A clear, cloudless atmosphere is made up of air, which is itself made up of 78% nitrogen, 21% oxygen and 1% of argon and elements called 'trace gases'. Among the latter, there is, by order of importance, water vapour, carbon dioxide, methane, sulphur dioxide and ozone. These gases absorb the infrared radiation emitted by the Earth, preventing this energy from escaping directly from the Earth's surface to space, and warming up the atmosphere.
Hence the name ' greenhouse gases '.
Despite their extremely small concentrations, these gases determine a large part of the climate. Although some of them take part in complex chemical reactions while others remain more passive, they are all transported by winds and clouds, thus establishing a very close coupling* between atmospheric composition and wind patterns. Ozone is found especially in the stratosphere. The destruction by CFCs of this gas, which absorbs incident solar radiation in the ultraviolet, caused the well-known ozone hole.
Hence the name ' greenhouse gases '.
Despite their extremely small concentrations, these gases determine a large part of the climate. Although some of them take part in complex chemical reactions while others remain more passive, they are all transported by winds and clouds, thus establishing a very close coupling* between atmospheric composition and wind patterns. Ozone is found especially in the stratosphere. The destruction by CFCs of this gas, which absorbs incident solar radiation in the ultraviolet, caused the well-known ozone hole.
This map shows the climatology of tropospheric humidity expressed in percentage. Measurement acquired on July 4th, 2005 by the so-called water-vapour channel of the SEVIRI instrument on board the Meteosat/MSG-1 satellite. The moist air filaments appear in blue/ red, mixed with dry air in green. They disclose the very fluid nature of atmosphere. The clouds, masked, are in the white part.
Courtesy Hélène Brogniez/LMD.
Political action at the end of the 1970s succeeded in halting this process, and the hole is slowly and naturally closing up again. In the troposphere one can also fi nd the aerosols. These can be of natural or anthropogenic origin. These solid particles, about a micron across, are suspended in the air, and can be mineral (dust from deserts that forms plumes) or organic (such as soot caused by the burning of wood, petroleum or coal). These particles block solar radiation, and by a parasol effect, cool the surface. Aerosols in suspension in the atmosphere are found mainly in the Northern hemisphere due to industry. The accumulated effect is visible on a global scale, and it is even thought that it may mask part of the expected climate warming caused by the increase of greenhouse gases in the atmosphere.
Courtesy Hélène Brogniez/LMD.
Political action at the end of the 1970s succeeded in halting this process, and the hole is slowly and naturally closing up again. In the troposphere one can also fi nd the aerosols. These can be of natural or anthropogenic origin. These solid particles, about a micron across, are suspended in the air, and can be mineral (dust from deserts that forms plumes) or organic (such as soot caused by the burning of wood, petroleum or coal). These particles block solar radiation, and by a parasol effect, cool the surface. Aerosols in suspension in the atmosphere are found mainly in the Northern hemisphere due to industry. The accumulated effect is visible on a global scale, and it is even thought that it may mask part of the expected climate warming caused by the increase of greenhouse gases in the atmosphere.
 Image taken by the AVHRR instrument on board the MetOp-A satellite, China. © EUMETSAT 2008 |
At lower altitudes, near the surface in ocean regions, measuring atmospheric humidity is easier. In addition, passive microwave sensors such as those on board the SSM/I satellites of the US defence enable us to go back about twenty years. This long, validated series of measurements confi rms the foundations of the theory of climate change.
However, the atmosphere is not restricted to these layers of air, whether they are dirty or humid, clean or dry. It is also a highly active part of the water cycle, which links the hydrosphere, the source of water, with the biosphere and the cryosphere. It is the realm of clouds, storms, fronts and squalls, pounding the surface of the planet with water, snow and hail, irrigating crops and feeding rivers, but also sometimes causing damage.
What happens to a water molecule as it travels round the water cycle? It is pulled out of the ocean and then transported by the wind in the form of vapour. It then rises through the atmosphere in a cloud. As it rises it cools, which changes the water vapour into liquid or solid water.
However, the atmosphere is not restricted to these layers of air, whether they are dirty or humid, clean or dry. It is also a highly active part of the water cycle, which links the hydrosphere, the source of water, with the biosphere and the cryosphere. It is the realm of clouds, storms, fronts and squalls, pounding the surface of the planet with water, snow and hail, irrigating crops and feeding rivers, but also sometimes causing damage.
What happens to a water molecule as it travels round the water cycle? It is pulled out of the ocean and then transported by the wind in the form of vapour. It then rises through the atmosphere in a cloud. As it rises it cools, which changes the water vapour into liquid or solid water.
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