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| {{dambigbox|Air|Air}}
| | ==Why the sky is blue== |
| {{Image|Clean Air.jpg|right|250px|Landscape with clear air and and clouds}}
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| {{main|Earth's atmosphere}}
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| '''Air''' is a colorless, odorless and tasteless mixture of [[gas]]es consisting mostly of [[nitrogen]] (N<sub>2</sub>) and [[oxygen]] (O<sub>2</sub>). It is the part of [[Earth's atmosphere]] that [[human]]s and all other [[animal]]s breathe in order to obtain the oxygen needed to sustain [[life]].
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| The Earth's atmosphere not only contains the air we breathe, it also holds clouds of moisture (water vapor) that become the [[water]] we drink. Furthermore, it protects us from meteors and harmful solar radiation and warms the Earth's surface by heat retention. In effect, the atmosphere is an envelope that protects all life on Earth.
| | [[Light]] is [[electromagnetic radiation]] that travels in waves of vibrating electric and [[magnetic field]]s and is a small part of a larger range of vibrating electromagnetic fields called the [[electromagnetic spectrum]]. Visible light is electromagnetic radiation visible to the human eye and is only a small part of the electromagnetic spectrum. |
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| The air may contain [[pollutant]]s that originate from a variety of sources such as our industries and our vehicles, and can directly or indirectly affect our health and the [[natural environment]]. These effects may be experienced near the [[Air pollution dispersion terminology|sources]] of air pollution and some air pollutants may be transported long distances by the [[wind]], even across political boundaries.<ref>{{cite book | author= Arya, S. Pal| title=Air Pollution Meteorology and Dispersion | edition=1st Edition | publisher=Oxford University Press | year=1998 | id=ISBN 0-19-507398-3}}</ref><ref>{{cite book | author=Barrat, Rod | title=Atmospheric Dispersion Modelling | edition=1st Edition | publisher=Earthscan Publications | year=2001 | id=ISBN 1-85383-642-7}}</ref><ref>{{cite book | author=Pielke, Roger A.| title=Mesoscale Modeling | edition=2nd Edition | publisher=Elsevier | year=2001 | id=ISBN 0-12-554766-8 }}</ref>
| | Light from the [[sun]] looks white, but is actually a combination of many [[color]]s. The colors blend continuously into one another. At one end of the spectrum are the red colors which have the highest [[wavelength]] and therefore the lowest [[frequency]]. At the other end of the spectrum are the blues and violets with the lowest wavelengths and therefore the highest frequency. |
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| == Composition of the atmospheric air ==
| | As light from the sun travels through Earth's atmosphere, it bumps into a bit of dust or a water droplet or a gas [[molecule]]. Dust particles and water droplets are very much larger than the wavelength of visible light. When light bumps into them, it gets reflected and bounced off in a different direction but the reflected light still appears white because it still contains all of the same colors it had before it was reflected. |
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| {{main|Atmospheric chemistry}}
| | However, gas molecules are much smaller than the wavelength of visible light. When light bumps into a gas molecule, it behaves differently than it does when it bumps into a dust particle or a water droplet in that some of the colors in the light are actually absorbed by the molecule. Subsequently, the molecule radiates or releases the light in a different direction. The colors in the radiated light are the same colors that were absorbed. |
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| |+ Composition of Dry Air <ref>[http://www.srh.noaa.gov/srh/jetstream/atmos/atmos_intro.htm The Atmosphere] From the website of the National Oceanic and Atmospheric Administration (NOAA) and the National Weather Service (NWS), Last updated May 5, 2009</ref>
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| !colspan="2"|Gas!!Colspan="2"|Concentration
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| !Name!!Symbol!!Volume %!!ppmv
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| |[[Nitrogen]]|| align="center"|N<sub>2</sub>|| align="center"| 78.084|| align="center"| 780,840
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| |[[Oxygen]]|| align="center"|O<sub>2</sub>|| align="center"| 20.947|| align="center"| 209,470
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| |[[Argon]]|| align="center"|Ar|| align="center"|0.934|| align="center"|9,340
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| |[[Carbon dioxide]]|| align="center"|CO<sub>2</sub>|| align="center"|0.033|| align="center"|330
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| |[[Neon]]|| align="center"|Ne|| align="center"|0.001820|| align="center"|18.20
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| |[[Helium]]|| align="center"|He|| align="center"|0.000520|| align="center"|5.20
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| |[[Methane]]|| align="center"|CH<sub>4</sub>|| align="center"|0.000200|| align="center"|2.00
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| |[[Krypton]]|| align="center"|Kr|| align="center"|0.000110|| align="center"|1.10
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| |[[Sulfur dioxide]]|| align="center"|SO<sub>2</sub>|| align="center"|0.000100|| align="center"|1.00
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| |[[Hydrogen]]|| align="center"|H<sub>2</sub>|| align="center"|0.000050|| align="center"|0.50
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| |[[Nitrous oxide]]|| align="center"|N<sub>2</sub>O|| align="center"|0.000050|| align="center"|0.50
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| |[[Xenon]]|| align="center"|Xe|| align="center"|0.000009|| align="center"|0.09
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| |[[Ozone]]|| align="center"|O<sub>3</sub>|| align="center"|0.000007|| align="center"|0.07
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| |[[Nitrogen dioxide]]|| align="center"|NO<sub>2</sub>|| align="center"|0.000002|| align="center"|0.02
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| |colspan="4"|<span style="font-size:0.85em;">Notes:<br/>
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| -- ppmv = [[Parts-per notation|parts per million parts by volume]]<br/>
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| -- Water vapor varies up to a maximum of 4 volume percent.<br/>
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| -- The total volume percent of the listed gases does not equal<br/> exactly 100 percent because of rounding of the numbers.</span>
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| |}
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| |}
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| The adjacent table lists the concentration of 14 gases present in filtered dry air. Two of the gases, nitrogen and oxygen make up 99.03 percent of the clean, dry air. The other listed gases total to 0.97 percent.
