Encyclopedia
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 are designated as
astronauts. An altitude of 120 km marks the boundary where atmospheric effects become noticeable during re-entry. The
Karman line, at 100 km , 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 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 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.
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.
Pressure
- Barometric Formula: barometric formula
- Main article: Atmospheric pressure
- One mathematical model: NRLMSISE-00
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 of air above the earth varies with location and time. Atmospheric pressure drops by ~50% at an altitude of about 5 km . The average atmospheric pressure, at
sea level, is about 101.3 kilopascals .
Thickness of the atmosphere
Even at heights of 1000 km and above, the atmosphere is still present .
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 .
- 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 by mass is below 100 km, although in the rarefied region above this there are auroras and other atmospheric effects.
Composition
Source for figures above: . carbon dioxide . Methane updated by IPCC TAR table 6.1 . 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 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.
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 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."
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 . 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.
The evolution of the Earth's atmosphere
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 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 . 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 . 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 , and
animal shells. As oxygen was released, it reacted with
ammonia to release 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 , but eventually oxygen accumulated in the atmosphere, resulting in
mass extinctions and further evolution. With the appearance of an
ozone layer 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 , 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.
References
See also
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