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1301.0 - Year Book Australia, 2006  
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Contents >> Chapter 1 - Geography and Climate >> Australia's climate

AUSTRALIA'S CLIMATE

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. Australia is the world’s second-driest continent (after Antarctica), with average (mean) annual rainfall below 600 millimetres (mm) per year over 80% of the continent, and below 300mm over 50%. Summers are hot through most of the country, with average January maximum temperatures exceeding 30 degrees Celsius (C) over most of the mainland except for the southern coastal fringe between Perth and Brisbane, and areas at high elevations. Winters are warm in the north and cooler in the south, with overnight frosts common in inland areas south of the Tropic of Capricorn; only at higher elevations do wintertime temperatures approach those found in much of northern Europe or North America.

Seasonal fluctuations in both rainfall and temperature can be large in parts of the country. In northern Australia, temperatures are warm throughout the year, with a ‘wet’ season from approximately November through April, when almost all the rainfall occurs, and a ‘dry’ season from May through October. Further south, temperature becomes more important in defining seasonal differences and rainfall is more evenly distributed through the year, reaching a marked winter peak in the south-west and along parts of the southern fringe.

Australia experiences many of nature’s more extreme phenomena; including droughts, floods, tropical cyclones, severe storms, bushfires, and the occasional tornado.

CLIMATIC CONTROLS

Australia’s climate is largely determined by its latitude, with the mainland lying between 10 degrees south (S) and 39S and Tasmania extending south to 44S. This places much of Australia under the influence of the sub-tropical high pressure belt (or ridge), which is a major influence on climate near, and poleward of, the tropics in both hemispheres. The aridity of much of Australia is largely a consequence of the subsiding air associated with this ridge of high pressure.

The sub-tropical ridge consists of areas of high pressure (anticyclones) which pass from west to east across the continent. Individual anticyclones, which can be up to 4,000 km across, can remain near-stationary for several days, bringing clear skies and fine conditions to large parts of the continent, before moving on. The latitude of the sub-tropical ridge varies seasonally. During winter, the ridge is normally centred between latitudes 30 and 35S, whereas in summer it moves south to between latitudes 35 and 40S (although individual systems can form significantly further north or south than these characteristic latitudes).

Winds circulate counter-clockwise around anticyclones in the Southern Hemisphere, and hence the flow on the southern side of the sub-tropical ridge tends to be westerly. This zone of westerly flow is generally strongest south of Australia (the so-called ‘Roaring Forties’), but the northern part of the zone can affect southern Australia, particularly in winter and spring. Extensive depressions (lows) over the Southern Ocean have associated frontal systems embedded in the westerlies, which bring periods of rain and showers to southern parts of the country. Tasmania is under the influence of westerly flow for much of the year.

North of the sub-tropical ridge the flow is generally easterly. In winter this easterly- to south-easterly flow is especially persistent over the northern half of the continent, bringing dry conditions to most locations, except along the east coast. In summer, hot air rising over northern Australia causes an area of low pressure, drawing moist oceanic air from north and west of the continent. Where this air collides with the air coming from the south and east it generates what is known as the intertropical convergence zone, otherwise known as the monsoon trough. This zone progressively moves southwards over northern Australia (the exact timing and location vary from year to year), allowing warm, moist monsoonal air from the north-west to penetrate into the northern reaches of the continent. Elsewhere, moist easterly flow from the Pacific Ocean and Tasman Sea brings summer rain to most of the east coast.

Australia’s generally low relief (map 1.4) means that topography has less impact on atmospheric systems that control the climate than is the case in other more mountainous continents. This lack of topographic obstruction, and the absence of cool ocean currents off the west coast (as are found at similar latitudes off Africa and the Americas) as a stabilising influence, allows the occasional penetration of tropical moisture deep into the continent. As a result, the Australian desert, while relatively dry, does not match the extreme aridity of deserts such as the Sahara where vast areas have average annual rainfalls below 25mm (see the article Australia's deserts). There are also no barriers to occasional bands of moisture and cloud extending from the warm waters of the Indian Ocean off north-western Australia right across the continent to the southern states. These ‘north-west cloud bands’, which are most common in late autumn and early winter, can produce good rainfall in their own right, sometimes in significant amounts, but their major influence is to provide an additional in-feed of moisture into frontal systems traversing southern Australia, enhancing the rainfall produced by those systems.

One area where topography does have a major influence on rainfall is in Tasmania. Westerly winds are intercepted by the island’s mountains, causing heavy rainfall on the western (windward) side, and leaving eastern and central Tasmania in a much drier so-called ‘rain-shadow’. The interaction of topography with westerly winds in winter also plays a role in locally enhancing rainfall in regions such as the Australian Alps and the Adelaide Hills. The Great Dividing Range and associated ranges in eastern Australia enhance rainfall over the east coast hinterland during periods of easterly flow, and partially block moisture from penetrating further inland.

