'Earth's atmosphere' is a layer of
gases surrounding the planet
Earth and retained by the Earth's
gravity. It contains roughly (by molar content/volume) 78%
nitrogen, 20.95%
oxygen, 0.93%
argon, 0.038%
carbon dioxide, trace amounts of
other gases, and a variable amount (average around 1%) of
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.
There is no definite boundary between the atmosphere and
outer space. It slowly becomes thinner and fades into space. Three quarters of the atmosphere's mass is within 11
km of the
planetary surface. In the
United States, people who travel above an
altitude of 80.5 km (50
statute miles) are designated
astronauts. An altitude of 120 km (400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. The
Kármán line, at 100 km (328,000 ft), is also frequently regarded as the boundary between atmosphere and outer space.
Temperature and layers
The
temperature of the Earth's atmosphere varies with
altitude; the
mathematical relationship between temperature and altitude varies among six different atmospheric layers:
# '
Troposphere': From the
Greek word "τρέπω" meaning to turn or mix. The troposphere is the lowest layer of the atmosphere; it begins at the surface and extends to between 7 km (23,000 ft) at the poles and 17 km (60,000 ft) 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 makes them less dense so they rise. When an air mass rises the pressure upon it decreases so it expands, doing work against the opposing pressure of the surrounding air. To do work is to expend
energy, so the temperature of the air mass decreases. As the temperature decreases, water vapor in the air mass may condense or solidify, releasing
latent heat that further uplifts the air mass. This process determines the maximum rate of decline of temperature with height, called the
adiabatic lapse rate.
# '
Stratosphere': From the
Latin word "stratus" meaning a spreading out. The stratosphere extends from the troposphere's 7 to 17 km (23,000 – 60,000 ft) range to about 50 km (160,000 ft). Temperature increases with height. The stratosphere contains the
ozone layer, the part of the Earth's atmosphere which contains relatively high concentrations of ozone. "Relatively high" means a few parts per million—much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from approximately 15 to 35 km (50,000 – 115,000 ft) above Earth's surface, though the thickness varies seasonally and geographically.
# '
Mesosphere': From the Greek word "μέσος" meaning middle. The mesosphere extends from about 50 km (160,000 ft) to the range of 80 to 85 km (265,000 – 285,000 ft), temperature decreasing with height.
# '
Thermosphere': from 80 – 85 km (265,000 – 285,000 ft) to 640+ km (400+ mi), temperature increasing with height.
# '
Ionosphere': is the part of the atmosphere that is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. It is located in the thermosphere and is responsible for
auroras.
# '
Exosphere': from 500 – 1000 km (300 – 600 mi) up to 10,000 km (6,000 mi), 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 15
°C (59
°F).
Pressure and thickness
Main articles: Atmospheric pressure
:''Barometric Formula: (used for
airplane flight)
barometric formula''
:''One mathematical model:
NRLMSISE-00''
The average atmospheric pressure, at
sea level, is about 101.3
kilopascals (about 14.7
psi); total atmospheric mass is 5.1361×10^18 kg
[2].
Atmospheric pressure is a direct result of the total weight of the air above the point at which the pressure is measured. 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 decreases with height, dropping by 50% at an altitude of about 5.6 km (18,000 ft). Equivalently, about 50% of the total atmospheric mass is within the lowest 5.6 km. This pressure drop is approximately exponential, so that each doubling in altitude results in an approximate decrease in pressure by half. However, because of changes in temperature throughout the atmospheric column, as well as the fact that the force of gravity begins to decrease at great altitudes, a single equation does not model atmospheric pressure through all altitudes (it is modeled in 7 exponentially decreasing layers, in the equations given above).
Even in the exosphere, the atmosphere is still present (as can be seen for example by the effects of atmospheric drag on
satellites).
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:
[1]
★ 50% of the atmosphere by mass is below an altitude of 5.6 km.
★ 90% of the atmosphere by mass is below an altitude of 16 km. The common cruising altitude of commercial airliners is about 10 km.
★ 99.99997% of the atmosphere by mass is below 100 km (almost all of it). The highest
X-15 plane flight in 1963 reached an altitude of 354,300 ft or 108 km.
Therefore, most of the atmosphere (99.9997%) is below 100 km, although in the rarefied region above this there are
auroras and other atmospheric effects.
Composition
Composition of Earth's atmosphere as at 1987.Dec. The lower pie represents the least common gases that compose 0.038% of the atmosphere. Values normalized for illustration.
