Atmosphere of Earth

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**1. Composition of Earth’s Atmosphere:**
– Consists of gases like nitrogen, oxygen, argon, and water vapor.
– Includes trace gases such as carbon dioxide, methane, nitrous oxide, and ozone.
– Contains noble gases like neon, helium, krypton, and xenon.
– Also includes pollutants like sulfur compounds and industrial gases.

**2. Layers of Earth’s Atmosphere:**
– Troposphere: Lowest layer where weather occurs, contains 80% of atmosphere mass.
– Stratosphere: Contains the ozone layer, stable with rising temperatures.
– Mesosphere: Coldest layer with phenomena like noctilucent clouds and lightning.
– Thermosphere: Contains the ionosphere, temperature increases with height.
– Exosphere: Outermost layer with particles escaping into space.

**3. Evolution of Earth’s Atmosphere:**
– First atmosphere: Primarily hydrogen from the solar nebula.
– Second atmosphere: Formed from outgassing and asteroid impacts, nitrogen dominant.
– Third atmosphere: Oxygen accumulation over time, leading to an oxidizing atmosphere.

**4. Atmospheric Properties:**
– Pressure and Thickness: Atmospheric pressure decreases with altitude, with distinct layers.
– Temperature: Varies with altitude, stabilizing in some layers due to specific factors.
– Density and Mass: Atmospheric mass and density change with altitude.
– Optical Properties: Interaction with solar radiation, scattering, absorption, and emission.

**5. Atmospheric Circulation and Pollution:**
– Circulation: Large-scale movement of air distributing heat around Earth.
Air Pollution: Introduction of harmful substances into the atmosphere, leading to issues like ozone depletion and global warming.
– Impact of Human Activity: Greenhouse gas emissions since 1750 have contributed to rising global temperatures.

The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth creates pressure, absorbs most meteoroids and ultraviolet solar radiation, warms the surface through heat retention (greenhouse effect), and reduces temperature extremes between day and night (the diurnal temperature variation), maintaining conditions allowing life and liquid water to exist on the Earth's surface.

Blue light is scattered more than other wavelengths by the gases in the atmosphere, surrounding Earth in a visibly blue layer at the stratosphere, above the clouds of the troposphere, when seen from space on board the ISS at an altitude of 335 km (208 mi) (the Moon is visible as a crescent in the far background).

As of 2023, by mole fraction (i.e., by number of molecules), dry air contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. Air composition, temperature, and atmospheric pressure vary with altitude. Within the atmosphere, air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere.[citation needed]

Earth's early atmosphere consisted of gases in the solar nebula, primarily hydrogen. The atmosphere changed significantly over time, affected by many factors such as volcanism, life, and weathering. Recently, human activity has also contributed to atmospheric changes, such as global warming, ozone depletion and acid deposition.

The atmosphere has a mass of about 5.15×1018 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes 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. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi). Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

The study of Earth's atmosphere and its processes is called atmospheric science (aerology), and includes multiple subfields, such as climatology and atmospheric physics. Early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann. The study of historic atmosphere is called paleoclimatology.

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