Structure of the Atmosphere

The atmosphere is a very thin and fragile skin around the earth. It consists of oxygen and other gases, held in place by the earth's gravity, that together form the air on which life depends.

Daily weather patterns are the most obvious examples of the planet's constantly changing atmosphere, but previous temperature changes over millions of years have resulted in a number of glacial periods. These periods have left a variety of legacies in the Tasmanian landscape, such as sharp mountain ranges, cirques, moraines and many lakes.

The composition of the atmosphere has also evolved over billions of years and it has not always allowed the earth to support life. Billions of years ago the atmosphere was extremely violent, and made up primarily of water vapour, carbon dioxide, nitrogen, hydrogen, sulfur dioxide, carbon monoxide, ammonia, methane, and hydrogen sulfide. These days, around 99% of the atmosphere near the earth's surface is made up of nitrogen and oxygen, with all the other gases making up the last 1%. Some of these gases are reasonably constant, while others may vary considerably as indicated in the table below.

Composition of Air Near the Earth's Surface

Gases	Formula	% by Volume
Nitrogen	N2	78.08
Oxygen	O2	20.95
Argon	Ar	0.93
Neon	Ne	0.001 8
Helium	He	0.000 5
Hydrogen	H2	0.000 05
Xenon	Xe	0.000 009
Carbon dioxide	CO2	variable (average 0.036)
Methane	CH4	variable (average 0.000 1)
Ozone	O2	variable (polluted air average 0.000 004)
Carbon monoxide	CO	variable (polluted air average 0.000 02)
Sulfur dioxide	SO2	variable (polluted air average 0.000 001)
Nitrogen dioxide	NO2	variable (polluted air average 0.000 001)
Particles (dust etc.)		variable (polluted air average 0.000 01)
Water vapour	H2O	variable (up to 4% in some areas)

Source: adapted from Crowder 1995

Much of the water vapour once found in the atmosphere condensed as the planet cooled billions of years ago, forming the seas and oceans that now cover 71% of the earth's surface. Most of the carbon dioxide has either dissolved into the oceans, been converted into such features as coal and limestone, or been incorporated into plants and animals.

Over 70% of the mass of the atmosphere is in the lower atmosphere - the troposphere (see figure below) - which extends from the ground to between 11 and 15 km above the earth's surface. It is primarily within this layer that clouds occur and most of our weather takes place. The atmosphere peters out several hundred kilometres from the earth's surface into interplanetary space, but the 'skin' it forms is thin when compared to the earth's diameter of over 12, 500 km.

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The ozone layer, which protects the earth's surface from harmful solar radiation (see figure below), is mainly concentrated between 15 and 30 km above the earth's surface within the stratosphere. Absorption of ultraviolet radiation by ozone in this layer causes the temperature to rise. The absorption of shorter wavelengths of ultraviolet radiation by oxygen molecules in the thermosphere causes the temperature to rise there too. Above the thermosphere, the lack of any air means that temperature has little meaning.

Nitrogen and oxygen, the main components of the atmosphere, are almost transparent to the shortwave solar radiation that reaches the earth from the sun. Much of this radiation is absorbed by the land and oceans (i.e. they are heated), but a significant amount is reflected by clouds, snow and ice. The hydrological cycle and the broad atmospheric and ocean circulation patterns help to redistribute the energy to areas receiving less shortwave radiation. Energy is also radiated back from the earth as longwave radiation (heat), maintaining the balance with the incoming solar radiation, as indicated in the figure below.

Water vapour, carbon dioxide and many other gases absorb some of the radiation reaching the earth from the sun, and much of the longwave infrared radiation emitted by the earth. This natural process known as the greenhouse effect stores heat in the atmosphere. Without it, the earth's average surface temperature would be about 33°C lower than at present (Lahane 1995).

Humans are now putting added pressure on the atmosphere so that changes are occurring at a faster rate than at any time in the past. The effects of human induced change are apparent through the Enhanced Greenhouse Effect and Ozone Depletion. Air quality has also degraded in particular areas of Tasmania and is discussed in detail within the Atmosphere Chapter.

Nature of Climate

The climate of the earth is driven by the sun. Differences in heating at the poles and the equator, and between land and sea, create winds, which transfer energy around the globe (as shown in the figure below).

