The earth’s atmosphere is a layer of gases surrounding the planet earth that is retained by the earth’s gravity. It has a mass of about 5 quadrillion metric tons. Dry air contains roughly (by volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbondioxide and trace amounts of other gases. Air also contains a variable amount of water vapour, on average of about 1%.The atmosphere protects life on earth by absorbing ultaviolet solar radiation, warming the surface through heat retention (greenhouse effect) and reducing temperature extremes between day and night.
3 quarters of the atmosphere’s mass is about 11kms. (6.8 mi;36,000ft) of the surface.The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundry bw the atmosphere and enter space. An altitude of 120km(75mi)marks the boundry where atmosphereic effects becomes noticeable during atmospheric reentry of space craft.The Karman line, at 100km(62mi)also is often regarded as the boundary bw atmosphere and outer space.
STRUCTURE OF THE ATMOSPHERE:-
Principal layers:
Earth’s atmosphere can be divided into five main layers. These layers are mainly determined by whether temperature increases or decreases with altitude. From lowest to highest, these layers are:
Troposphere:
The troposphere begins at the surface and extends to between 7 km (23,000 ft) at the poles and 17 km (56,000 ft) at the equator, with some variation due to weather. Solar heating at the ground causes temperatures to decrease with altitude in the troposphere. This promotes vertical mixing (hence the origin of its name in the Greek word “trope” meaning turn or overturn). The troposphere contains roughly 80% of the mass of the atmosphere. The tropopause is the boundary between the troposphere and stratosphere.
Stratosphere:
The stratosphere extends from the tropopause to about 51 km (32 mi; 170,000 ft). Temperature increases with height, which restricts turbulence and mixing. The stratopause, which is the boundary between the stratosphere and mesosphere, typically is at 50 to 55 km (31 to 34 mi; 160,000 to 180,000 ft). The pressure here is 1/1000th sea level.
Mesosphere:
The mesosphere extends from about 50 km (31 mi; 160,000 ft) to 8085 km (5053 mi; 260,000280,000 ft). Temperature decreases with height, reaching 100 C (148.0 F; 173.1 K) in the upper mesosphere. This is also where most meteors burn up when entering the atmosphere. The mesopause is the temperature minimum that marks the boundary between the thermosphere and the mesosphere. It is the coldest place on Earth, with a temperature of 100 C (148.0 F; 173.1 K).
Thermosphere:
This layer begins at the mesopause and continues to more than 640 km (400 mi; 2,100,000 ft). Temperature increases with height up to the thermopause and then is constant with height. The temperature of this layer can rise to 1,500 C (2,730 F), though the gas molecules are so far apart that temperature in the usual sense is not well defined. The International Space Station orbits in this layer, between 320 and 380 km (200 and 240 mi). The top of the thermosphere is the bottom of the exosphere, called the exobase. Its height varies with solar activity and ranges from about 350800 km (220500 mi; 1,100,0002,600,000 ft).
Exosphere:
The outermost layer of Earth’s atmosphere extends from the exobase upward, and contains free-moving particles that may migrate into and out of the magnetosphere or the solar wind. It is mainly composed of hydrogen and helium. The density of the exosphere is so low that particles can travel hundreds of km without colliding with one another.
OTHER LAYERS:-
Within the five principal layers determined by temperature are several layers determined by other properties.
The stratosphere contains the ozone layer. In this layer ozone concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from about 1535 km (9.322 mi; 49,000110,000 ft), though the thickness varies seasonally and geographically. About 90% of the ozone in our atmosphere is contained in the stratosphere.
The ionosphere, the part of the atmosphere that is ionized by solar radiation, stretches from 50 to 1,000 km (31 to 620 mi; 160,000 to 3,300,000 ft) and typically overlaps both the exosphere and the thermosphere. It forms the inner edge of the magnetosphere. It has practical importance because it influences, for example, radio propagation on the Earth. It is responsible for auroras.
The atmosphere can also be defined by whether the gases in it are well mixed. In the homosphere the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence.Above the the turbopause at about 100 km (62 mi; 330,000 ft) (essentially corresponding to the mesopause), the composition which varies with altitude. This is because the distance that particles can move without colliding with one another is much greater than the size of motions that can cause mixing. This lack of mixing allows the gases to stratify by molecular weight, with the heavier ones such as oxygen and nitrogen present only near the bottom of the heterosphere. The upper part of the heterosphere is composed almost completely of hydrogen, the lightest element.