EPISODIC WEATHER EVENTS

Tropical cyclones are the most dramatic episodic weather events to affect Australia. Tropical cyclones are strong, well-organised low pressure systems that form poleward of about 5 of the Equator, over water that is warmer than approximately 26C. (The weak Coriolis force near the Equator, which is important in inducing the rotation required for the development of a tropical cyclone, accounts for the lack of cyclones in that region.) Tropical cyclones can vary significantly in size, and once formed are classified as category 1 (weakest) to 5 (strongest) according to their intensity at any given time. Category 4 and 5 cyclones have wind gusts exceeding 225 kilometres/hour (km/h) and can be exceptionally damaging, as in the near-total destruction of Darwin by Tropical Cyclone Tracy on 25 December 1974. The strongest wind gust instrumentally measured in a tropical cyclone on the Australian mainland is 267 km/h, at Learmonth (Western Australia) during Tropical Cyclone Vance on 22 March 1999, but it is believed that gusts in excess of 320 km/h have occurred away from instruments. The zone of most destructive winds associated with tropical cyclones is normally quite narrow, only about 50km wide in the case of Tracy, and rarely more than 300km.

Tropical cyclones bring heavy rain as well as strong winds, and are the cause of most of Australia’s highest-recorded daily rainfalls (table 1.7). Warm water acts as the cyclone’s energy source, and hence is required to maintain the strength of the winds. As a result, tropical cyclones rapidly lose their intensity on moving over land, although the rainfall with former cyclones often persists well after the destructive winds have eased, occasionally bringing heavy rains deep into the inland and causing widespread flooding. (Such flooding can also occur from tropical depressions that never reach sufficient intensity to be classified as cyclones.) Parts of inland Western Australia receive 30-40% of their average annual rainfall from these systems, and it is not unheard of for places to receive their average annual rainfall within a one or two-day period as a tropical cyclone (or ex-cyclone) passes by.

On average, about three tropical cyclones directly approach the Queensland coast during the season between November and May, and three affect the north and north-west coasts, but the number and location of cyclones vary greatly from year to year. The most susceptible areas are north of Carnarvon on the west coast and north of Rockhampton on the east, but on occasions tropical cyclones have reached as far south as Perth and northern New South Wales.

Away from the tropics, 'heatwaves' can occur over many parts of Australia. In southern Australia, they are normally associated with slow-moving anticyclones. A large anti-cyclone remaining stationary ('blocking') over the Tasman Sea will result in northerly or north-westerly flow on its western flank, bringing hot air from the centre of the continent over the south-east coastal regions (and sometimes to Tasmania). In south-western Australia, summer heatwaves are more commonly associated with the characteristic north-south trough of low pressure along the west coast moving offshore, suppressing sea breezes and causing hot north-easterly winds to blow from the interior to the coast.

'Cold outbreaks' can occur over southern Australia when intense south to south-west flow associated with strong cold fronts or large depressions directs cold air from the Southern Ocean over the continent. These outbreaks are most common in the south-east of the country and can result in low temperatures and snow falling to low elevations. While principally a winter and early spring phenomenon, cold outbreaks can occur at other times of year, and the fact that the air originates over the Southern Ocean (where there is only about a 4C change in temperature from winter to summer) means that they can also bring cold air and 'unseasonable' snowfalls at high elevations at any other time of year.

Intense low pressure systems can also form outside the tropics, most commonly off the east coast where they are known as 'east coast lows'. These systems can bring very strong winds and heavy rain, particularly where they direct moist easterly winds on their southern flank onto the coastal ranges of southern Queensland, New South Wales, eastern Victoria and north-eastern Tasmania. Examples of systems of this type include two, a fortnight apart, in June 1967 off southern Queensland which caused major flooding and severe beach erosion in the Gold Coast region, and an intense low in Bass Strait that sank or damaged many yachts in the 1998 Sydney-Hobart race.

INTERANNUAL VARIABILITY

The major driver of interannual climate variability in Australia, particularly eastern Australia, is the El Nio-Southern Oscillation phenomenon. El Nio is an anomalous large warming of the central and eastern tropical Pacific Ocean, while La Nia, the reverse phase of the system, is an anomalous cooling. The Southern Oscillation refers to a see-sawing of atmospheric pressure between the northern Australian-Indonesian region and the central Pacific Ocean. El Nio events are strongly associated with abnormally high pressures in the northern Australian-Indonesian region and abnormally low pressures over the central Pacific, while the reverse is true during La Nia events.

The Southern Oscillation Index (SOI) is an index of the pressure differences between Darwin and Tahiti and has traditionally been used as an indicator of El Nio events (which are very often, but not always, associated with a strongly negative SOI). However, with modern satellite and floating buoy observations developed over the last 30 years, ocean temperature anomalies, both at and below the surface, can be monitored directly and hence proxy measurements, such as the SOI, are less important than they once were.

El Nio events characteristically develop during the southern autumn, and continue for about 9-12 months until the following autumn. The most recent El Nio followed this pattern, developing in May-June 2002 and dissipating in February-March 2003. On occasions El Nio events are followed immediately by La Nia events (or vice versa), but it is more common for them to be followed by near-normal (neutral) ocean conditions. Events lasting for more than one year are rare, but not unknown. There are typically two to three El Nio events per decade, but there is large variation from decade to decade in their frequency and the balance of El Nio and La Nia events; since 1980, El Nio events have been predominant, whereas La Nia events were frequent in the 1950’s and 1970’s.

El Nio events are generally associated with a reduction in winter and spring rainfall across much of eastern, northern and southern Australia. This can lead to widespread and severe drought, particularly in eastern Australia, as well as increased daytime temperatures and bushfire risk. Conversely, La Nia events are generally associated with wetter-than-normal conditions and have contributed to many of Australia’s most notable floods. There is considerable variation, however, in the way each El Nio and La Nia event affects rainfall patterns from the time of onset through its developmental stages to eventual decay.

