|यो एउटा अनाथ (एक्लो) पृष्ठको रुपमा छ, यो पृष्ठ कुनै पनि पृष्ठसित नजोडिएको अथवा एक-दुइवटा वटासित मात्र जोडिएको हुन सक्छ। कृपया सम्बन्धित लेखबाट; यस पृष्ठसित लिङ्क जोड़्न सहायता गर्नुहोस् सुझाव यहाँ उपलब्ध हुनसक्छ। (सेप्टेम्बर २०११)|
पृथ्वीलाई बेरेर राख्ने जुन स्थानमा वायु छ त्यसलाई वायुमंडल भनिन्छ। वायुमंडलका अतिरिक्त पृथ्वीको स्थलमंडल (Litho sphere) ठोस पदार्थले बनिएको, र जलमंडल (Hydro sphere) जलले बनिएको छ। वायुमंडल कति टाडासम्म फैलेकोछ यो कुराको सही जानकारी मिल्न नसकेता पनि यो निश्चित छ कि पृथिवीको चारैतिर सयौं मायलसम्म यो फैलेको छ। वायुमंडलको तल्लो भाग (जो प्राय: चारदेखि आठ माइलसम्म फैलेकोछ) क्षोभमंडल (Troposphere), त्यसमाथिको भाग समतापमंडल (Stratosphere) र त्यसमाथि अझ माथिको भागलाई आयनमंडल (Ionosphere) भनिन्छ। क्षोभमंडल र समतापमंडलको बीचको बीच भागलाई "शांतमंडल" (Topopause) र समतापमंडल र आयनमंडलको बीचलाई स्ट्रैटोपज (Stratopause) भनिन्छ। साधारणतया माथिको तह धेरै शान्त रहन्छ।
प्राणी र वनस्पतिको जीवनपोषणका निम्ति वायु अत्यावश्यक हुन्छ। पृथ्वीतलका अपक्षय (weathering)माथि पनि यसको गहिरो प्रभाव पर्छ। नाना प्रकारका भौतिक र रासायनिक क्रियाहरु वायुमंडलको वायुको कारणले नैं हुन्छन्। वायुमंडलका अनेक दृश्य, जस्तै इन्द्रेणी, बिजुली चमकिनु र अकाश गढ्गढाउनु, उत्तर ध्रुवीय ज्योति (aurora borealis), दक्षिण ध्रुवीय ज्योति (aurora australis) प्रभामंडल (halo), किरीट (corona), मरीचिका इत्यादि प्रकाश वा विद्युतको कारण उत्पन्न हुनेगर्छ्न।
वायुमंडलको घनत्व एक समान रहँदैन। समुद्रतलमा वायुको दबाव यति धेरै हुन्छ कि यसले पाराको खाँबालाई २९.९२ इंच वा ७६ सेंटीमीटर उठाउन सक्छ। प्रति वर्ग इंच १४.५ पाउंड दबावको बराबरी हुन्छ। माथि उठ्ता दबावमा कमी हुँदैजान्छ। ताप अथवा स्थान परिवर्तन हुँदा दबावमा अन्तर आउने गर्छ।
सूर्यको लघुतरंग विकिरण ऊर्जाले पृथ्वी तात्छ। पृथ्वीबाट दीर्घतरंग भौमिक ऊर्जाको विकिरण वायुमंडलमा अवशोषित हुन्छ। जसबाट वायुमंडलको ताप - ६८रू सें. ५५रू सें. को बीचमा रहन्छ। ६० माइलदेखि माथि पराबैंगनी (ultraviolet) प्रकाशको कारणले आक्सीजन अणु आयनमा परिणत हुन्छ र परमाणु इलेक्ट्रोनमा। यस कारण यस मंडललाई आयन मंडल भनिन्छ। रात्रिको समयमा यो आयन वा इलेक्ट्रोन फेरि परस्पर मिलेर अणु अथवा परमाणुमा परिणत हुनेगर्छ जसबाट राति प्रकाशको वर्णपटमा हरिया र राता रेखाहरु देखिन्छन्।।
वायुमंडल संगठन [सम्पादन गर्ने]
वायुमंडलको वायु ग्याँसको मिश्रण हो। यस वायुमण्दलाई पृथ्वीको गुरुत्वाकर्षण शक्तिले थामेर राखेकोछ। पृथ्वीको वायुमण्डलले सम्पूर्ण प्राणी र उद्भिद्हरुलाई जीवित राख्न पराबैंगनी (ultraviolet) सौर विकिरण आदिबाट जोगाउनका साथै पृथ्वीलाई दिउँसो हुने अत्यधिक गर्मी र रातिको अत्यधिक जाडोलाई सन्तुलन गराई बचाउने काम गर्दछ। सुख्खा हावामा ७८.०९% नाइट्रोजन, २०.९५% अक्सिजन, ०.९३% अर्गोन, ०.०३९% कार्बन डाइ अक्साइड, र केही मात्रामा अन्य ग्यासहरु र केही मात्रा पानीको वाष्पकणहरु हुन्छन्।
|ppmv: parts per million by volume (note: volume fraction is equal to mole fraction for ideal gas only, see volume (thermodynamics))|
|नाइट्रोजन (N2)||७८०,८४० ppmv (७८.०८४%)|
|अक्सिजन (O2)||२०९,४६० ppmv (२०.९४६%)|
|अर्गोन (Ar)||९,३४० ppmv (०.९३४०%)|
