter months are generally cloudier 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 and higher levels of humidity favour the formation of 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 cover is greater near coasts and on the windward slopes of the eastern uplands of Australia and less over the dry interior.
The formation of radiation fogs, in which air near the ground is cooled by overnight radiation from the ground, is determined by the occurrence of a favourable blend of temperature, humidity, wind and overlying cloud cover. The nature of the local terrain can also be important for the development of fog, and there is a tendency for it to be particularly prevalent and persistent in valleys and hollows. The incidence of such fogs can vary significantly over short distances. Other types of fogs occur when low cloud covers high ground (hill fog), particularly where highlands are close to the coast, and more rarely, near some coastlines when warm moist air moves over relatively cool waters near the shore (sea fog).
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. Fog is more likely to occur in the colder months, particularly in the eastern uplands. Radiation fogs normally develop overnight and dissipate during the morning or early afternoon, although on rare occasions they persist through the day, particularly in inland Tasmania. 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 between May and August. Brisbane averages 20 days of fog per year. Darwin averages only two days per year, mostly in July and August.
The mid-latitude anticyclone belt is the chief determinant of Australia's two main prevailing wind streams. These streams tend to be easterly to the north of this belt and westerly to the south. The cycles of development, motion and decay of low-pressure systems that form to the north and south of the anticyclone belt and also intersperse between individual anticyclones result in a great diversity of wind flow patterns. Wind variations are greatest around the coast where diurnal land and sea-breeze effects also come into play. Sea breezes play a prominent role in modifying coastal climates in many parts of Australia, particularly along the west coast of Western Australia where they are a major feature of the summer climate. In Perth the sea breeze is known as the 'Fremantle Doctor'.
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 measured in Australia have been associated with tropical cyclones. The highest recorded gust was 267 km/h at Learmonth (Western Australia) on 22 March 1999 (with Cyclone Vance), while gusts reaching 200 km/h have been recorded on several occasions in northern Australia with cyclone visitations. The highest gusts recorded at Australian capitals have been 217 km/h at Darwin (during Cyclone Tracy), 185 km/h at Brisbane Airport and 156 km/h at Perth.
Dust storms are a regular occurrence on windy days in many of the arid zones of Australia. During drought years, they can extend to the more densely settled areas of the south-east, particularly when strong north- to north-westerly winds occur in advance of an approaching cold front. Well-known examples include those of February 1983, which plunged central Melbourne into darkness, and October 2002, which covered a vast area of eastern Queensland and New South Wales, including Brisbane and Sydney. These occurred in the later part of the severe El Niño-related droughts of 1982-83 and 2002-03 respectively.
While bushfires are not strictly a climatic phenomenon, both weather and climate are strong determinants in their occurrence and intensity. Provided vegetation is sufficiently abundant and dry, the spread of bushfires is most rapid in windy conditions with low humidity. In southern Australia such conditions are also normally associated with high temperatures. A Fire Danger Index, which combines expected wind speed, humidity, temperature and a measure of pre-existing dryness, is frequently used to assess the risk of rapid fire spread on any given day.
The most favoured season for bushfires varies in different parts of Australia. In south-eastern Australia (including Tasmania) the most favoured season is summer and early autumn; in coastal New South Wales and southern Queensland it is spring and early summer; and in much of northern Australia it is winter and spring (or the later part of the 'dry' season). In the arid zones of Australia large fires most commonly occur in the months following an abnormally wet season, when there is enough vegetation to provide fuel.
The south-east Australian bushfires which occurred at the end of 2002 and the beginning of 2003 were among the most protracted and extensive of the last century. The 2002-03 bushfire season and its impact was discussed in the Environment chapter in Year Book Australia 2004. There were also protracted and extensive fires, particularly in Victoria, in 2006-07.