Temperatures in the tropical Indian Ocean also have an influence on Australia’s climate, particularly in the south-west of Western Australia, where the influences of El Nio and La Nina events are more limited. Indian Ocean conditions also have a bearing on winter rainfall in south-eastern Australia through their effects on the frequency of northwest cloud bands (see earlier section).

The article 'Climate variability and El Nio' in the Geography and climate chapter of Year Book Australia 1998 provides further details.

CLIMATE CHANGE

Temperatures in Australia were relatively stable from 1910 until 1950, and since then have followed an increasing trend, with an overall increase during 1910 to 2004 of approximately 0.7C. Overnight minimum temperatures have warmed more quickly than daytime maximum temperatures, but both have increased over almost the entire continent, with the largest increases occurring in north-eastern Australia. In conjunction with this trend, the frequencies of frosts and other extreme low temperatures have decreased, while the frequency of extreme high temperatures has increased, although at a slower rate. Over Australia the observed warming has accelerated in recent years, and the late-20th century warming has been largely attributed to the enhanced greenhouse effect.

Over the continent as a whole, rainfall has increased over the 1900-2004 period, with the largest increases occurring over northern and north-western Australia. Since 1960, however, there have been substantial decreases in rainfall over three relatively small, but economically and agriculturally important, regions: south-western Western Australia; Victoria (particularly southern Victoria), and the eastern coastal fringe (particularly south-eastern Queensland).

Table 1.5 shows temperatures and rainfall averaged over Australia since the commencement of comprehensive national records. The article ' A hundred years of science and service - Australian meteorology through the twentieth century' in Year Book Australia 2001 provides further details, including maps of temperature and rainfall trends to 1999.

While some temperature and rainfall data exist prior to the starting dates used in table 1.5, they have not been used in analyses of climate change. This is because large parts of the Australian continent had no observations before that time. In the case of temperatures, most pre-1910 data is also not comparable with post-1910 data, because the louvred, white-painted screen (the ‘Stevenson screen’) which is used for sheltering thermometers from direct solar radiation was only introduced as a national standard around that time. Many pre-1910 temperatures were measured in locations such as underneath tin verandahs or even indoors, and cannot be validly compared with more recent data (see the article 'Temperature measurement and the Stevenson screen' in Year Book Australia 2005 for further details).


1.5 MEAN TEMPERATURES(a) AND RAINFALL

Temperature deviation
Rainfall
Period(b)
C
mm

10-YEAR PERIODS - ANNUAL AVERAGE

1900-09
n.a.
425
1910-19
-0.33
449
1920-29
-0.40
430
1930-39
-0.28
418
1940-49
-0.41
436
1950-59
-0.27
468
1960-69
-0.22
431
1970-79
-0.12
527
1980-89
0.23
463
1990-99
0.39
485

YEARS

1990
0.50
418
1991
0.68
469
1992
0.15
452
1993
0.30
499
1994
0.25
341
1995
0.18
523
1996
0.60
470
1997
0.23
527
1998
0.84
565
1999
0.21
584
2000
-0.21
727
2001
-0.10
559
2002
0.63
341
2003
0.62
487
2004
0.45
512

(a) Temperatures are shown as anomalies (or deviations) from 1961-90 base period.
(b) The full annual time series since 1900 (rainfall) and 1910 (temperature) are available via <http://www.bom.gov.au/climate/change>.

Source: Australian Bureau of Meteorology.


RAINFALL AND OTHER PRECIPATION

Annual

Map 1.6 shows average annual rainfall over the Australian continent.

The driest section of Australia, with an average of less than 200 mm per year, extends over a large area from the west coast near Shark Bay, across the interior of Western Australia and northern South Australia into south-western Queensland and north-western New South Wales. The driest part of this region is in the vicinity of Lake Eyre in South Australia, where average annual rainfall is below 150 mm. This region is not normally exposed to moist air masses and rainfall is irregular, averaging rain on only around 20 days per year. Very occasionally, favourable synoptic situations (usually, but not always, disturbances of tropical origin) can bring heavy rains to many parts of this normally arid to semi-arid region, with falls of up to 400 mm over a few days being recorded in the most extreme cases. Such heavy rainfalls often lead to widespread flooding and a subsequent short-lived ‘blooming’ of the desert regions.

The region with the highest average annual rainfall is the east coast of Queensland between Cairns and Cardwell, where mountains are very close to the tropical coast. The summit of Bellenden Ker has an average of 7,996 mm over 32 years of records, while at lower elevations, Topaz has an average of 4,382 mm over 25 years, and Babinda 4,231 mm over 94 years. The mountainous region of western Tasmania also has a high annual rainfall, with Lake Margaret having an average of 2,956 mm over 59 years, and short-term records suggest that other parts of the region have an average near 3,500 mm.

The Snowy Mountains area in New South Wales also has a particularly high rainfall. While there are no official rain gauges in the wettest areas on the western slopes above 1,800 metres elevation, runoff data suggests that the average annual rainfall in parts of this region exceeds 3,000 mm. Small pockets with averages exceeding 2,500 mm also occur in the north-east Victorian highlands and some parts of the east coastal slopes.

1.6 AVERAGE ANNUAL RAINFALL - 1961-1990

1.6: AVERAGE ANNUAL RAINFALL - 1961-1990

Source:Australian Bureau of Meteorology.


