Earth's Atmosphere

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Meteorology

 

The Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by the Earth's gravity. It has a mass of about five quadrillion metric tons. Dry air contains roughly (by volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, and trace amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), 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. An altitude of 120 km (75 mi) marks the boundary where atmospheric effects become noticeable during atmospheric reentry. The Kármán line, at 100 km (62 mi), is also frequently regarded as the boundary between atmosphere and outer space. Three quarters of the atmosphere's mass is within 11 km (6.8 mi; 36,000 ft) of the surface.

 

picture of Earth's Atmosphere

Atmospheric gases scatter blue light more than other wavelengths, giving the Earth a blue halo when seen from space

 

 

Temperature and layers

 

The temperature of the Earth's atmosphere varies with altitude; the mathematical relationship between temperature and altitude varies among five different atmospheric layers (ordered highest to lowest, the ionosphere is part of the thermosphere):

 

layers of the atmosphere

Layers of the Atmosphere (not to scale)

 

Exosphere

From 500–1,000 km (310–620 mi; 1,600,000–3,300,000 ft) up to 10,000 km (6,200 mi; 33,000,000 ft); contain free-moving particles that may migrate into and out of the magnetosphere or the solar wind.

 

Exobase

Also known as the 'critical level', it is the lower boundary of the exosphere.

 

Ionosphere

The part of the atmosphere that is ionized by solar radiation stretches from 50 to 1,000 km (31 to 620 mi; 160,000 to 3,300,000 ft) and typically overlaps both the exosphere and the thermosphere. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. Because of its charged particles, it has practical importance because it influences, for example, radio propagation on the Earth. It is responsible for auroras.

 

Thermopause

The boundary above the thermosphere, it varies in height from 500–1,000 km (310–620 mi; 1,600,000–3,300,000 ft).

 

Thermosphere

From 80–85 km (50–53 mi; 260,000–280,000 ft) to over 640 km (400 mi; 2,100,000 ft), temperature increasing with height. The temperature of this layer can rise to 1,500 °C (2,730 °F). The International Space Station orbits in this layer, between 320 and 380 km (200 and 240 mi).

 

Mesopause

The temperature minimum at the boundary between the thermosphere and the mesosphere. It is the coldest place on Earth, with a temperature of −100 °C (−148.0 °F; 173.1 K).

 

Mesosphere

From the Greek word "μέσος" meaning middle. The mesosphere extends from about 50 km (31 mi; 160,000 ft) to the range of 80–85 km (50–53 mi; 260,000–280,000 ft). Temperature decreases with height, reaching −100 °C (−148.0 °F; 173.1 K) in the upper mesosphere. This is also where most meteors burn up when entering the atmosphere.

 

Stratopause

The boundary between the mesosphere and the stratosphere, typically 50 to 55 km (31 to 34 mi; 160,000 to 180,000 ft). The pressure here is 1/1000th sea level.

 

Stratosphere

From the Latin word "stratus" meaning spreading out. The stratosphere extends from the troposphere's 7–17 km (4.3–11 mi; 23,000–56,000 ft) range to about 51 km (32 mi; 170,000 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 15–35 km (9.3–22 mi; 49,000–110,000 ft) above Earth's surface, though the thickness varies seasonally and geographically.

 

Ozone Layer

Though part of the Stratosphere, the ozone layer is considered as a layer of the Earth's atmosphere in itself because its physical and chemical composition is far different from the Stratosphere. Ozone (O3) in the Earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O2 to create O3. O3 is unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle. This occurs in the ozone layer, the region from about 10 to 50 km (33,000 to 160,000 ft) above Earth's surface. About 90% of the ozone in our atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about 20 and 40 km (66,000 and 130,000 ft), where they range from about 2 to 8 parts per million.

 

Tropopause

The boundary between the stratosphere and troposphere.

 

Troposphere

From the Greek word "τρέπω" meaning to turn or change. 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 (56,000 ft) at the equator, with some variation due to weather factors. The troposphere has a great deal of vertical mixing because of solar heating at the area. This heating makes air masses 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. The troposphere contains roughly 80% of the total mass of the atmosphere. Fifty percent of the total mass of the atmosphere is located in the lower 5.6 km (18,000 ft) of the troposphere.

