What is air?
The atmosphere and air
There is one thing that everyone does 24 hours a day without even thinking—breathe.
The atmosphere, essential for all life, is something rarely considered and generally taken for granted However, although its importance has risen greatly in the public perception over recent decades from the traditional concerns of local air pollution to the global issues of climate change, global warming and stratospheric ozone depletion. The air people breathe is also described as an 'Ecosystem Service'.
What is the atmosphere?
The atmosphere is the thin envelope of gases surrounding the Earth consisting primarily of nitrogen (78 per cent) and oxygen (21 per cent), with argon, carbon dioxide, trace gases and air pollutants making up the remainder as shown in the diagram (right). This is what is known as air.
Air is bound to the Earth's surface by gravitational force and is at its densest at sea level. The density of the atmosphere decreases as the altitude increases, as does atmospheric pressure. At an altitude of about 5600 m the pressure is half that at sea level while at the summit of Everest (8850 m) the pressure is less than one-third, which is why most mountaineers require additional oxygen to continue climbing.
The higher the altitude, the colder it gets. The mean temperature at sea level is 15 ºC while at 8850 m it is a very cold -40 ºC. This decrease continues up to 17 km above the Earths surface.
The structure of the atmosphere
While there is no definite boundary between the atmosphere and outer space, it becomes gradually thinner as altitude increases, and is usually described as a series of spherical atmospheric layers. A profile of the Earth's atmosphere is shown at right.
Of particular importance to human health and the environment are the troposphere and the stratosphere.
The troposphere is the lowest layer of the atmosphere in which people live and extends out to approximately 12 km. Most weather conditions are confined to this layer, where the atmosphere is relatively unstable and is also the layer where most air pollutants of health concern are found.
Above this is the stratosphere where the temperature rises once again, reaching surface levels at about 50 km. Within the stratosphere is the ozone layer which provides protection from the suns ultraviolet radiation.
Beyond the stratosphere is the mesosphere, a region of decreasing temperature and pressure and the thermosphere where the temperature once again rises. Further out again are the ionosphere, important in the propagation of radio waves, and the exosphere at the edge of space.
Of the Sun's radiation, the atmosphere absorbs around 15 per cent, and reflects around 30 per cent, mainly from clouds, back into space. At the surface, a further 5 per cent is reflected back by snow and ice, while most of the energy (50 per cent) is absorbed by the Earth's surface.
Approximately 20—30 km above the Earth's surface is a part of the atmosphere containing elevated concentrations of ozone. This is known as the ozone layer. The thickness of the layer is not uniform and varies over the seasons and also geographically.
This layer has the capacity to strongly absorb the shorter wavelengths of incoming solar radiation and by doing so provides a protective blanket that shields the Earth's surface from the damaging effects of short-wave ultraviolet radiation (UV-B and UV-C) from the Sun.
Last updated 14 February 2011