Earth's atmosphere
 |
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 and 21%
oxygen, trace amounts of
other gases, and 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.
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 at the poles and 17 km 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 buoys 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 7â€"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 80â€"85 km to 640+ km, temperature increasing with height.
The boundaries between these regions are named the
tropopause,
stratopause, and
mesopause.
The average temperature of the atmosphere at the surface of earth is 14 °C.
Various atmospheric regions
Atmospheric regions are also named in other ways:
*
ionosphere â€" the region containing
ions: approximately the mesosphere and thermosphere up to 550 km.
*
exosphere â€" above the ionosphere, where the atmosphere thins out into
space. This is the last major atmosphere. (
"Exo" means
"outside" in Greek.)
*
magnetosphere â€" the region where the
Earth's magnetic field interacts with the
solar wind from the
Sun. It extends for tens of thousands of kilometers, with a long tail away from the Sun.
*
ozone layer â€" or ozonosphere, approximately 10 - 50 km, where
stratospheric ozone is found. Note that even within this region, ozone is a minor constituent by volume.
* upper atmosphere â€" the region of the atmosphere above the
mesopause.
*
Van Allen radiation belts â€" regions where
particles from the Sun become concentrated.
Barometric Formula: (used for airplane flight) barometric formulaMain article: Atmospheric pressure
One mathematical model: NRLMSISE-00Atmospheric 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).
Although the atmosphere exists at heights of 1000 km and more, it is so thin as to be considered nonexistent.
*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 32800 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 of Earth's atmosphere. The lower pie represents the least common gases that compose 0.038% of the atmosphere. Values normalized for illustration. |
|
Mean Atmospheric Water Vapor. |
Source for figures above: NASA. carbon dioxide (updated 2006). Methane updated (to 1998) by IPCC TAR table 6.1 [1]. 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.Minor components of air not listed above include:*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.[
2] 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.[
3]
Main article: Density of air
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.
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 Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480 x 10
18 kg with an annual range due to water vapor of 1.2 or 1.5 x 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 x 10
16 kg and the dry air mass as 5.1352 ±0.0003 x 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.
 |
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) dissipated this atmosphere.
About 3.5 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 "second atmosphere", which was primarily carbon dioxide and
water vapor, with some
nitrogen but virtually no
oxygen (though very recent simulations run at the University of Waterloo and University of Colorado in 2005 suggested that it may have had up to 40% hydrogen [
4]). This second atmosphere had approximately 100
times as much
gas as the current atmosphere. It is generally believed that the
greenhouse effect, caused by high levels of carbon dioxide, kept the Earth from
freezing.
During the next few million years, water vapor
condensed to form
rain and
oceans, which began to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. One of the earliest types of bacteria were the
cyanobacteria. Fossil evidence indicates that these bacteria 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). 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 create nitrogen; in addition,
bacteria would also convert ammonia into nitrogen.
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. O2 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 CO2 tends to be produced...but CO2 dissolves easily in water, while O2 tends, relatively, to be expelled by it. So as CO2 builds up in the atmosphere, it dissolves in the ocean, where its presence stimulates algae to consume it, producing O2, which is expelled into the atmosphere. This strikes a balance, where the amount of O2 and CO2 tend to be that which will keep the algae moderately active. Too much CO2 makes the algae more active, driving the amount down, and too little makes it less active, allowing the amount to rise.
The thermosphere: a part of the heterosphere, by J. Vercheval.
*
Air car*
Air glow*
Atmosphere (for information on atmospheres in general).
*
Atmospheric chemistry*
Atmospheric dispersion modeling*
Atmospheric electricity*
Atmospheric models*
Compressed air*
Global warming*
Greenhouse effect*
Historical temperature record*
Intergovernmental Panel on Climate Change (IPCC)
*
US Standard Atmosphere*
NASA atmosphere models*
NASA's Earth Fact Sheet*
American Geophysical Union: Atmospheric Sciences*
Layers of the Atmosphere*
The AMS Glossary of Meteorology