Mean Atmospheric Water Vapor
'Composition of
dry atmosphere, by volume'[2]| ''ppmv: parts per million by volume'' |
| Gas | Volume |
|---|---|
| Nitrogen (N2) | 780,840 ppmv (78.084%) |
| Oxygen (O2) | 209,460 ppmv (20.946%) |
| Argon (Ar) | 9,340 ppmv (0.9340%) |
| Carbon dioxide (CO2) | 383 ppmv (0.0383%) |
| Neon (Ne) | 18.18 ppmv |
| Helium (He) | 5.24 ppmv |
| Methane (CH4) | 1.745 ppmv |
| Krypton (Kr) | 1.14 ppmv |
| Hydrogen (H2) | 0.55 ppmv |
| 'Not included in above dry atmosphere:' |
| Water vapor (H2O) | ~0.25% over full atmosphere, typically 1% to 4% near surface |
'Minor components of air not listed above include'
The mean molar mass of air is 28.97 g/mol. Note that the composition figures above are by volume-fraction (V%), which for
ideal gases is equal to mole-fraction (that is, fraction of total molecules). By contrast, ''mass-fraction'' abundances of gases, particularly for gases with significantly different molecular (molar) mass from that of air will differ from those by volume. For example, in air, helium is 5.2 ppm by ''volume-fraction'' and ''mole-fraction'', but only about (4/29) × 5.2 ppm = 0.72 ppm by ''mass-fraction''.
Heterosphere
Below the
turbopause at an altitude of about 100 km (not far from the mesopause), the Earth's atmosphere has a more-or-less uniform composition (apart from water vapor) as described above; this constitutes the 'homosphere'.
[3] 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.
[4]
Density and mass
Main articles: Density of air
Earth's atmosphere from space
The density of air at sea level is about 1.2 kg/m
3(1.2 g/L). 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.
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 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 kg with an annual range due to water vapor of 1.2 or 1.5 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 kg and the dry air mass as 5.1352 ±0.0003 kg."
Evolution on Earth
The history of the Earth's atmosphere prior to one billion years ago is poorly understood and an active area of scientific research. The following discussion presents a plausible scenario.
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 largely nitrogen and carbon dioxide. However, simulations run at the University of Waterloo and University of Colorado in 2005 suggest that it may have had up to 40% hydrogen.
[5] It is generally believed that the
greenhouse effect, caused by high levels of carbon dioxide and
methane, kept the Earth from
freezing.
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 produce oxygen while sequestering carbon dioxide in organic molecules, playing a major role in
oxygenating the atmosphere.
Photosynthesising plants would later
evolve and continue releasing oxygen and sequestering carbon dioxide. 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 nitrogen currently present in the atmosphere results from 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". 200 – 250 million years ago, up to 35 percent of the atmosphere was oxygen (bubbles of ancient atmosphere were found in an amber).
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 (see ''
Gaia theory'').
Air pollution
Main articles: Air pollution
'Air pollution' is a
chemical,
physical (e.g.
particulate matter) or
biological agent that modifies the natural characteristics of the atmosphere in an unwanted way.
Stratospheric ozone depletion due to air pollution (chiefly from
chlorofluorocarbons) has long been recognized as a threat to human health as well as to the earth's
ecosystems.
Worldwide air pollution is responsible for large numbers of deaths and cases of
respiratory disease. Enforced air quality standards, like the
Clean Air Act in the United States, have reduced the presence of some pollutants. While
major stationary sources are often identified with air pollution, the greatest
source of emissions are actually mobile sources, principally the
automobile. Gases such as
carbon dioxide,
methane, and
fluorocarbons contribute to
global warming, and these gases, or excess amounts of some of emitted from fossil fuel burning, have recently been identified by the United States and many other countries (see
Kyoto accord), as pollutants.
References
1. Lutgens, Frederick K. and Edward J. Tarbuck (1995) ''The Atmosphere'', Prentice Hall, 6th ed., pp14-17, ISBN 0-13-350612-6
2. Source for figures: Carbon dioxide, NASA Earth Fact Sheet, (updated 2007.01). 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.
3. ''homosphere''—AMS Glossary
4. The thermosphere: a part of the heterosphere (offline, see Internet Archive copy)
5. Early Earth atmosphere favourable to life: study
See also
★
Aerial perspective
★
Air glow
★
Atmosphere (for information on atmospheres in general)
★
Atmospheric chemistry
★
Atmospheric dispersion modeling
★
Atmospheric electricity
★
Atmospheric models
★
Aviation
★
Biosphere
★
Compressed air
★
Global warming
★
Greenhouse effect
★
Historical temperature record
★
Hydrosphere
★
Intergovernmental Panel on Climate Change (IPCC)
★
Lithosphere
★
Meteorology
★
US Standard Atmosphere
External links
★
NASA atmosphere models
★
NASA's Earth Fact Sheet
★
American Geophysical Union: Atmospheric Sciences
★
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
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