Atmosphere

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A WORLD OF AIR

When we talk about the atmosphere, the word meteorology immediately comes to mind. This is in fact a fundamental part of the Climate Engine that we shall explore together.

The atmosphere is a very thin envelope of gas around the Earth (equivalent to a few tenths of a millimetre if we downscale the planet to the size of an orange). This thin outer skin separates us from the vacuum of space. It acts as a screen between the Sun's rays, which are full of energy, and the Earth's surface. The way in which this screen works and its role in radiative exchange with space affects the temperature of the Earth's surface. The composition of the atmosphere therefore defines the climate of our planet, and any alteration to this composition modifies the climate.
Satellites are powerful tools that can be used to characterise* what this thin layer contains, and that can help us to understand how the radiative energy is exchanged between the surface and space via the atmosphere.

californiaImage acquired on October 22nd, 2007 by the MERIS instrument (ENVISAT satellite) above California.
Fierce easterly desert winds blowing smoke from wildfires have fed more than a dozen fires from Santa Barbara to the Mexican border, generating wide clouds of smoke and forcing the evacuation of a quarter of a million people.
© ESA.

Put simply, our atmosphere is made up of two layers: the troposphere, which goes from the surface up to an altitude of around 15 kilometres, and the stratosphere, which goes from 15 to 30 kilometres. The main differences between these two layers concern temperature, chemical composition, wind patterns and clouds. In the troposphere, temperature decreases with altitude, whereas it increases in the stratosphere. Winds are very strong in the stratosphere and mainly horizontal (stratifi ed atmosphere), whereas atmospheric circulation in the troposphere causes mixing (tropo means 'which changes' in Greek) between the top and bottom of the troposphere. Finally, there are no clouds in the stratosphere, whereas they are massively present in the lower part of the atmosphere.
desertThe desert's puff Sand and dust from the Sahara Desert blowing across the Atlantic Ocean along the coasts of Mauritania (top), Senegal (middle) and Guinea Bissau (bottom).
The cloud-covered Cape Verde islands are visible off the coast of Senegal.

This image was acquired on March 29th2008 by the MERIS instrument (ENVISAT satellite).
© ESA.
A sphere difficult to observe
Each sphere of the Earth system has its own time scales. In the hydrosphere, change is measured in years or centuries, while in the cryosphere and the biosphere it is measured in seasons or years. For the atmosphere, change occurs from one hour, day or week to the next. It is the rapid component of the climate engine, and this is what makes it so difficult to observe.

A major advantage of satellites for the study of the atmosphere is the repeated nature of their measurements. More or less operational for nearly thirty years, the series of observations from space dedicated to meteorology thus provides a mine of information for the study of the operation of the climate engine and of ongoing climate change. This operational meteorological flotilla is made up of five geostationary* and two low Earth orbit* satellites operated by meteorological agencies (NOAA, EUMETSAT, JMA), and recently reinforced by a series of experimental satellites prefi guring the climate monitoring systems of the coming decades (such as TRMM dedicated to measuring rainfall).

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