Seasonal

Australia’s rainfall pattern is strongly seasonal in character, with a winter rainfall regime in parts of the south, a summer regime in the north and generally more uniform or erratic throughout the year elsewhere. Major rainfall zones include:
  • the marked wet summer and dry winter of northern and north-western Australia. In this region winters are almost completely dry, except near exposed eastern coastlines (e.g. Darwin in table 1.7).
  • the wet summer and relatively (but not completely) dry winter of south-eastern Queensland and north-eastern New South Wales (e.g. Brisbane in table 1.7).
  • fairly uniform rainfall in south-eastern Australia, including most of New South Wales, parts of Victoria and eastern Tasmania. The exact seasonal distribution can be influenced by local topography; for example, winter is the wettest season at Albury on the windward side of the Snowy Mountains, but the driest season at Cooma on the leeward side (e.g. Sydney, Melbourne, Canberra and Hobart in table 1.7).
  • a marked wet winter and dry summer (sometimes called a 'Mediterranean' climate). This climate is most prominent in south-western Western Australia and southern South Australia, but there is also a winter rainfall maximum in some other parts of the south-east, particularly those areas exposed to westerly or south-westerly winds, such as western Tasmania and south-western Victoria (e.g. Adelaide, Perth in table 1.7).
  • low and erratic rainfall through much of the western and central inland. Rainfall events are irregular and can occur in most seasons, but are most common in summer (e.g. Alice Springs in table 1.7).

1.7 AVERAGE MONTHLY RAINFALL AND TEMPERATURES(a), Capital cities and Alice Springs

Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual

AVERAGE DAILY MAXIMUM TEMPERATURE (C)

Sydney
26.1
26.4
25.2
23.1
20.4
17.7
17.2
18.5
20.7
22.4
23.6
25.6
22.3
Melbourne
25.8
26.5
24.0
20.5
17.3
14.4
13.9
15.3
17.3
19.7
21.8
24.2
20.1
Brisbane
29.2
28.8
28.0
26.1
23.5
21.1
20.6
21.6
23.9
25.5
27.1
28.6
25.3
Adelaide
28.7
29.3
26.1
22.2
18.8
16.0
15.2
16.5
18.7
21.7
24.7
26.8
22.1
Perth
31.9
32.2
29.8
25.9
21.8
18.9
17.9
18.4
20.2
22.5
25.8
29.2
24.5
Hobart
21.8
22.0
20.2
17.9
15.1
12.3
12.2
13.4
15.3
17.2
18.6
20.3
17.2
Darwin
31.8
31.4
31.8
32.8
32.2
30.7
30.7
31.5
32.7
33.3
33.3
32.6
32.1
Canberra
27.7
27.3
24.5
20.0
15.9
12.3
11.5
13.2
16.2
19.4
22.6
26.3
19.7
Alice Springs
36.4
35.1
32.8
27.8
23.2
19.7
20.0
23.0
27.5
30.9
33.9
35.8
28.8

AVERAGE DAILY MINIMUM TEMPERATURE (C)

Sydney
19.4
19.6
18.1
15.2
12.5
9.6
8.6
9.5
11.7
14.2
16.0
18.3
14.4
Melbourne
15.4
15.8
14.3
11.7
9.8
7.6
6.8
7.6
9.0
10.5
12.2
13.9
11.2
Brisbane
21.2
20.9
19.5
16.8
14.2
10.8
9.5
9.9
12.4
15.5
18.0
19.9
15.7
Adelaide
16.8
17.1
15.2
12.1
10.2
8.1
7.4
8.2
9.6
11.5
13.8
15.5
12.1
Perth
17.2
17.8
16.3
13.4
10.8
9.1
8.4
8.5
9.3
10.5
13.0
15.2
12.5
Hobart
12.5
12.7
11.4
9.6
7.6
5.2
4.7
5.5
6.9
8.3
9.8
11.3
8.8
Darwin
24.8
24.9
24.6
24.2
22.4
20.1
19.4
20.9
23.4
25.1
25.6
25.5
23.4
Canberra
13.3
13.3
10.9
6.7
3.7
0.8
-0.1
1.0
3.6
6.3
8.9
11.6
6.7
Alice Springs
21.3
20.7
17.4
12.3
8.2
4.8
3.8
6.2
10.4
14.6
17.9
20.2
13.2

AVERAGE RAINFALL (mm)

Sydney
136.3
130.9
151.2
127.7
110.0
126.8
69.6
92.0
68.8
88.1
101.7
73.4
1,276.5
Melbourne
52.4
49.0
40.0
52.1
58.8
48.6
45.1
54.6
59.2
69.5
64.2
61.1
654.4
Brisbane
158.6
174.3
125.3
108.7
115.7
53.1
60.1
37.2
34.8
96.8
106.0
119.6
1,194.0
Adelaide
19.4
12.7
26.6
42.0
61.2
79.7
79.9
68.0
62.2
347.5
29.7
27.8
563.0
Perth
12.7
18.2
15.9
36.5
92.8
145.5
154.1
117.3
76.7
44.2
26.5
7.2
745.3
Hobart
47.3
40.0
41.9
44.2
38.6
37.5
53.7
59.2
48.7
48.3
50.6
56.5
576.4
Darwin
499.8
336.2
376.3
104.4
23.2
1.6
0.5
8.0
15.5
76.6
134.0
270.9
1,847.1
Canberra
66.3
52.7
50.3
49.3
44.6
38.4
46.4
49.2
56.7
60.9
67.4
47.8
630.0
Alice Springs
41.3
48.5
47.9
24.1
20.6
15.2
14.3
9.2
11.3
23.2
29.8
40.1
325.6

(a) Averages are for the period (1971-2000) except for Adelaide (1977-2000). Brisbane, Perth, Darwin, Canberra and Alice Springs averages are for observations taken at airports, others are at locations in or near the central city.

