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Earth's atmosphere

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   Layers of Atmosphere (NOAA)
   Enlarge
   Layers of Atmosphere (NOAA)

   Earth's atmosphere is a layer of gases surrounding the planet Earth and
   retained by the Earth's gravity. It contains roughly 78% nitrogen, 21%
   oxygen, 0.97% argon, 0.04% carbon dioxide, and trace amounts of other
   gases, in addition to water vapor. This mixture of gases is commonly
   known as air. The atmosphere protects life on Earth by absorbing
   ultraviolet solar radiation and reducing temperature extremes between
   day and night.

   The atmosphere has no abrupt cut-off. It slowly becomes thinner and
   fades away into space. There is no definite boundary between the
   atmosphere and outer space. Three-quarters of the atmosphere's mass is
   within 11 km of the planetary surface. In the United States, persons
   who travel above an altitude of 50.0 miles (80.5 km) are designated as
   astronauts. An altitude of 120 km (75 mi or 400,000 ft) marks the
   boundary where atmospheric effects become noticeable during re-entry.
   The Karman line, at 100 km (62 mi), is also frequently used as the
   boundary between atmosphere and space.

Temperature and the atmospheric layers

   The temperature of the Earth's atmosphere varies with altitude; the
   mathematical relationship between temperature and altitude varies
   between the different atmospheric layers:
     * troposphere: From the Greek word "tropos" meaning to turn or mix.
       The troposphere is the lowest layer of the atmosphere starting at
       the surface going up to between 7 km (4.4 mi) at the poles and 17
       km (10.6 mi) at the equator with some variation due to weather
       factors. The troposphere has a great deal of vertical mixing due to
       solar heating at the surface. This heating warms air masses, which
       then rise to release latent heat as sensible heat that further
       uplifts the air mass. This process continues until all water vapor
       is removed. In the troposphere, on average, temperature decreases
       with height due to expansive cooling.
     * stratosphere: from that 7–17 km range to about 50 km, temperature
       increasing with height.
     * mesosphere: from about 50 km to the range of 80 km to 85 km,
       temperature decreasing with height.
     * thermosphere: from 80–85 km to 640+ km, temperature increasing with
       height.
     * exosphere: from 500-1000 km up to 10,000 km, free-moving particles
       that may migrate into and out of the magnetosphere or the solar
       wind.

   The boundaries between these regions are named the tropopause,
   stratopause, mesopause, thermopause and exobase.

   The average temperature of the atmosphere at the surface of earth is 14
   °C.

Pressure

   Atmospheric pressure is a direct result of the weight of the air. This
   means that air pressure varies with location and time, because the
   amount (and weight) of air above the earth varies with location and
   time. Atmospheric pressure drops by 50% at an altitude of about 5 km
   (equivalently, about 50% of the total atmospheric mass is within the
   lowest 5 km). The average atmospheric pressure, at sea level, is about
   101.3 kilopascals (about 14.7 pounds per square inch).

Thickness of the atmosphere

   Even at heights of 1000 km and above, the atmosphere is still present
   (as can be seen for example by the effects of atmospheric drag on
   satellites).

   However:
     * 57.8% of the atmosphere by mass is below the summit of Mount
       Everest.
     * 72% of the atmosphere by mass is below the common cruising altitude
       of commercial airliners (about 10000 m or 33000 ft).
     * 99.99999% of the atmosphere by mass is below the highest X-15 plane
       flight on August 22, 1963, which reached an altitude of 354,300 ft
       or 108 km.

   Therefore, most of the atmosphere (99.9999%) by mass is below 100 km,
   although in the rarefied region above this there are auroras and other
   atmospheric effects.

Composition

   Composition of Earth's atmosphere. The lower pie represents the least
   common gases that compose 0.038% of the atmosphere. Values normalized
   for illustration.
   Enlarge
   Composition of Earth's atmosphere. The lower pie represents the least
   common gases that compose 0.038% of the atmosphere. Values normalized
   for illustration.