The climate of Tasmania is largely determined by its position at the northern edge of the band of westerly winds known as the 'Roaring Forties'. However, the continental landmass of mainland Australia to the north also has a significant influence. Cold air from the Southern Ocean can bring freezing winds and snow to the State, while air heated over the Australian mainland can result in hot, dry winds.

Nowhere in Tasmania is further than 115 km from the coast, and the surrounding ocean moderates the island's climate. Thus, the climate is classed as temperate maritime, where summers are generally mild and winters are cool to cold.

The lowest temperature recorded by the Bureau of Meteorology in Tasmania (-13.0°C) was at Shannon, Tarraleah and Butlers Gorge on 30 June 1983, while the highest temperature was 40.8°C at Hobart on 4 January 1976 (BoM 1999).

Air flow over the State is generally from the west, which results in cloudy and wet conditions in the south and west. Although more likely in winter, snow is possible at all times of the year. The north and east are normally much drier under these conditions.

However, low and high-pressure systems (cyclonic and anticyclonic systems) embedded in the flow can change the winds, providing a variety of weather conditions. Easterly winds usually result in wet conditions through the east, and finer conditions in the west. These are more common during spring and summer. Northerly airflow can occur during the summer months and produces hot, dry winds over much of the State.

During spring in particular, northerly winds are sometimes associated with low-pressure systems approaching the State. These winds can bring heavy rains to the north, often causing flooding in rivers such as the Mersey, Forth, and North and South Esk rivers.

Variations in topography (mountains, hills and valleys) also affect wind direction. For example, Hobart and Launceston rarely experience pure westerly winds, as the Derwent and Tamar valleys respectively change the wind's direction (see figures below). Understanding the surface wind pattern is important for determining the dispersion patterns of, for example, factory emissions, vehicle exhaust and agricultural sprays, and the effect of windbreaks on evaporation rates over agricultural areas.

The topography also modifies other aspects of the climate, especially temperature and precipitation. The prevailing westerly airflow from the Southern Ocean is generally moisture-laden. When it is forced to rise over the western, central and southern highlands, it cools and releases much of its moisture as rain (and snow). This pattern occurs throughout the year, resulting in high precipitation levels, but is more pronounced during winter and early spring. By the time the air flow gets to the eastern side of the State, it has released most of its moisture, and the chance of rain is reduced (see figure below). Hence, there is a marked difference in average cloudiness and rainfall across the State. Some areas in the east receive an annual average of around 500 mm, while parts of the west receive over 3,000 mm. At times, the east coast and adjacent ranges experience heavy rainfall. Typically, this happens when a significant low-pressure system is located in Bass Strait or the Tasman Sea, and moisture-laden winds are forced to rise over the land.

The topography also has an important influence in summer, especially during bushfires. The northerly sides of hills receive more sun, so tend to be drier than south-facing slopes. In addition, fires move much more quickly uphill than they do downhill. When combined with the northerly winds that are often experienced in summer, the result can be a very fast spread of fire. These conditions were particularly evident during the 1967 bushfires when the spread of fires was extremely rapid.

The topography also affects temperature. Generally, temperatures drop about 1°C for every 100 m rise in altitude. However, this can vary at different sites and with the time of day and year, causing features such as cold air drainage and frost hollows. Such features are particularly evident during still winter conditions when temperature inversions (see figure below) are created.

The inversion acts as a lid, keeping any pollutants released into the lower layer close to the surface, which can be exacerbated when inversions occur in valleys (see figure below). Further information may be obtained on temperature inversions and their influence on air quality problems in the Meteorological Conditions Issue Report.

Natural climate variability through time

The climate is not just variable across space (geographically); it is also variable over time. Climate is defined as the weather conditions experienced in a place, averaged over a 30 year period, while climate variability is generally measured over much longer time periods. Significant variations have been measured over the last million years (see graphs below), with alternation between lower temperatures (glacial periods) and warmer times (interglacials). The changes are caused by a complex interaction of many factors, but mainly by variations in the amount of solar radiation reaching the earth, which affects the amount of heating of the earth's surface. Volcanic activity can affect the amount of radiation reaching the earth's surface, because volcanic dust blocks out sunshine and causes temperatures to cool.

During the last glacial, which had its peak around 20,000 years ago, global average temperatures were probably only around 5°C lower than they are now (Clark and Cook 1994). While that may not seem much, it was enough to leave most of the Central Highlands covered by an icecap (see map below). Many other areas also had small icecaps and glaciers, or were subject to periglacial processes such as freeze-thaw cycles.