Source: Australian Bureau of Meteorology 2003.


Rain days and extreme rainfalls

The frequency of rain days (defined as days when 0.2 mm or more of rainfall is recorded in a 24-hour period) is greatest near the southern Australian coast, exceeding 150 per year in much of Tasmania, southern Victoria and the far south-west of Western Australia, peaking at over 250 per year in western Tasmania. Values exceeding 150 per year also occur along parts of the north Queensland coast. At the other extreme, a large part of inland western and central Australia has fewer than 25 rain days per year, and most of the continent away from the coasts has fewer than 50 per year. In the high rainfall areas of northern Australia away from the east coast the number of rain days is typically about 80 to 120 per year, but rainfall events are typically heavier in this region than in southern Australia.

The highest daily rainfalls have occurred in the northern half of Australia and along the east coast, most of them arising from tropical cyclones, or further south east coast lows, near the coast in mountainous areas. Daily falls in excess of 500 mm have occurred at scattered locations near the east coast as far south as the Illawarra, south of Sydney, and falls exceeding 300 mm have occurred in north-eastern Tasmania and the Otway Ranges of southern Victoria. Most locations in temperate Australia away from the east coast have highest recorded daily rainfalls in the 75-150 mm range, although some locations have exceeded 200 mm. In these regions, extreme daily rainfalls are often associated with thunderstorms, for which rainfall recordings can vary dramatically over short distances.

The highest daily and annual rainfalls for each state and territory are listed in tables 1.8 and 1.9.


1.8 HIGHEST DAILY RAINFALLS(a)

mm
Date

New South Wales
Dorrigo (Myrtle Street)
809
21.2.1954
Cordeaux River
573
14.2.1898
Victoria
Tanybryn
375
22.3.1983
Rotamah Island
300
27.11.1988
Queensland(b)
Beerwah (Crohamhurst)
907
3.2.1893
Finch Hatton PO
878
18.2.1958
South Australia
Motpena
273
14.3.1989
Nilpena
247
14.3.1989
Western Australia
Roebourne (Whim Creek)
747
3.4.1898
Fortescue
593
3.5.1890
Tasmania
Cullenswood
352
22.3.1974
Mathinna
337
5.4.1929
Northern Territory
Roper Valley Station
545
15.4.1963
Angurugu (Groote Eylandt)
513
28.3.1953
Australian Capital Territory
Lambrigg
182
27.5.1925

(a) The standard daily rainfall period is 9 am to 9 am.
(b) Bellenden Ker (Top Station) has recorded a 48-hour total of 1,947 mm on 4-5 January 1979, including 960 mm from 3 pm on the 3rd to 3 pm on the 4th. No observation was made at 9 am on the 4th.

Source: Australian Bureau of Meteorology.

1.9 HIGHEST ANNUAL RAINFALLS

Station
Year
mm

NSWTallowwood Point
1950
4,540
Vic.Falls Creek SEC(a)
1956
3,739
QldBellenden Ker (Top Station)
2000
12,461
SAAldgate State School
1917
1,853
WAKimberley Coastal Camp
2000
2,334
Tas.Lake Margaret
1948
4,504
NTDarwin Botanic Gardens
1998
2,906
ACTBendora Dam
1974
1,831

(a) State Electricity Commission.

Source: Australian Bureau of Meteorology.


Floods

Heavy rainfall conducive to widespread flooding can occur anywhere in Australia, but is most common in the north and in the eastern coastal areas. There are three main flood types:
  • short-lived floods lasting a few days that occur in shorter coastal streams, and inundate the natural or modified flood plain. These are the most economically damaging floods, affecting the relatively densely-populated coastal river valleys of New South Wales and Queensland (e.g. the Burdekin, Brisbane, Tweed, Richmond, Clarence, Macleay, Hunter and Nepean-Hawkesbury valleys), and the major river valleys of the tropics. While these floods are chiefly caused by summer rains, they can occur in any season. Floods of similar duration also occur in Tasmania, Victoria (particularly rivers draining the north-east ranges) and the Adelaide Hills, although in these latter regions they are more common in winter and spring.
  • long-lived floods of the major inland basins. These floods usually arise from heavy summer rains in inland Queensland and New South Wales, and move slowly downstream, some ultimately draining into the lower Murray-Darling system or towards Lake Eyre. Floods of this type can take several months to move from the upper catchments to the lower Darling or to Lake Eyre. They often cover an extensive area and gradually disappear through a combination of seepage into the sandy soils and evaporation; it is only occasionally that floodwaters of Queensland origin actually reach Lake Eyre. Floodwaters can also cover large areas in situ when heavy rains occur in a region of uncoordinated drainage such as much of western and central Australia. (There is no evidence that Lake Eyre flooding leads to increased rainfall in eastern Australia, with recent research indicating that any observed linkage is an artefact of the tendency of Lake Eyre floods to occur during La Nia years).

