Atmospheric stratification describes the structure of the atmorphere, dividing it into distinct layers, each with specific characteristics such as temperature or composition. The atmosphere has a mass of about 5x1018 kg, three quarters of which is whitin about 11 km (6.8 mi; 36,000ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. An altitude of 120 km (75 mi) is where atmospheric effects become noticeable during atmospheric reentry of spacecraft. The karman line, at 100 km (62 mi), also is aften regarded as the boundary between atmosphere and outer space. This altitude amounts to 1.57% of the earth's radius.
Air is the name given to the atmosphere used in breathing and photosynthesis. Dry air contains roughly (by volume) 78.09% nitrogen, 20,9% oxygen, 0.93% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. While air content and atmospheric pressure vary at different layers, air suitable for the survival of terrestrial plants and terrestrial animals is currently only known to be found in earth's troposphere and artificial atmospheres.
Composition
Air is manly composed of nirtogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases are often referred to as traces gases, among which are the greenhouse gases such as water vapor , carbon dioxide, methane, nitrous oxide, and ozone. Filtered air icludes trace amounts of many other chemical compounds. Many natural substances may be present in tiny amount in an unfiltered air simple, including dust, pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), flluorine compound, elemental mercury, and sulfur compounds such as sulfur dioxide.
Preassure and thickness
The average atmospheric pressure at sea level is about 1 atmosphere (atm)= 101.3kPa (kilopascals)= 14.7 psi (pounds per square inch)= 760 ton= 29.92 inches of mercury (symbol Hg). Total atmospheric mass is
5.1480×10 18 kg (1.135×10 19 lb), [ 15 ] about 2.5% less than would be inferred from the average sea level pressure and the earth's area of 51007.2 megahectares, this protion being displaced by the earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the poin where the pressure varies with location and weather.
If the atmosphere had a unifrom density, it would terminate a bruptly at an altidude ao 8.50 km (27,900 ft). It a actually decreases exponentially with altidude, dropping by half every 5.6km (18,000 ft) or by a factor of 1/e every 7.64 km (25,100 ft), the average scale height of the atmosphere below 70 km (43 mi; 230,000 ft). However, the atmosphere is more a accurately modeled with a customized equation for ea chlayer that takes gradients of temperature, molecular coomposition, solar radiation and gravity into account.
in summary, the mass of the earth's atmosphere is distributed approximately as follows:
- 50% is below 5.5 km (18,000 ft)
- 90% is below 16 km (52,000 ft)
- 99.99997% is below 100 km (62 mi; 330,000 ft), the karman line. By international convention, this marks the beginning of space where human travelers are considered astronauts.
Even above the karman line, significant atmospheric effects such as auroras still occur. Meteors begin to glow in this region though the larger ones may not bum up until they penetrate more deeply. The various layers of the earth's ionosphere, important to HF radio propagation, begin below 100km and extend beyon 500 km. By comparison, the international space station and space shuttle typically orbit at 350-400km, within F-layer of ionosphere where they encounter enough atmospheric drag to require reboosts every few months. Depending on solar activity, satellites can still experience noticeable atmospheric drag at altidudes as high as 700-800 km.
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