|कार्बन डाइ अक्साइड (CO2)||३९० ppmv (०.०३९%)|
|निओन (Ne)||१८.१८ ppmv (०.००१८१८%)|
|हिलियम् (He)||५.२४ ppmv (०.०००५२४%)|
|मिथेन (CH4)||१.७९ ppmv (०.०००१७९%)|
|क्रिप्टन (Kr)||१.१४ ppmv (०.०००११४%)|
|हाइड्रोजन (H2)||०.५५ ppmv (०.००००५५%)|
|नाइट्रोस् अक्साईड (N2O)||०.३ ppmv (०.००००३%)|
|कार्बन मोनो अक्साइड (CO)||०.१ ppmv (०.००००१%)|
|जिनोन् (Xe)||०.०९ ppmv (9×10−६%)|
|ओजोन (O3)||०.० देखि ०.०७ सम्म ppmv (०% देखि ७×10−६%)|
|नाइट्रोजन् अक्साइड (NO2)||०.०२ ppmv (२×10−६%)|
|आयोडिन (I)||0.01 ppmv (1×10−6%)|
|Not included in above dry atmosphere:|
|वाष्पकण (H2O)||~०.४०% सम्पूर्ण वायुमण्डलमा, विशेष गरी १% देखि ४% सतहमा|
वायुमण्डलको बनावट [सम्पादन गर्ने]
मुख्य परत [सम्पादन गर्ने]
Earth's atmosphere can be divided into five main layers. These layers are mainly determined by whether temperature increases or decreases with altitude. From highest to lowest, these layers are:
- The outermost layer of Earth's atmosphere extends from the exobase upward. Here the particles are so far apart that they can travel hundreds of kilometres without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the magnetosphere or the solar wind. The exosphere is mainly composed of hydrogen and helium.
- Temperature increases with height in the thermosphere from the mesopause up to the thermopause, then is constant with height. The temperature of this layer can rise to १,५०० °C (२,७३० °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 ३२० and ३८० km (२०० and २४० 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 ३५०–८०० km (२२०–५०० mi; १,१००,०००–२,६००,००० ft).
- The mesosphere extends from the stratopause to ८०–८५ km (५०–५३ mi; २६०,०००–२८०,००० ft). It is the layer where most meteors burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The mesopause, the temperature minimum that marks the top of the mesosphere, is the coldest place on Earth and has an average temperature around −८५ °C (−१२१.० °F; १८८.२ K).
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night. Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%.
The atmosphere has a mass of about 5×10१८ kg, three quarters of which is within about ११ km (६.८ mi; ३६,००० ft) 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 १२० km (७५ mi) is where atmospheric effects become noticeable during atmospheric reentry of spacecraft. The Kármán line, at १०० km (६२ mi), also is often regarded as the boundary between atmosphere and outer space.
Composition [सम्पादन गर्ने]
Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases are often referred to as trace gases, among which are the greenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Many natural substances may be present in tiny amounts in an unfiltered air sample, including dust, pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine compounds, elemental mercury, and sulfur compounds such as sulfur dioxide [SO2].