Droughts

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 factors such as evaporation and soil moisture are also significant and can be dominant in some situations. Demands for water are very diverse, and droughts therefore can be considered on a variety of timescales. Rainfall in a single year is important for unirrigated crop and pasture growth, while for large water storages and irrigation variations on a multi-year timescale are more important, and a succession of relatively dry years that are not exceptional individually can cause severe water storages when aggregated over an extended period.

While droughts can occur in all parts of Australia, they are most economically damaging in south-eastern Australia (southern Queensland, New South Wales, Victoria, Tasmania and the settled parts of South Australia), an area encompassing about 75% of Australia’s population and much of its agriculture. In south-western Western Australia, another economically and agriculturally significant area, interannual variability of rainfall is smaller than it is in the south-east and severe widespread droughts in individual years are a less important issue, although, in recent decades, this area has experienced a general decline in rainfall (see Climate change).

In terms of rainfall deficits over a 1-2 year period, the most severe droughts on record for eastern Australia have been those of 1901-02, 1982-83, 1994-95 and 2002-03, all of which were associated with El Nio. Occasionally, severe droughts are embedded within more extensive dry periods; the 1901-02 drought was contained within a persistently dry period from 1895-1903 (the so-called 'Federation Drought'). The 2002-03 drought, while not quite as dry over most of eastern Australia as those of 1901-02 or 1982-83, was particularly severe in its impacts for two reasons. First, it was accompanied by record high average maximum temperatures and, consequently, increased evaporation in many areas. Secondly, it affected virtually the entire continent. During earlier droughts the effects over Western Australia were more limited or non-existent. The direct effect of the 2002-03 drought on agricultural production is that it had a downward impact on gross domestic product growth of almost one percentage point between 2001-02 and 2002-03 (see the article in the National Accounts chapter in Year Book Australia 2005). Other notable droughts on the 1-2 year timescale include those of 1888, 1914, 1919-20, 1940-41, 1944, 1946, 1965, 1967 and 1972.

Longer-term periods of persistent below-average rainfall are also often loosely referred to as 'droughts', and apart from that of 1895-1903, have generally been more regional in nature. A typical example of such a long-term drought has occurred over large parts of eastern Australia since 2001, and in some areas, such as southern Victoria (including Melbourne), since 1997. The Sydney region and eastern Queensland have been affected since 1999-2000. The south-west of Western Australia has also experienced a marked downturn in rainfall since 1970. Other extended dry periods of this type affected much of inland Australia between 1958 and 1968, the south-east from 1937-45, and Queensland from 1991-95.

Typically, these multi-year dry episodes are not ones of continuous below-normal rainfall, but rather periods of near-normal rainfall over several months, alternating with drier periods, and few, if any, times of sustained above-normal rainfall to offset the dry periods. Large water storages are particularly susceptible to such events, as they typically rely on a relatively small number of wet years to offset losses during drier periods. The Sydney water supply catchments provide an example of this, with about 40% of the total inflows into the Warragamba catchment since 1910 occurring in the wettest 10% of years.

The period since 2001 has been the driest on record over parts of eastern Australia (see map 1.10), meaning that many large water storages have not recovered from the 2002-03 drought. While rainfall returned to near-normal levels in the second half of 2003 following the severe drought of 2002-03, there have been no periods of sustained above-average rainfall in most of the region since early 2001. For eastern Australia as a whole (defined as the combined areas of Queensland, New South Wales, Victoria and Tasmania), the four-year period from June 2001 to May 2005 was the driest June to May four-year period on record. For Australia’s cropping regions only the period 1911-15 was drier. Conditions in the period 2001-05 are comparable to those of the lengthy drought of the 1940s, although (to date) they have not persisted for as long.

1.10 AUSTRALIAN RAINFALL DECILES - 1 June 2001 to 31 May 2005
1.10: AUSTRALIAN RAINFALL DECILES - 1 June 2001 to 31 May 2005
Source: Australian Bureau of Meteorology.


Adding to the impact of recent dry conditions has been the accompanying increase in temperature. The period from the start of the 2002-03 El Nio event (March 2002) to May 2005 was clearly the warmest such period on record for eastern Australia. Maximum temperatures averaged over Australia were 0.99C above the 1961-90 normal. In contrast, temperatures averaged through the driest periods of the 1940s were near the 1961-90 normal.

Drought definitions, and the area of coverage and length of droughts to that time, together with related information, may be obtained from the article 'Drought in Australia' in Year Book Australia 1988.

Thunderstorms, hail and tornadoes

Thunderstorms are most frequent over northern Australia. Thunder is heard at least once on 80 days or more per year near Darwin, largely as a result of convectional processes during the summer wet season. High frequencies (30 to 50 per year) also occur over the eastern uplands of New South Wales as a result of orographic uplift of moist air streams. Some parts of southern Australia receive fewer than ten thunderstorms per year, with eastern Tasmania receiving fewer than five. Through most of Australia thunderstorms are more common during the warmer half of the year, but along the southern fringe they also occur in winter as a result of low-level instability in cold air masses of Southern Ocean origin.

