Atmosphere, definition and study. A-Z index of Cognitio.
The Earth’s atmosphere is the gas shell that covers the planet Earth. It has a rather complex structure and is divided into several layers, called spheres, which are low in height order: troposphere, stratosphere, mesosphere, thermosphere, ionosphere and exosphere. The blue color of the sky is due to the blue component of the visible spectrum of solar radiation, increasingly as the air density increases.
The Earth’s atmosphere does not have a well-defined outer boundary: it slowly fades into the interplanetary space, with the color passing progressively from blue to blue to the intense black of the almost empty interplanetary.
It is the layer where almost all-weather phenomena occur and contains 80% of the total gas mass and 99% of the water vapor: the air of the troposphere is heated by the earth’s surface. The air of the lower layers, which tends to rise, generates large convective currents from which constant equatorial winds originate, the rest of the atmospheric circulation and atmospheric disturbances. Ascent to the altitude, in addition to pressure and temperature, also decreases the water vapor content of the air. At some point the temperature stabilizes at about -55 ° C. The word troposphere comes from the Greek τρόπος (trópos) which means “variation, change” precisely because within this sphere are all those vertical and horizontal air motions that scatter the atmosphere itself and characterize the changing weather. The troposphere is also the place of life as well as meteorological phenomena: all plants and all living things live in it using some of the gases that make up it, in addition to benefiting from solar radiation.
It is the atmospheric layer that is above the troposphere and reaches a height of 50-60 km. Here is a phenomenon called thermal inversion: while in the troposphere the temperature decreases with the height, in the stratosphere increases, to the temperature of 0 ° C. This phenomenon is due to the presence of a layer of ozone, the ozone layer, which absorbs most ultraviolet (UV) radiation (about 99%). At some points in the ozone layer, the ozone layer has tapered (a phenomenon of the ozone hole discovered in the Antarctic area) to the point where it no longer offers effective protection against ultraviolet rays, which in these conditions can reach great amount of terrestrial soil. These rays cause serious damage to plants and, in general, to all living beings. Man damage can be skin and blindness due to irreversible retinal damage. In the stratosphere the components are increasingly rare fact, the water vapor and the atmospheric dust diminish; there are still some rare meteorological phenomena and certain types of clouds (such as the mother-of-pearl clouds).
In this area, ranging from 50 to 90 km of altitude, the atmosphere no longer has the influence of the Earth’s surface and is constant at all latitudes. It is characterized by an accentuated rarefaction of gaseous elements and a gradual increase in lighter ones at the expense of the heaviest ones. In this part of the atmosphere the temperature begins to decrease with the height and reaches the minimum value, varying from -70 to -90 ° C, around 80 km; at this point you can observe sometimes cloudy clouds, probably made up of ice crystals and very small dusts: they are visible in the summer, at dusk and appear as thin and brilliant clouds, intensely illuminated by the last rays of the Sun.
Observation of these clouds shows that in the high mesosphere there is a complex system of variable airflow currents, which should reach speeds up to 300 km / h. In this layer originate the “falling stars”, that is the little meteorites that usually can not reach the Earth’s surface and burn before reaching Earth, leaving bright skies. Beyond the mesopause, at about 100 km, the air is so rare that it does not oppose a tangible resistance to the motion of the bodies, and it becomes possible to move with the orbital motion. For this reason, in astronautics, mesosphere is considered the boundary with interplanetary space.
The thermosphere is the layer next to the Mesosphere. The temperature, after lowering in the mesosphere, returns to rise with the quota. Some experimental data state that at a height of about 300 km the temperature would be 1000 ° C. Paradoxically, astronauts at this height need to wear warmed suits so they do not die cold despite the surrounding gas having a temperature higher than that of the jacket and therefore gives it heat. This is because of the reduced density of the gas, the amount of heat that this is able to provide to the suit is significantly lower than what the suit loses by irradiation.
The ionosphere is the atmosphere layer in which the atmospheric gases are strongly ionized: it consists of the outer layers of the atmosphere, exposed to direct solar radiation that tears electrons from atoms and molecules. Contains, as a whole, a minimum fraction of atmospheric gaseous mass, about 1% only (it is extremely rarefied) but has a thickness of several hundred kilometers and absorbs much of the ionizing radiation coming from the space. The temperature in this layer rises to the altitude, for solar irradiation, and reaches 1700 ° C at its outer limit. At the border between the mesopause and the ionosphere, the polar aurora. The ionosphere is of great importance in telecommunications because it is able to reflect radio waves, helping them to propagate beyond the visible range. Between 60 and 80 km, long waves, between 90 and 120, are reflected in the mid-waves 200 and 250 short waves, between 400 and 500 km long waves.
It is the most external part of the Earth’s atmosphere, where chemical composition changes radically. Within it, an ideal line can be identified as a gravitational boundary (atmospheric fringe): in an exact correspondence of this line, an object without its own kinetic force remains stationary (neither falls to the Earth nor ascends to the space); an object present under this line is still subject to the earthly severity of a landfall; the closer you approach this line, the more the fall is slow, to become almost imperceptible; passing the object, the object ascends to the space or moves away from Earth.
Its constituents, as mentioned above, are mostly hydrogen and helium, mostly particles of solar wind captured by earth magnetosphere. Indirect observation methods and theoretical calculations show that the temperature of the exosphere increases with the height to reach, if not even exceed, 2000 ° C. Due to this temperature, some of the present molecules reach the Earth’s escape speed (11.2 km / s) and escape from the atmosphere, losing space.
The present chemical composition of the atmosphere is the result of an evolution of the same from the primordial times: volcanic activity, photosynthesis, solar radiation action, oxidation processes and microbial activity have changed over time composition until the achievement of the current equilibrium. Up to 2.45 billion years ago, the earth’s atmosphere was devoid of oxygen (O2): its presence in the modern atmosphere is due to the photosynthesis initially operated by cyan bacteria, to which algae and plants have been added. As the first photosynthetic organisms released oxygen, this went to oxidize the rocks of the Earth’s surface. Once the easily oxidize substances are exhausted, oxygen has begun to accumulate in the Earth’s atmosphere, initially in modest quantities, then (from 850 million years ago) the oxygen concentration rises (with different fluctuations) to the values current.
Atmosphere, definition and study. A-Z index of Cognitio.
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