Due to the cold temperature of the mesophere, water vapor is frozen, forming ice clouds (or Noctilucent clouds). A type of lightning referred to as either sprites or ELVES, form many miles above thunderclouds in the troposphere.
- The stratosphere extends from the tropopause to about ५१ km (३२ mi; १७०,००० ft). Temperature increases with height, which restricts turbulence and mixing. The stratopause, which is the boundary between the stratosphere and mesosphere, typically is at ५० to ५५ km (३१ to ३४ mi; १६०,००० to १८०,००० ft). The pressure here is 1/1000th sea level.
- The troposphere begins at the surface and extends to between ७ km (२३,००० ft) at the poles and १७ km (५६,००० ft) at the equator, with some variation due to weather. The troposphere is mostly heated by transfer of energy from the surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude. This promotes vertical mixing (hence the origin of its name in the Greek word "τροπή", trope, meaning turn or overturn). The troposphere contains roughly 80%[स्रोत नखुलेको] of the mass of the atmosphere. The tropopause is the boundary between the troposphere and stratosphere.
Other layers [सम्पादन गर्ने]
Within the five principal layers determined by temperature are several layers determined by other properties.
- The ozone layer is contained within the stratosphere. 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 १५–३५ km (९.३–२२ mi; ४९,०००–११०,००० 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 ५० to १,००० km (३१ to ६२० mi; १६०,००० to ३,३००,००० 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 homosphere and heterosphere are defined by whether the atmospheric gases 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. The homosphere includes the troposphere, stratosphere, and mesosphere. Above the turbopause at about १०० km (६२ mi; ३३०,००० ft) (essentially corresponding to the mesopause), the composition varies with altitude. This is because the distance that particles can move without colliding with one another is large compared with the size of motions that cause mixing. This 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 planetary boundary layer is the part of the troposphere that is nearest the Earth's surface and is directly affected by it, mainly through turbulent diffusion. During the day the planetary boundary layer usually is well-mixed, while at night it becomes stably stratified with weak or intermittent mixing. The depth of the planetary boundary layer ranges from as little as about 100 m on clear, calm nights to 3000 m or more during the afternoon in dry regions.
Physical properties [सम्पादन गर्ने]
Pressure and thickness [सम्पादन गर्ने]
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 5.1480×1018 kg (1.135×1019 lb), about 2.5% less than would be inferred naively from the average sea level pressure and the Earth's area of 51007.2 megahectares, this defect having been displaced by the Earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and time, because the amount of air above the Earth's surface varies.
If atmospheric density were to remain constant with height the atmosphere would terminate abruptly at ८.५० km (२७,९०० ft). Instead, density decreases with height, dropping by 50% at an altitude of about ५.६ km (१८,००० ft). As a result the pressure decrease is approximately exponential with height, so that pressure decreases by a factor of two approximately every ५.६ km (१८,००० ft) and by a factor of e = 2.718… approximately every ७.६४ km (२५,१०० ft), the latter being the average scale height of Earth's atmosphere below ७० km (४३ mi; २३०,००० 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 ५.६ km (१८,००० ft).
- 90% of the atmosphere by mass is below an altitude of १६ km (५२,००० ft). The common altitude of commercial airliners is about १० km (३३,००० ft) and Mt. Everest's summit is ८,८४८ m (२९,०२९ ft) above sea level.
- 99.99997% of the atmosphere by mass is below १०० km (६२ mi; ३३०,००० 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 ३५४,३०० ft (१०८.० km).
Density and mass [सम्पादन गर्ने]
The density of air at sea level is about 1.2 kg/m3 (1.2 g/L). Density is not measured directly but is calculated from measurements of temperature, pressure and humidity using the equation of state for air (a form of the ideal gas law). Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula. More sophisticated models are used to predict orbital decay of satellites.
The average mass of the atmosphere is about 5 quadrillion (5×10१५) tonnes or 1/1,200,000 the mass of Earth. According to the National Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480×10१८ kg with an annual range due to water vapor of 1.2 or 1.5×10१५ kg depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27×10१६ kg and the dry air mass as 5.1352 ±0.0003×10१८ kg."