Some thunderstorms can become severe, with flash flooding, large hail and damaging winds. These storms can be very destructive. The Sydney hailstorm of 14 April 1999, in which hailstones up to nine centimetres (cm) in diameter were observed, was Australia’s most costly natural disaster, with losses estimated at $1.7b. Flash flooding associated with severe thunderstorms has caused loss of life, notably when seven deaths occurred in Canberra on 26 January 1971, and thunderstorms have also been implicated in numerous air crashes, such as when a plane crashed into Botany Bay on 30 November 1961 with the loss of 15 lives.

While thunderstorms in general are most common in northern Australia, the most damaging thunderstorms, in terms of hail and wind gusts, occur in the eastern halves of New South Wales and southern Queensland. Smaller hail (less than 1 cm in diameter) commonly occurs in southern coastal Australia in cold unstable air in the wake of cold frontal passages.

Tornadoes are also associated with severe thunderstorms, although they do not occur with the same frequency or severity as can occur in the United States of America. As tornado paths are narrow it is rare, but not unknown, for them to strike major population centres, with notable examples occurring in Brighton (Melbourne) in February 1918, the southern suburbs of Brisbane in November 1973, and several Perth suburbs in May 2005.

Snow

During most years, 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 those seasons. The area, depth and duration of snow cover are highly variable from year to year. These areas can experience light snowfalls at any time of year. Small patches of snow can occasionally persist through summer in sheltered areas near the highest peaks, but there are no permanent snowfields.

Snowfalls at lower elevations are more irregular, although areas above 600 metres in Victoria and Tasmania, and above 1,000 metres in the New South Wales highlands, receive snow at least once in most winters, as do the highest peaks of Western Australia’s Stirling Ranges. In most cases snow cover is light and short-lived. In extreme cases, snow has fallen to sea level in Tasmania and parts of Victoria, and to 200 metres in other parts of southern Australia, but this is extremely rare. The only major Australian cities to have received a significant snow cover at any time in the last century are Canberra and Hobart, although Melbourne experienced a heavy snowfall in 1849, and there are anecdotal reports of snowflakes in Sydney in 1836.

The heaviest snowfall in Australian history outside the alpine areas was that of 4-5 July 1900, when 50-100 cm fell around Bathurst and in the Blue Mountains, and 25 cm as far west as Forbes (only 240 metres above sea level). Other major widespread low-elevation snow events occurred in July 1901, July 1949 and July 1984. In August 2005, the heaviest low-level snowfalls since 1951 occurred in parts of southern Victoria, with snow falling to sea level in parts of south Gippsland and accumulations of 5 to 20 cm at elevations above 150 metres in the Strzelecki Ranges and Latrobe Valley.

TEMPERATURE

Average temperatures

Average annual air temperatures range from 28C along the Kimberley coast in the extreme north of Western Australia to 4C in the alpine areas of south-eastern Australia. Although annual temperatures can 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. In the south, the months with the highest average temperature are January or February. Due to the increase in cloudiness during the wet season, the month of highest average temperature in the north of the continent is December or, in the extreme north and north-west, November.

Temperature differences between winter and summer are least in tropical Australia. They are greatest in the southern inland, with seasonal differences along the coast being moderated by the ocean’s proximity.

Maps 1.11 to 1.14 show average monthly maximum and minimum temperatures for January and July.

Average monthly maxima

In January, average maximum temperatures exceed 35C over a vast area of the interior and exceed 40C over parts of the north-west. The highest summer maxima occur in the Pilbara and Gascoyne regions of north-western Western Australia, where average January maxima are around 41C; in some years daily maxima exceed 40C for several weeks at a time. Marble Bar experienced 160 consecutive days above 37.8C (100 Fahrenheit) in 1923-24, and Nyang had an average maximum of 44.8C for the months of February 1998 and January 2005, an Australian record. At the other extreme, average January maxima are near 15C on the highest peaks of the south-east ranges and near 20C in much of Tasmania.

In July, a more regular latitudinal distribution of average maxima is evident, ranging from 30C near the north coast to below 3C in the alpine areas of the south-east.

1.11 AVERAGE MAXIMUM TEMPERATURE(a) - January

1.11: AVERAGE MAXIMUM TEMPERATURE(a) - January

(a) Based on the 30-year period 1961-1990.

Source: Australian Bureau of Meteorology.

1.12 AVERAGE MAXIMUM TEMPERATURE(a) - July
1.12: AVERAGE MAXIMUM TEMPERATURE(a) - July
(a) Based on the 30-year period 1961-1990.

Source: Australian Bureau of Meteorology.


Average monthly minima

Average minimum temperatures in all seasons are highest in northern Australia and near the coasts, and are lowest in the mountainous areas of the south-east. The highest average January minimum temperatures (near 27C) are found near the north-west coast, while in winter they exceed 20C 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 5C in January and -5C in July, while most inland areas south of the tropics have average July minima between 0 and 6C.

1.13 AVERAGE MINIMUM TEMPERATURE(a) - January
1.13: AVERAGE MINIMUM TEMPERATURE(a) - January
(a) Based on the 30-year period 1961-1990.

Source: Australian Bureau of Meteorology.


1.14 AVERAGE MINIMUM TEMPERATURE(a) - July
1.14: AVERAGE MINIMUM TEMPERATURE(a) - July
(a) Based on the 30-year period 1961-1990.

Source: Australian Bureau of Meteorology.