| | Gas molecules are smaller than the wavelength of visible light. If light bumps into them, it acts differently. When light hits a gas molecule, some of it may get absorbed. After awhile, the molecule radiates (releases, or gives off) the light in a different direction. The color that is radiated is the same color that was absorbed. The different colors of light are affected differently. All of the colors can be absorbed. But the higher frequencies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering. (It is named after Lord John Rayleigh, an English physicist, who first described it in the 1870's.) |
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| Note the amounts of [[greenhouse gas]]es that are present: water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Additional gases (not listed in the table) are also present in very minute amounts.
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| The atmospheric air is rarely, if ever, dry. Water vapor is nearly always present up to about 4% of the total volume. In the deserts regions, when dry winds are blowing, the water vapor content will be near zero. This climbs to near 3% on extremely hot/humid days. The upper limit of 4% is for tropical climates.
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| Unfiltered air contains minute amounts of various types of [[particulate matter]] derived from sources such as from dust, pollen and spores, sea spray, volcanoes, meteoroids and industrial activities.
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| ==The nitrogen cycle==
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| {{main|Nitrogen cycle}}
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| The nitrogen in the air is also essential for life on Earth. It is incorporated into [[amino acid]]s and [[protein]]s, and is part of the [[nucleic acid]]s, such as [[DNA]] and [[RNA]]. In plants, nitrogen is used in [[chlorophyll]] which is essential for [[photosynthesis]] and further growth.
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| However, the free nitrogen in the atmosphere is mostly unusable by plants. It must first be transformed through [[Bacteria|bacterial]] conversion processes into forms usable by living organisms. That transformation (referred to as ''[[nitrogen fixation]]'') is part the [[nitrogen cycle]] which is the circulating flow of nitrogen through the atmosphere, the soil of the Earth's [[crust]] and the Earth's water bodies.<ref>[http://www.saburchill.com/IBbiology/chapters03/images/03%20N-CYCLE.ppt The Nitrogen Cycle] 2008, Paul Billiet. From the [[Open Door Web Site ]], a reference source for both students and teachers.</ref><ref>[http://soil.gsfc.nasa.gov/NFTG/nitrocyc.htm The Nitrogen Cycle: Nitrogen Transformations in Soil, Water, and Air.] From a website page of the [[National Aeronautics and Space Administration]] (NASA).</ref>
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| ==References==
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| {{reflist}}
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Why the sky is blue
Light is electromagnetic radiation that travels in waves of vibrating electric and magnetic fields and is a small part of a larger range of vibrating electromagnetic fields called the electromagnetic spectrum. Visible light is electromagnetic radiation visible to the human eye and is only a small part of the electromagnetic spectrum.
Light from the sun looks white, but is actually a combination of many colors. The colors blend continuously into one another. At one end of the spectrum are the red colors which have the highest wavelength and therefore the lowest frequency. At the other end of the spectrum are the blues and violets with the lowest wavelengths and therefore the highest frequency.
As light from the sun travels through Earth's atmosphere, it bumps into a bit of dust or a water droplet or a gas molecule. Dust particles and water droplets are very much larger than the wavelength of visible light. When light bumps into them, it gets reflected and bounced off in a different direction but the reflected light still appears white because it still contains all of the same colors it had before it was reflected.
However, gas molecules are much smaller than the wavelength of visible light. When light bumps into a gas molecule, it behaves differently than it does when it bumps into a dust particle or a water droplet in that some of the colors in the light are actually absorbed by the molecule. Subsequently, the molecule radiates or releases the light in a different direction. The colors in the radiated light are the same colors that were absorbed.
Gas molecules are smaller than the wavelength of visible light. If light bumps into them, it acts differently. When light hits a gas molecule, some of it may get absorbed. After awhile, the molecule radiates (releases, or gives off) the light in a different direction. The color that is radiated is the same color that was absorbed. The different colors of light are affected differently. All of the colors can be absorbed. But the higher frequencies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering. (It is named after Lord John Rayleigh, an English physicist, who first described it in the 1870's.)