Optical properties [सम्पादन गर्ने]
Solar radiation (or sunlight) is the energy the Earth receives from the Sun. The Earth also emits radiation back into space, but at longer wavelengths that we cannot see. Part of the incoming and emitted radiation is absorbed or reflected by the atmosphere.
Scattering [सम्पादन गर्ने]
When light passes through our atmosphere, photons interact with it through scattering. If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the Sun. Indirect radiation is light that has been scattered in the atmosphere. For example, on an overcast day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue, you are seeing scattered blue light. This is also why sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.
Absorption [सम्पादन गर्ने]
Different molecules absorb different wavelengths of radiation. For example, O2 and O3 absorb almost all wavelengths shorter than 300 nanometers. Water (H2O) absorbs many wavelengths above 700 nm. When a molecule absorbs a photon, it increases the energy of the molecule. We can think of this as heating the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below.
The combined absorption spectra of the gases in the atmosphere leave "windows" of low opacity, allowing the transmission of only certain bands of light. The optical window runs from around 300 nm (ultraviolet-C) up into the range humans can see, the visible spectrum (commonly called light), at roughly 400–700 nm and continues to the infrared to around 1100 nm. There are also infrared and radio windows that transmit some infrared and radio waves at longer wavelengths. For example, the radio window runs from about one centimeter to about eleven-meter waves.
Emission [सम्पादन गर्ने]
Emission is the opposite of absorption, it is when an object emits radiation. Objects tend to emit amounts and wavelengths of radiation depending on their "black body" emission curves, therefore hotter objects tend to emit more radiation, with shorter wavelengths. Colder objects emit less radiation, with longer wavelengths. For example, the Sun is approximately ६,००० K (५,७३० °C; १०,३४० °F), its radiation peaks near 500 nm, and is visible to the human eye. The Earth is approximately २९० K (१७ °C; ६२ °F), so its radiation peaks near 10,000 nm, and is much too long to be visible to humans.
Because of its temperature, the atmosphere emits infrared radiation. For example, on clear nights the Earth's surface cools down faster than on cloudy nights. This is because clouds (H2O) are strong absorbers and emitters of infrared radiation. This is also why it becomes colder at night at higher elevations. The atmosphere acts as a "blanket" to limit the amount of radiation the Earth loses into space.
The greenhouse effect is directly related to this absorption and emission (or "blanket") effect. Some chemicals in the atmosphere absorb and emit infrared radiation, but do not interact with sunlight in the visible spectrum. Common examples of these chemicals are CO2 and H2O. If there are too much of these greenhouse gases, sunlight heats the Earth's surface, but the gases block the infrared radiation from exiting back to space. This imbalance causes the Earth to warm, and thus climate change.
Refractive index [सम्पादन गर्ने]
The refractive index of air is close to, but just greater than 1. Systematic variations in refractive index can lead to the bending of light rays over long optical paths. One example is that, under some circumstances, observers onboard ships can see other vessels just over the horizon because light is refracted in the same direction as the curvature of the Earth's surface.
The refractive index of air depends on temperature, giving rise to refraction effects when the temperature gradient is large. An example of such effects is the mirage.
Circulation [सम्पादन गर्ने]
Atmospheric circulation is the large-scale movement of air, and the means (with ocean circulation) by which heat is distributed around the Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant as it is determined by the Earth's rotation rate and the difference in solar radiation between the equator and poles.
Evolution of Earth's atmosphere [सम्पादन गर्ने]
Second atmosphere [सम्पादन गर्ने]
Water related sediments have been found dating from as early as 3.8 billion years ago. About 3.4 billion years ago, nitrogen was the major part of the then stable "second atmosphere." An influence of life has to be taken into account rather soon in the history of the atmosphere, since hints of early life forms are to be found as early as 3.5 billion years ago. The fact that this is not perfectly in line with the - compared to today 30% lower - solar radiance of the early Sun has been described as the "Faint young Sun paradox".
The geological record however shows a continually relatively warm surface during the complete early temperature record of the Earth with the exception of one cold glacial phase about 2.4 billion years ago. In the late Archaean era an oxygen-containing atmosphere began to develop, apparently from photosynthesizing algae which have been found as stromatolite fossils from 2.7 billion years ago. The early basic carbon isotopy (isotope ratio proportions) is very much in line with what is found today, suggesting that the fundamental features of the carbon cycle were established as early as 4 billion years ago.