Extreme maxima

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 48C. 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 48C. Most Tasmanian sites away from the north coast have extreme maxima between 35 and 40C. The lowest extreme maxima are found along the north coast of Tasmania (e.g. 29.5C at Low Head) and at high elevations (e.g. 27.0C 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 50.5C at Mardie and 49.1C at Roebourne in Western Australia, and 49.4C at Whyalla and 47.9C at Ceduna in South Australia.

Extreme maximum temperatures recorded at selected locations, including the highest recorded in each state/territory, are shown in table 1.15.

Prolonged heat waves, with a number of successive days over 40C, 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 30C in February 2004.

Such prolonged heatwaves are rare in coastal regions, except in Western Australia. The record number of consecutive days in Melbourne over 40C, for example, is five, with Brisbane and Sydney each registering two.

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 (46.1C on the 12th), Melbourne (45.6C on the 13th) and Sydney (45.3C 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.


1.15 EXTREME MAXIMUM TEMPERATURES

Station
C
Date

New South Wales
Wilcannia
50.0
11.1.1939
Victoria
Swan Hill(a)
49.4
18.1.1908
Boort
48.3
13.1.1939
Queensland
Cloncurry(a)
53.1
16.1.1889
Birdsville
49.5
24.12.1972
South Australia
Oodnadatta
50.7
2.1.1960
Western Australia
Mardie
50.5
20.2.1998
Tasmania
Bushy Park(a)
40.8
26.12.1945
Hobart
40.8
4.1.1976
Northern Territory
Finke
48.3
1 & 2.1.1960
Australian Capital Territory
Canberra (Acton)
42.8
11.1.1939

(a) Temperatures known not to have been measured in a Stevenson screen (see Temperature measurement and the Stevenson screen, Year Book 2005).

Source: Australian Bureau of Meteorology.


Extreme minima

The lowest recorded temperatures in Australia have been in the Snowy Mountains of New South Wales, where Charlotte Pass recorded -23.0C on 28 June 1994 (table 1.16), with a number of other locations recording temperatures below -15C. 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 -10C or lower, including Gudgenby, Australian Capital Territory (-14.6C) and Woolbrook, New South Wales (-14.5C). At lower elevations, most inland places south of the tropics have extreme minima between -3C and -7C, and such low temperatures have also occurred in favoured locations within a few kilometres of southern and eastern coasts, such as Sale, Victoria (-5.6C), Bega, New South Wales (-8.1C), Grove, Tasmania (-7.5C) and Taree, New South Wales (-5.0C).

In the tropics, extreme minima near or below 0C have occurred at many places away from the coast, as far north as Herberton, Queensland (-5.0C). Some locations near tropical coasts, such as Mackay (-0.8C), Townsville (0.1C) and Kalumburu, Western Australia (0.3C) have also recorded temperatures near 0C. In contrast, some exposed near-coastal locations, such as Darwin, have never fallen below 10C, and Thursday Island, in the Torres Strait, has an extreme minimum of 16.1C.

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.

Frosts can occur at any time of year over most of Tasmania, much of inland Victoria and south-eastern South Australia, and the higher parts of the tablelands of New South Wales. In these regions the median frost period generally exceeds 200 days, extending out to 300 days in central Tasmania.


1.16 EXTREME MINIMUM TEMPERATURES

Station
C
Date

New South Wales
Charlotte Pass
-23.0
28.6.1994
Victoria
Mount Hotham
-12.8
30.7.1931
Queensland
Stanthorpe
-11.0
4.7.1895
South Australia
Yongala
-8.2
20.7.1976
Western Australia
Booylgoo Springs
-6.7
12.7.1969
Tasmania
Shannon
-13.0
30.6.1983
Butlers Gorge
-13.0
30.6.1983
Tarraleah
-13.0
30.6.1983
Northern Territory
Alice Springs
-7.5
12.7.1976
Australian Capital Territory
Gudgenby
-14.6
11.7.1971

Source: Australian Bureau of Meteorology.


OTHER ASPECTS OF CLIMATE

Humidity

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:
  • over the interior of the continent there is a marked dryness during most of the year, which extends towards the northern coast in the dry season (May-October).
  • the coastal fringes are comparatively moist, although this is less so along the north-west coast of Western Australia where airflow is predominantly off the continent.
  • in northern Australia, the highest values of humidity occur during the summer wet season (December-February) and the lowest during the winter dry season (June-August).
  • in most of southern Australia the highest values are experienced in the winter rainy season (June-August) and the lowest in summer (December-February).

By way of a 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.

Global radiation

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

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.

Evaporation

Average annual pan evaporation exceeds rainfall over most of Australia. It is highest in the north of Western Australia, reaching around 3,400mm around Wyndham, and exceeds 3,000mm 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 800mm per year in the west to 1,200mm in the east. Over the mainland it is below 1,400mm over southern Victoria and adjacent parts of South Australia and New South Wales, and around 1,500mm 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.

Cloud

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.

Fog

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.

Winds

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 coasts 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'.

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. The high frequency of north-west winds at Hobart, for example, is caused by the north-west to south-east orientation of the Derwent River valley, while wave effects on the lee side of the Adelaide Hills can lead to very strong local winds ('gully winds') in the eastern suburbs of Adelaide during periods of general easterly flow.

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 (with Tropical 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 Tropical Cyclone Tracy), 185 km/h at Brisbane Airport and 156 km/h at Perth.

Dust storms

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 Nio-related droughts of 1982-83 and 2002-03 respectively.

Fire weather

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 southeast 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.

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