 

The average temperature of the atmosphere at the surface of Earth is 14 °C (57 °F; 287 K) or 15 °C (59 °F; 288 K), depending on the reference.

 

 

Pressure and thickness

 

The average atmospheric pressure, at sea level, is about 1 atmosphere (atm) = 101.3 kPa (kilopascals) = 14.7 psi (pounds per square inch) = 760 torr = 29.9 inches of mercury (symbol Hg). Total atmospheric mass is 5.1480×1018 kg (1.135×1019 lb).

 

Atmospheric pressure is a direct result of the total weight of the air above the point at which the pressure is measured. Air pressure varies with location and time, because the amount (and weight) of air above the earth varies with location and time. However, the average mass of the air above a square meter of the Earth's surface can be calculated from the total amount of air and the surface area of the Earth. The total air mass is 5148.0 teratonnes and area is 51007.2 megahectares. Thus 5148.0/510.072 = 10.093 tonnes (9.934 LT; 11.126 ST) per square meter or 14.356 pounds per square inch (98.98 kPa). This is about 2.5% below the officially standardized unit atmosphere (1 atm) of 101.325 kPa or 14.696 psi, and corresponds to the mean pressure not at sea level, but at the mean base of the atmosphere as contoured by the Earth's terrain.

 

Were atmospheric density to remain constant with height the atmosphere would terminate abruptly at 7.81 km (25,600 ft). Instead, density decreases with height, dropping by 50% at an altitude of about 5.6 km (18,000 ft). For comparison the highest mountain, Mount Everest, is higher, at 8.8 km (29,000 ft), so air is less than half as dense at the summit than at sea level. This is why it is so difficult to climb without supplemental oxygen.

 

This pressure drop is approximately exponential, so that pressure decreases by approximately half every 5.6 km (18,000 ft) and by 63.2% (1 − 1 / e = 1 − 0.368 = 0.632) every 7.64 km (25,100 ft), the average scale height of Earth's atmosphere below 70 km (43 mi; 230,000 ft). However, because of changes in temperature, average molecular weight, and gravity throughout the atmospheric column, the dependence of atmospheric pressure on altitude is modeled by separate equations for each of the layers listed above. Even in the exosphere, the atmosphere is still present. This can be seen by the effects of atmospheric drag on satellites.

 

In summary, 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:

§  50% of the atmosphere by mass is below an altitude of 5.6 km (18,000 ft).

§  90% of the atmosphere by mass is below an altitude of 16 km (52,000 ft). The common altitude of commercial airliners is about 10 km (33,000 ft) and Mt. Everest's summit is 8,848 m (29,030 ft) above sea level.

§  99.99997% of the atmosphere by mass is below 100 km (62 mi; 330,000 ft); although in the rarefied region above this there are auroras and other atmospheric effects. The highest X-15 plane flight in 1963 reached an altitude of 354,300 ft (108.0 km).

 

 

Composition

 

Filtered air includes trace amounts of many of the chemical elements. Substantial amounts of argon, nitrogen, and oxygen are present as elementary gases. Note the major greenhouse gases: water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Many additional elements from natural sources may be present in tiny amounts in an unfiltered air sample, including contributions from dust, pollen and spores, sea spray, volcanism, and meteoroids. Various industrial pollutants are also now present in the air, such as chlorine (elementary or in compounds), fluorine (in compounds), elementary mercury, and sulfur (in compounds such as sulfur dioxide [SO2]).

