Stratosphere

see also: Etymology of Stratosphere

The stratosphere is the second layer of the Terrestrial atmosphere, being located above the Troposphère and under the Mésosphère. The temperatures reached in the atmosphere are laminated, i.e. the layers furthest away from the Earth have temperatures lower than those which are brought closer. Stratosphere is located between ten and fifty kilometers of altitude compared to the surface of the Earth according to moderated latitudes, while it starts with approximately eight kilometers of altitude to the poles.

The temperature in stratosphere varies according to altitude, because this one is heated by the absorption of the rays Ultraviolet S coming from the Sun. Inside this layer, the temperature increases as one rises there in altitude (see the article Couche of inversion). At the point highest of stratosphere, the temperature turns around 270 K, which borders the freezing point of water. This part of the layer names the Stratopause, where the temperature starts again to fall when one goes up. This vertical stratification makes so that stratosphere is dynamically stable: there is no regular Convection nor of turbulences associated with this part of the atmosphere. The warming is caused by the ozonosphere, which absorbs the ultraviolet Radiations Sun, which results in to heat the roadbases of stratosphere. The bottom of stratosphere is characterized by a balance between the transmitted heat of the layer of ozone by Conduction and the transmitted heat of troposphere by convection. That implies that stratosphere starts at low altitude close to the poles, because the Température is colder there.

The commercial aircraft fly typically at an altitude close to ten kilometers in moderated latitudes, with the short-nap cloth of stratosphere. This makes it possible to avoid the Turbulence S of the convection present in the Troposphère. The turbulences met during the phase of flight are frequently caused by strong variations of convections coming from troposphere. The majority of the sailplanes fly on Volute S thermics which go up through troposphere above hot currents. These volutes finish at the base of stratosphere, which fixes a limit of altitude at the sailplanes on the whole of the sphere. Some sailplanes manage nevertheless to reach stratosphere by a method called Ridge Lift , which consists in using the strong mountainous winds to rise naturally towards the skies.

Stratosphere is an area where occur of intense radiative processes, dynamic and chemical in which the horizontal mixture of the gas components occurs much more quickly than with the vertical. The quasi biennial Oscillation (OQB) in the tropical latitudes, which is led by the generated waves of gravity in a convective way in troposphere, is an interesting characteristic of the circulation of stratosphere. The OQB induces a secondary, important Circulation for the total stratospheric transport of the tracers such as the Ozone or the Steam.

In the winters of the boreal hemisphere, the sudden stratospheric Avertissements can often be observed, because they are caused by the absorption of the waves of Rossby in stratosphere.

Thinning of the layer of ozone

The leading cause of the thinning of the layer of ozone deferred is the presence of Chlorofluorocarbone S (also known under initials CFC - CCl₂F₂, CCl₃F) in the stratosphere of the Earth. Chlorofluorocarbons are composed of chlorine, fluorine and carbon. Because CFC are stable, economic, not poisons, nonflammable and not corrodents, they are used like propellents, cooling agents, solvents, etc However, it is this stability which causes the omnipresence of CFC in the environment. These molecules reach possibly stratosphere, where they undergo a series of chain reactions which carries out in end of line to the destruction of the Couche of ozone.

The US government banished, in 1980, the use of CFC in the form of aerosol. The world efforts to reduce the use of CFC started in 1987, and an international banishment followed in 1996 to prevent the effects of the industrial production of CFC. These efforts were drastiquement disappointing because of the black-markets in China and Russia, where the value of CFC illegally manufactured climbed to 500 million American dollars. The quantities of CFC in stratosphere continued to increase until the beginning of the year 2000 and one estimates that they will reach an acceptable level towards half of this century.

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