   CAPTION: Composition of
   dry atmosphere, by volume

   ppmv: parts per million by volume
   Gas                    Volume
   Nitrogen (N[2])        780,840 ppmv (78.084%)
   Oxygen (O[2])          209,460 ppmv (20.946%)
   Argon (Ar)             9,340 ppmv (0.9340%)
   Carbon dioxide (CO[2]) 381 ppmv
   Neon (Ne)              18.18 ppmv
   Helium (He)            5.24 ppmv
   Methane (CH[4])        1.745 ppmv
   Krypton (Kr)           1.14 ppmv
   Hydrogen (H[2])        0.55 ppmv
   Not included in above dry atmosphere:
   Water vapor (H[2]O)    typically 1% to 4%(highly variable)
   Mean Atmospheric Water Vapor.
   Enlarge
   Mean Atmospheric Water Vapor.

   Source for figures above: NASA. carbon dioxide (updated 2006). Methane
   updated (to 1998) by IPCC TAR table 6.1 . The NASA total was 17 ppmv
   over 100%, and CO[2] was increased here by 15 ppmv. To normalize, N[2]
   should be reduced by about 25 ppmv and O[2] by about 7 ppmv.

   Minor components of air not listed above include:
   Gas              Volume
   nitrous oxide    0.5 ppmv
   xenon            0.09 ppmv
   ozone            0.0 to 0.07 ppmv
   nitrogen dioxide 0.02 ppmv
   iodine           0.01 ppmv
   carbon monoxide  trace
   ammonia          trace
     * The mean molar mass of air is 28.97 g/mol.

Heterosphere

   Below the turbopause at an altitude of about 100 km, the Earth's
   atmosphere has a more-or-less uniform composition (apart from water
   vapor) as described above; this constitutes the homosphere. However,
   above about 100 km, the Earth's atmosphere begins to have a composition
   which varies with altitude. This is essentially because, in the absence
   of mixing, the density of a gas falls off exponentially with increasing
   altitude, but at a rate which depends on the molar mass. Thus higher
   mass constituents, such as oxygen and nitrogen, fall off more quickly
   than lighter constituents such as helium, molecular hydrogen, and
   atomic hydrogen. Thus there is a layer, called the heterosphere, in
   which the earth's atmosphere has varying composition. As the altitude
   increases, the atmosphere is dominated successively by helium,
   molecular hydrogen, and atomic hydrogen. The precise altitude of the
   heterosphere and the layers it contains varies significantly with
   temperature.

Density and mass

   The density of air at sea level is about 1.2 kg/m^3. Natural variations
   of the barometric pressure occur at any one altitude as a consequence
   of weather. This variation is relatively small for inhabited altitudes
   but much more pronounced in the outer atmosphere and space due to
   variable solar radiation.
   Temperature and pressure against altitude from the NRLMSISE-00 standard
   atmosphere model
   Enlarge
   Temperature and pressure against altitude from the NRLMSISE-00 standard
   atmosphere model

   The atmospheric density decreases as the altitude increases. This
   variation can be approximately modeled using the barometric formula.
   More sophisticated models are used by meteorologists and space agencies
   to predict weather and orbital decay of satellites.

   The average mass of the atmosphere is about 5,000 trillion metric tons.
   According to the National Centre for Atmospheric Research, "The total
   mean mass of the atmosphere is 5.1480×10^18 kg with an annual range due
   to water vapor of 1.2 or 1.5×10^15 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
   1.27×10^16 kg and the dry air mass as 5.1352 ±0.0003×10^18 kg."

   The above composition percentages are done by volume. Assuming that the
   gases act like ideal gases, we can add the percentages p multiplied by
   their molar masses m, to get a total t = sum (p·m). Any element's
   percent by mass is then p·m/t. When we do this to the above
   percentages, we get that, by mass, the composition of the atmosphere is
   75.523% nitrogen, 23.133% oxygen, 1.288% argon, 0.053% carbon dioxide,
   0.001267% neon, 0.00029% methane, 0.00033% krypton, 0.000724% helium,
   and 0.0000038 % hydrogen.