Third atmosphere [सम्पादन गर्ने]
The accretion of continents about 3.5 billion years ago added plate tectonics, constantly rearranging the continents and also shaping long-term climate evolution by allowing the transfer of carbon dioxide to large land-based carbonate storages. Free oxygen did not exist until about 1.7 billion years ago and this can be seen with the development of the red beds and the end of the banded iron formations. This signifies a shift from a reducing atmosphere to an oxidising atmosphere. O2 showed major ups and downs until reaching a steady state of more than 15%. The following time span was the Phanerozoic era, during which oxygen-breathing metazoan life forms began to appear.
Air pollution [सम्पादन गर्ने]
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to organisms into the atmosphere. Stratospheric ozone depletion is believed to be caused by air pollution (chiefly from chlorofluorocarbons).[स्रोत नखुलेको]
See also [सम्पादन गर्ने]
- Aerial perspective
- Air glow
- Atmosphere (for information on atmospheres in general)
- Atmospheric dispersion modeling
- Atmospheric electricity
- Atmospheric models
- Atmospheric Radiation Measurement (ARM) (in the U.S.)
- Atmospheric stratification
- Carbon dioxide in the Earth's atmosphere
- Compressed air
- Global dimming
- Historical temperature record
- Intergovernmental Panel on Climate Change
- Kyoto Protocol
- Ozone layer
- Standard Dry Air
- US Standard Atmosphere
- Warm period
- Water vapor in Earth's atmosphere
References [सम्पादन गर्ने]
- Source for figures: Carbon dioxide, NOAA Earth System Research Laboratory, (updated 2010.06). Methane, IPCC TAR table 6.1, (updated to 1998). The NASA total was 17 ppmv over 100%, and CO2 was increased here by 15 ppmv. To normalize, N2 should be reduced by about 25 ppmv and O2 by about 7 ppmv.
- States, Robert J. & Gardner, Chester S. (January 2000), "Thermal Structure of the Mesopause Region (80–105 km) at 40°N Latitude. Part I: Seasonal Variations", Journal of the Atmospheric Sciences 2000 57: 66–77, <http://journals.ametsoc.org/doi/full/10.1175/1520-0469%282000%29057%3C0066%3ATSOTMR%3E2.0.CO%3B2>
- homosphere—AMS Glossary
- "Earth's Atmosphere"
- NASA - Earth Fact Sheet
- "Global Surface Temperature Anomalies"
- "Earth's Radiation Balance and Oceanic Heat Fluxes"
- "Coupled Model Intercomparison Project Control Run"
- The Mass of the Atmosphere: A Constraint on Global Analyses
- Lutgens, Frederick K. and Edward J. Tarbuck (1995) The Atmosphere, Prentice Hall, 6th ed., pp14-17, ISBN 0-13-350612-6
- B. Windley: The Evolving Continents. Wiley Press, New York 1984
- J. Schopf: Earth’s Earliest Biosphere: Its Origin and Evolution. Princeton University Press, Princeton, N.J., 1983
- Celestial climate driver: a perspective from 4 billion years of the carbon cycle Geoscience Canada, March, 2005 by Jan Veizer
- Veizer in B. F. Windley (ed.), The Early History of the Earth, John Wiley and Sons, London, p. 569., 1976
- Christopher R. Scotese, Back to Earth History : Summary Chart for the Precambrian, Paleomar Project
- "Summary for Policymakers" (PDF), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Intergovernmental Panel on Climate Change, 5 February 2007
- Starting from  Pollution - Definition from the Merriam-Webster Online Dictionary
|विकिमीडिया कमन्समा अरु धेरै सामाग्रीहरू छन्: Earth's atmosphere|
- NASA atmosphere models
- NASA's Earth Fact Sheet
- American Geophysical Union: Atmospheric Sciences
- Stuff in the Air Find out what the atmosphere contains.
- Layers of the Atmosphere
- Answers to several questions of curious kids related to Air and Atmosphere
- The AMS Glossary of Meteorology
- Paul Crutzen Interview Free video of Paul Crutzen Nobel Laureate for his work on decomposition of ozone talking to Harry Kroto Nobel Laureate by the Vega Science Trust.
- Slides describing the Earth's modern atmosphere
- Air Property