With so much water locked up in ice and snow during the glacial periods, the level of the oceans was lower, and the atmosphere generally drier. Many of the sand dune complexes in the north-east were formed during this time, when the dry, sandy plains were blown around by the more frequent and stronger winds. Plants and animals unable to adapt to such different climatic conditions survived in small areas that still remained suitable (refugia), from which most expanded their range again during the interglacial periods.

The earth's climate has also varied over the last few hundred years. From the 15th to 18th centuries there was a 'little ice-age', with average temperatures marginally lower than either before or after. Most of this variation occurred for reasons that are natural. However, there is evidence to suggest that some of the climatic cycles are now being modified by emissions produced by humans (see the Enhanced Greenhouse Effect and Ozone Depletion Issue Reports).

The Southern Oscillation and El NiÑo

Australia is affected by the climatic phenomenon known as the Southern Oscillation (BoM 1998). Much drier conditions are experienced in the Australian region during the most extreme events of the Oscillation, known as El NiÑo events (see map below), with droughts throughout most of the country as indicated in the map of El NiÑo-related drought areas. Tasmania is at the edge of the area affected by the Southern Oscillation, but nevertheless experiences similar problems to the rest of the east coast of mainland Australia.

The Southern Oscillation is a major air pressure shift between the Asian and east Pacific regions, resulting in changes to the circulation of air, known as the 'Walker circulation' (see figure below). The Walker circulation is named after Sir Gilbert Walker, a Director-General of British observatories in India. Early this century, he identified a number of relationships between seasonal climate variations in the Asia and Pacific region.

The Humbolt Current brings cold water northward along the South American coast. It then moves to the west along the equator, where it is heated by the sun, raising the temperature by as much as 8°C. The air moving over these waters is warm and moist, and rises to high levels in the atmosphere. The rising air is associated with a region of low pressure, towering cumulonimbus clouds and heavy rains. The air then travels eastward, cools, and sinks over the eastern Pacific Ocean.

During the other extreme of this circulation pattern, known as El NiÑo, the Walker circulation patterns weaken, and the seas around Australia cool. The trade winds slacken and feed less moisture into the Australian/Asian region, resulting in a high probability of drought in Australia.

However, the weaker East Australian Current means that the interface between the warmer waters and the colder Antarctic waters (the Sub-Tropical Convergence) moves northward into the Tasman Sea. The Sub-Tropical Convergence is a zone of high biological productivity, important for many commercial fisheries. For example, during El NiÑo events, jack mackerel become more abundant around Tasmania.

El NiÑo episodes also affect people in other countries. Along the South American coast, off Ecuador and Peru, there is normally an upwelling of colder, nutrient-rich waters from the deeper ocean to the surface. El NiÑo ('the boy child' in Spanish-a reference to Christ) comes around Christmas time. The upwelling weakens and warmer currents spread along the coast, bringing rain to one of the driest coastlines in the world. Nevertheless, for many people in Peru and Ecuador, El NiÑo episodes are not as welcome as the name may suggest. The cold, nutrient-rich waters support abundant plankton, which in turn support a large fishing industry. The warmer currents lack nutrients, and fish and other marine life die due to the lack of food. Severe El NiÑos can devastate the fishing industry in the region.

The status of the Walker circulation pattern is measured by the Southern Oscillation Index. It is calculated monthly, based on the difference in air pressure between Tahiti and Darwin. The long-term average of the Southern Oscillation Index is zero. When it is positive, the Walker circulation is at its strongest, and stronger Pacific trade winds and warmer seas to the north of Australia mean that eastern and northern Australia will probably have wetter than normal conditions. The Southern Oscillation Index is strongly negative during an El NiÑo episode, and the Walker circulation is at its weakest. The seas around Australia cool, the trade winds slacken, and less moisture is fed into the Australasian region, increasing the probability of drought in Australia. The Southern Oscillation Index is measured as one of the indicators of the Climate Variability Issue Report.

The nature of Tasmania's climate is highly variable across space and time. Understanding the way in which the many climatic factors interact on both global and local scales aids the sustainable management of the Tasmanian environment. Indicators of climate variability are discussed in relation to improving environmental management in Tasmania in the Climate Variability Issue Report.