Evolution of the Earth's atmosphere

   Diagram of chemical and transport processes related to atmospheric
   composition.
   Enlarge
   Diagram of chemical and transport processes related to atmospheric
   composition.

   The history of the Earth's atmosphere prior to one billion years ago is
   poorly understood, but the following presents a plausible sequence of
   events. This remains an active area of research.

   The modern atmosphere is sometimes referred to as Earth's "third
   atmosphere", in order to distinguish the current chemical composition
   from two notably different previous compositions. The original
   atmosphere was primarily helium and hydrogen. Heat from the
   still-molten crust, and the sun, plus a probably enhanced solar wind,
   dissipated this atmosphere.

   About 4.4 billion years ago, the surface had cooled enough to form a
   crust, still heavily populated with volcanoes which released steam,
   carbon dioxide, and ammonia. This led to the early "second atmosphere",
   which was primarily carbon dioxide and water vapor, with some nitrogen
   but virtually no oxygen. This second atmosphere had approximately 100
   times as much gas as the current atmosphere, but as it cooled much of
   the carbon dioxide was dissolved in the seas and precipitated out as
   carbonates. The later "second atmosphere" contained nitrogen, carbon
   dioxide, and very recent simulations run at the University of Waterloo
   and University of Colorado in 2005 suggest that it may have had up to
   40% hydrogen. It is generally believed that the greenhouse effect,
   caused by high levels of carbon dioxide and methane, kept the Earth
   from freezing. In fact temperatures were probably very high, over 70
   degrees C, until some 2.7 billion years ago.

   One of the earliest types of bacteria were the cyanobacteria. Fossil
   evidence indicates that bacteria shaped like these existed
   approximately 3.3 billion years ago and were the first oxygen-producing
   evolving phototropic organisms. They were responsible for the initial
   conversion of the earth's atmosphere from an anoxic state to an oxic
   state (that is, from a state without oxygen to a state with oxygen)
   during the period 2.7 to 2.2 billion years ago. Being the first to
   carry out oxygenic photosynthesis, they were able to convert carbon
   dioxide into oxygen, playing a major role in oxygenating the
   atmosphere.

   Photosynthesizing plants would later evolve and convert more carbon
   dioxide into oxygen. Over time, excess carbon became locked in fossil
   fuels, sedimentary rocks (notably limestone), and animal shells. As
   oxygen was released, it reacted with ammonia to release nitrogen; in
   addition, bacteria would also convert ammonia into nitrogen. But most
   of the modern day level of nitrogen are due mostly to sunlight-powered
   photolysis of ammonia released steadily over the aeons from volcanoes.

   As more plants appeared, the levels of oxygen increased significantly,
   while carbon dioxide levels dropped. At first the oxygen combined with
   various elements (such as iron), but eventually oxygen accumulated in
   the atmosphere, resulting in mass extinctions and further evolution.
   With the appearance of an ozone layer (ozone is an allotrope of oxygen)
   lifeforms were better protected from ultraviolet radiation. This
   oxygen-nitrogen atmosphere is the "third atmosphere".

   This modern atmosphere has a composition which is enforced by oceanic
   blue-green algae as well as geological processes. O[2] does not remain
   naturally free in an atmosphere, but tends to be consumed (by inorganic
   chemical reactions, as well as by animals, bacteria, and even land
   plants at night), while CO[2] tends to be produced by respiration and
   decomposition and oxidation of organic matter. Oxygen would vanish
   within a few million years due to chemical reactions and CO[2]
   dissolves easily in water and would be gone in millennia if not
   replaced. Both are maintained by biological productivity and geological
   forces seemingly working hand-in-hand to maintain reasonably steady
   levels over millions of years.
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