The average temperature of the atmosphere at the surface of Earth is 14 C (57 F; 287 K) or 15 C (59 F; 288 K), depending on the reference.
PRESSURE AND THICKNESS:-
The average atmospheric pressure, at sea level, is about 1 atmosphere (atm) = 101.3 kPa (kilopascals) = 14.7 psi (pounds per square inch) = 760 torr = 29.9 inches of mercury (symbol Hg). Total atmospheric mass is 5.14801018 kg (1.1351019 lb).
Atmospheric pressure is the total weight of the air above the point at which the pressure is measured. Air pressure varies with location and time, because the amount (and weight) of air above the earth varies with location and time. However, the average mass of the air above a square meter of the Earth’s surface can be calculated from the total amount of air and the surface area of the Earth. The total air mass is 5148.0 teratonnes and area is 51007.2 megahectares. Thus 5148.0/510.072 = 10.093 tonnes (9.934 LT; 11.126 ST per square meter or 14.356 pounds per square inch (98.98 kPa). This is about 2.5% below the officially standardized unit atmosphere (1 atm) of 101.325 kPa or 14.696 psi, and corresponds to the mean pressure not at sea level, but at the mean base of the atmosphere as contoured by the Earth’s terrain.
Were atmospheric density to remain constant with height the atmosphere would terminate abruptly at 7.81 km (25,600 ft). Instead, density decreases with height, dropping by 50% at an altitude of about 5.6 km (18,000 ft). For comparison the highest mountain, Mount Everest, is higher, at 8.8 km (29,000 ft), so air is less than half as dense at the summit than at sea level. This is why it is so difficult to climb without supplemental oxygen.
This pressure drop is approximately exponential, so that pressure decreases by approximately half every 5.6 km (18,000 ft) and by 63.2% (1 1 / e = 1 0.368 = 0.632) every 7.64 km (25,100 ft), the average scale height of Earth’s atmosphere below 70 km (43 mi; 230,000 ft). However, because of changes in temperature, average molecular weight, and gravity throughout the atmospheric column, the dependence of atmospheric pressure on altitude is modeled by separate equations for each of the layers listed above. Even in the exosphere, the atmosphere is still present. This can be seen by the effects of atmospheric drag on satellites.
In summary, the equations of pressure by altitude in the above references can be used directly to estimate atmospheric thickness. However, the following published data are given for reference:
50% of the atmosphere by mass is below an altitude of 5.6 km (18,000 ft). 90% of the atmosphere by mass is below an altitude of 16 km (52,000 ft). The common altitude of commercial airliners is about 10 km (33,000 ft) and Mt. Everest’s summit is 8,848 m (29,030 ft) above sea level. 99.99997% of the atmosphere by mass is below 100 km (62 mi; 330,000 ft), although in the rarefied region above this there are auroras and other atmospheric effects. The highest X-15 plane flight in 1963 reached an altitude of 354,300 ft (108.0 km).
Composition of dry atmosphere, by volume ppmv: parts per million by volume GasVolume Nitrogen (N2) 780,840 ppmv (78.084%) Oxygen (O2) 209,460 ppmv (20.946%) Argon (Ar) 9,340 ppmv (0.9340%) Carbon dioxide (CO2) 383 ppmv (0.0383%) Neon (Ne) 18.18 ppmv (0.001818%) Helium (He) 5.24 ppmv (0.000524%) Methane (CH4) 1.745 ppmv (0.0001745%) Krypton (Kr) 1.14 ppmv (0.000114%) Hydrogen (H2) 0.55 ppmv (0.000055%) Nitrous oxide (N2O) 0.3 ppmv (0.00003%) Xenon (Xe) 0.09 ppmv (9×106%) Ozone (O3) 0.0 to 0.07 ppmv (0% to 7×106%) Nitrogen dioxide (NO2) 0.02 ppmv (2×106%) Iodine (I) 0.01 ppmv (1×106%) Carbon monoxide (CO) 0.1 ppmv Ammonia (NH3) trace Not included in above dry atmosphere: Water vapor (H2O) ~0.40% over full atmosphere, typically 1%-4% at surface
