By volume, dry air contains 78.09% nitrogen, 20.95% oxygen ,0.93% argon , 0.039% carbon dioxide , and small amounts of other gases.And the atmosphere has a mass of about 5.15 × 10 18 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space. There are five main layers and From lowest to highest, the five main layers are: Troposphere: 0 to 12 km (0 to 7 miles), Stratosphere: 12 to 50 km (7 to 31 miles), Mesosphere: 50 to 80 km (31 to 50 miles),Thermosphere: 80 to 700 km (50 to 440 miles), and Exosphere: 700 to 10,000 km (440 to 6,200 miles).

The troposphere is the lowest layer of Earth's atmosphere. It extends from Earth's surface to an average height of about 12 km, although this altitude actually varies from about 9 km (30,000 ft) at the poles to 17 km (56,000 ft) at the equator , with some variation due to weather. The lowest part of the troposphere (i.e. Earth's surface) is typically the warmest section of the troposphere. The troposphere contains roughly 80% of the mass of Earth's atmosphere. The name comes from a Greek word that refers to mixing. And mixing is exactly what happens within the troposphere. The troposphere is denser than all its overlying atmospheric layers because a larger atmospheric weight sits on top of the troposphere and causes it to be most severely compressed. Fifty percent of the total mass of the atmosphere is located in the lower 5.6 km (18,000 ft) of the troposphere. The stratosphere is the second-lowest layer of Earth's atmosphere. It lies above the troposphere and is separated from it by the tropopause . This layer extends from the top of the troposphere at roughly 12 km (7.5 mi; 39,000 ft) above Earth's surface to the stratopause at an altitude of about 50 to 55 km (31 to 34 mi; 164,000 to 180,000 ft). The ionosphere is a region of the atmosphere that is ionized by solar radiation. It is responsible for auroras . During daytime hours, it stretches from 50 to 1,000 km. The atmospheric pressure at the top of the stratosphere is roughly 1/1000 the pressure at sea level. Also although the temperature may be −60 °C (−76 °F; 210 K) at the tropopause, the top of the stratosphere is much warmer, and may be near 0 °C. The mesosphere is the third highest layer of Earth's atmosphere.It extends from the stratopause at an altitude of about 50 km (31 mi; 160,000 ft) to the mesopause at 80–85 km (50–53 mi; 260,000–280,000 ft) above sea level. It is the coldest place on Earth and has an average temperature around −85 °C (−120 °F ; 190 K ). The thermosphere is the second-highest layer of Earth's atmosphere. It extends from the mesopause,at an altitude of about 80 km (50 mi; 260,000 ft) up to the thermopause at an altitude range of 500–1000 km (310–620 mi; 1,600,000–3,300,000 ft). The height of the thermopause varies considerably due to changes in solar activity. The temperature of this layer can rise as high as 1500 °C (2700 °F).Also,this layer is completely cloudless and free of water vapor. The exosphere is the outermost layer of Earth's atmosphere (the upper limit of the atmosphere). It extends from the exobase, which is located at the top of the thermosphere at an altitude of about 700 km above sea level, to about 10,000 km. One of the most essential thing in atmosphere is the Ozone layer. The ozone layer is contained within the stratosphere and 90% of the ozone in Earth's atmosphere is contained in the stratosphere. when sulphur dioxides or nitrogen oxygen oxides combine with water vapours in the atmosphere. As a result they form acids which kills plants, animals, and entire forests. Magnetosphere is located above ionosphere, at the external limit of the Earth’s magnetic field. It behaves like a giant magnet, retaining high energy particles thus protecting Earth. This layer has lowest density of all, as air density gradually decreases as we go further up from the Earth. Atmosphere’s balance is menaced by human activity, which causes green house effect, global warming, air contamination, ozone belt destruction and acid rains. These effects are mainly due to development of industries. The burning of the fossil fuel produces carbon dioxide, the main green house effect gas. Aurorae, one of the most beautiful phenomena in nature is shown here at the bottom of the thermosphere can actually form at any altitude in this atmospheric layer. Auroras are shimmering curtains of light seen at night in the polar regions. They are known as the Northern Lights in the Arctic, and as the Southern Lights in the Antarctic. These spectacular displays are caused by charged particles from the Sun striking the upper atmosphere above the poles. Blue light is scattered more than other wavelengths by the gases in the atmosphere, giving Earth a blue halo when seen from space onboard ISS at a height of 402–424 km. About 20% of the Earth’s population breathes severely contaminated air, especially with carbon monoxide and sulfur dioxide resulting from industrial processes. This increases the number of respiratory conditions, especially among children and elders. The frequency of skin cancer is causes by thinning of ozone layer. This is due to chlorinated hydrocarbons and fluorocarbons from aerosols, fridges, detergents, and polystyrene. These gases rise into the atmosphere and decomposed into chloride ions that destroys the ozone layer. In 1985, a hole in the ozone layer is observed in Antarctica. In 1995, a similar hole was spotted into the ozone layer over the Arctic. Now chlorinated hydrocarbons and fluorocarbons are replaced with other chemicals in the industrial uses.

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