 

Composition of dry atmosphere, by volume

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 (0.001818%)

Helium (He)

5.24 ppmv (0.000524%)

Methane (CH4)

1.745 ppmv (0.0001745%)

Krypton (Kr)

1.14 ppmv (0.000114%)

Hydrogen (H2)

0.55 ppmv (0.000055%)

Nitrous oxide (N2O)

0.3 ppmv (0.00003%)

Xenon (Xe)

0.09 ppmv (9x10-6%)

Ozone (O3)

0.0 to 0.07 ppmv (0%-7x10-6%)

Nitrogen dioxide (NO2)

0.02 ppmv (2x10-6%)

Iodine (I)

0.01 ppmv (1x10-6%)

Carbon monoxide (CO)

0.1 ppmv

Ammonia (NH3)

trace

Not included in above dry atmosphere:

Water vapor (H2O)

~0.40% over full atmosphere, typically 1%-4% at surface

 

Composition of Earth's Atmosphere

Composition of Earth's atmosphere as of Dec. 1987. The lower pie represents the least common gases that compose 0.038% of the atmosphere. Values normalized for illustration

 

 

PPMV

 

The parts per million by volume figures above are by volume-fraction (V%), which for ideal gases is equal to mole-fraction (that is, the fraction of total molecules). Although the atmosphere is not an ideal gas, nonetheless the atmosphere behaves enough like an ideal gas that the volume-fraction is the same as the mole-fraction for the precision given.

 

 

By contrast, mass-fraction abundances of gases will differ from the volume values. The mean molar mass of air is 28.97 g/mol, while the molar mass of helium is 4.00, and krypton is 83.80. Thus helium is 5.2 ppm by volume-fraction, but 0.72 ppm by mass-fraction ([4/29] × 5.2 = 0.72), and krypton is 1.1 ppm by volume-fraction, but 3.2 ppm by mass-fraction ([84/29] × 1.1 = 3.2).

 

Heterosphere

 

Below the turbopause, at an altitude of about 100 km (62 mi; 330,000 ft) (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. However, above the turbopause, the Earth's atmosphere begins to have a composition which varies with altitude. This is 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 and hydrogen. Thus there is a layer, called the heterosphere, in which the Earth's atmosphere has varying composition.

 

 

Density and mass

 

The density of air at sea level is about 1.2 kg/m3 (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 because of 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.

 

Temperature and mass density

Temperature and mass density against altitude from the NRLMSISE-00 standard atmosphere model

 

 

The average mass of the atmosphere is about 5 quadrillion 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 × 1018 kg with an annual range due to water vapor of 1.2 or 1.5 × 1015 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 × 1016 kg and the dry air mass as 5.1352 ±0.0003 × 1018 kg."

 

 

Optical properties

 

Solar radiation (or sunlight) is the energy the Earth receives from the Sun. The Earth also emits radiation back into space, but at longer wavelengths that we cannot see. Depending on its condition, the atmosphere can block radiation from coming in or going out. Important examples of this are clouds and the greenhouse effect.

 

Scattering

 

When light passes through our atmosphere, photons interact with it through scattering. If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the sun. Indirect radiation is light that has been scattered in the atmosphere. For example, on an overcast day when you can't see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue; you are seeing scattered blue light. This is also why sunsets are red. Because the sun is close to the horizon, the sun rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.

 

Absorption

 

Absorption is another important property of the atmosphere. Different molecules absorb different wavelengths of radiation. For example, O2 and O3 absorb almost all wavelengths shorter than 300 nanometers. Water (H2O) absorbs many wavelengths above 700 nm, but this depends on the amount of water vapor in the atmosphere. When a molecule absorbs a photon, it increases the energy of the molecule. We can think of this as heating the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below.

 

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When you combine the absorption spectra of the gasses in the atmosphere, you are left with "windows" of low opacity, allowing the transmission of only certain bands of light. The optical window runs from around 300 nm (ultraviolet-C) up into the range humans can see, the visible spectrum (commonly called light), at roughly 400–700 nm and continues to the infrared to around 1100 nm. There are also infrared and radio windows that transmit some infrared and radio waves at longer wavelengths. For example, the radio window runs from about one centimeter to about eleven-meter waves.

 

Rough plot of Earth's atmospheric transmittance

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light

 

Emission

 

Emission is the opposite of absorption; it is when an object emits radiation. Objects tend to emit amounts and wavelengths of radiation depending on their "black body" emission curves, therefore hotter objects tend to emit more radiation, with shorter wavelengths. Colder objects emit less radiation, with longer wavelengths. For example, the sun is approximately 6,000 K (5,730 °C; 10,340 °F), its radiation peaks near 500 nm, and is visible to the human eye. The Earth is approximately 290 K (17 °C; 62 °F), so its radiation peaks near 10,000 nm, and is much too long to be visible to humans.

