Friday, June 17, 2011

layers of the atmosphere

The Earth's atmosphere

The Earth has an atmosphere more complex and divided into layers, which are in order of height: Troposphere, Stratosphere, Mesosphere, Thermosphere, exosphere, and its average chemical composition and the layers of the atmosphere - 27 Kla follows:

    Nitrogen (N2): 78.08%
    Oxygen (O2): 20.95%
    Argon (Ar): 0.93%
    Water vapor (H2O): 0.33%
    Carbon dioxide (CO2): 0.032% (320 ppm)
    Neon (Ne): 0.00181% (18 ppm)
    Helium (He): 0.0005% (5 ppm)
    Methane (CH4): 0.0002% (2 ppm)
    Hydrogen (H2): 0.00005% (0.5 ppm)
    Krypton (Kr): 0.000011% (0.11 ppm)
    Xenon (Xe) 0.000008% (0.08 ppm)
    Ozone (O3) 0.000004% (0.04 ppm)

They are also present in trace amounts of nitrogen oxides (NO, NO2, N2O), carbon monoxide (CO), ammonia (NH3), sulfur dioxide (SO2), hydrogen sulfide (H2S).

Not all layers have the same concentration of gas such as water vapor is present almost exclusively in the troposphere, the lowest layer, and is practically absent in the thermosphere and exosphere, which contain almost all the helium back and ' hydrogen, and ozone is contained mostly in the stratosphere, which is an important layer.


It is the layer where they occur almost every weather and contains 80% of the total gas mass and 99% of water vapor the air in the troposphere is heated by the earth's surface and has a global average temperature of 15 ° C at sea level, which decreases with altitude (0.65 ° C per 100m above sea level) down to about -60 ° C of the tropopause. The lower layers of air that tends to generate large convection currents rising from which they originate constant equatorial winds (trade winds) and the atmospheric disturbances. The troposphere has a variable thickness depending on the latitude to the poles is only 8 km thick and reaches 18 km at the equator. The atmospheric pressure decreases with altitude according to an exponential law, beyond the 7-8 km altitude the pressure is so low that you can not breathe without the use of masks attached to oxygen tanks.
Climbing upwards, as well as pressure and temperature, also decreases the water vapor content of air. At one point the temperature stabilizes at -60 º C. It is the tropopause, the transition zone between the troposphere and stratosphere.


It is the atmospheric layer that lies above the troposphere, and reaches a height of 50-60 km. Here is a phenomenon called temperature inversion: that is, while in the troposphere temperature decreases with height, increases in the stratosphere, up to a temperature between -3 and 0 ° C. This is due to the presence of an ozone layer (triatomic oxygen molecule), the ozone layer, which absorbs most solar UV radiation. In the stratosphere the components have increasingly rarefied, water vapor and dust fall, there are still some rare meteorological phenomena, and certain special types of clouds (cirrus).


In this area, ranging from 50 to 80 km altitude, the atmosphere is no longer sustained the influence of the earth's surface, and is constant at all latitudes. There are no more and no wind or air currents, or clouds or disturbances: the air was completely calm. Under these conditions, the gases are stratified by diffusion, and the average chemical composition of the air begins to vary as you climb. Carbon dioxide and water vapor rapidly disappears even more quickly, and the percentage of oxygen begins to decrease with altitude. Increase the percentage of light gases like helium and hydrogen. The warming effect of ozone is finished, and the temperature decreases with increasing altitude until it stabilizes, the upper limit of the mesosphere, at about -80 º C (mesopause).
This layer produces the shooting stars, ie the small meteorites that usually fail to reach the surface and burn before they reach Earth, leaving bright trails. Beyond the mesopause, at an altitude of 100 km, the air is so thin as not to put up a resistance to the motion of visible bodies, and it becomes possible to move with the orbital motion. For this reason, the mesopause in astronautics is considered the edge of space.


Beyond the mesopause begins the thermosphere. In this layer the gases are so rarefied that it is more appropriate to speak of atoms and molecules, which receive almost all of the direct solar radiation and are therefore mostly ionized state (together with the upper layers of the mesosphere, the thermosphere is the ionosphere) . The temperature in this layer rises with altitude, the sunlight, and can reach 1727 º C at its outer edge. Mesopause and thermosphere on the border between the northern lights occur.
The chemical composition is still similar to the average, with a predominance of nitrogen and oxygen, but it changes with increasing altitude. At about 550 km altitude, these two gases cease to be the main components of the atmosphere, and are displaced by helium and hydrogen.


And 'the outer part of Earth's atmosphere, where the chemical composition changes dramatically. The exosphere has no real upper limit, coming to understand even the Van Allen belts. His constituents, as already mentioned, are mostly hydrogen and helium, the majority of solar wind particles captured by the Earth's magnetosphere.


It is the layer of atmosphere where the atmospheric gases are highly ionized: it is constituted by the outer layers of the atmosphere, exposed to direct solar radiation which tears electrons from atoms and molecules. Contains, as a whole, only a small fraction of the atmospheric gaseous mass, about 1% only (extremely thin), but has a thickness of some hundreds of kilometers and absorbs most of the ionizing radiation from space.
The ionosphere is of great importance in telecommunications, because it can reflect radio waves helping to spread beyond the range visible on the ground, this phenomenon is particularly strong for the radio frequency range between 3 and 30 MHz, the so-called shortwave. With increasing frequency also increases the degree of ionization (ion concentration) necessary to support propagation.
The ionosphere has a band structure, divided during the day with strong solar radiation that ionizes preferentially different gases at different heights: During the night some of these bands come together, increasing the reflectivity of the ionosphere radio.

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