 

Because of its temperature, the atmosphere emits infrared radiation. For example, on clear nights the Earth's surface cools down faster than on cloudy nights. This is because clouds (H2O) are strong absorbers and emitters of infrared radiation. This is also why it becomes colder at night at higher elevations. The atmosphere acts as a "blanket" to limit the amount of radiation the Earth loses into space.

 

The greenhouse effect is directly related to this absorption and emission (or "blanket") effect. Some chemicals in the atmosphere absorb and emit infrared radiation, but do not interact with sunlight in the visible spectrum. Common examples of these chemicals are CO2 and H2O. If there are too much of these greenhouse gasses, sunlight heats the Earth's surface, but the gasses block the infrared radiation from exiting back to space. This imbalance causes the Earth to warm, and thus climate change.

 

Refractive index

 

The refractive index of air is close to, but just greater than 1. Systematic variations in refractive index can lead to the bending of light rays over long optical paths. One example is that under some circumstances ships can see other vessels just over the horizon because light is refracted in the same direction as the curvature of the earth's surface.

 

Circulation

 

three large circulation cells

An idealized view of three large circulation cells

 

Atmospheric circulation is the large-scale movement of air, and the means (with ocean circulation) by which heat is distributed on the surface of the Earth.

 

The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant. However, individual weather systems - midlatitude depressions or tropical convective cells - occur "randomly". It is accepted that weather cannot be predicted beyond a fairly short limit; perhaps a month in theory, or about ten days in practice. Nonetheless, the average of these systems (the climate) is stable over longer periods of time.

 

Evolution of Earth's atmosphere

 

Earliest atmosphere

 

The out gassings of the Earth were stripped away by solar wind early in the history of the planet until a steady state was established, the first atmosphere. Based on today's volcanic evidence, this atmosphere would have contained 80% water vapor, 10% carbon dioxide, 5 to 7% hydrogen-sulfur, and smaller amounts of nitrogen, carbon monoxide, hydrogen, methane and inert gases.

 

A major rainfall led to the buildup of a vast ocean, enriching the other agents, first carbon dioxide and later nitrogen and inert gases. A major part of carbon dioxide exhalations were soon dissolved in water and built up carbonatic sediments.

 

Second atmosphere

 

Water related sediments have been found dating from as early as 3.8 billion years ago. About 3.4 billion years ago, nitrogen was the major part of the then stable "second atmosphere." An influence of life has to be taken into account rather soon in the history of the atmosphere, since hints of early life forms are to be found as early as 3.5 billion years ago. The fact that this is not perfectly in line with the - compared to today 30% lower - solar radiance of the early sun has been described as the "Faint young Sun paradox".

 

The geological record however shows a continually relatively warm surface during the complete early temperature record of the Earth with the exception of one cold glacial phase about 2.4 billion years ago. Sometime during the late Archaean era an oxygen-containing atmosphere began to develop, apparently from photosynthesizing algae which have been found as stromatolite fossils from 2.7 billion years ago. The early basic carbon isotopy (isotope ratio proportions) is very much in line with what is found today,  suggesting that the fundamental features of the carbon cycle were established as early as 4 billion years ago.

 

Third atmosphere

 

The accretion of continents about 3.5 billion years ago added plate tectonics, constantly rearranging the continents and also shaping long-term climate evolution by allowing the transfer of carbon dioxide to large land-based carbonate storages. Free oxygen did not exist until about 1.7 billion years ago and this can be seen with the development of the red beds and the end of the banded iron formations. This signifies a shift from a reducing atmosphere to an oxidizing atmosphere. O2 showed major ups and downs until reaching a steady state of more than 15%. The following time span was the Phanerozoic era, during which oxygen-breathing metazoan life forms began to appear.

 

oxygen content of the atmosphere

Oxygen content of the Atmosphere since one Billion years

 

Currently, anthropogenic greenhouse gases are increasing in the atmosphere. According to the Intergovernmental Panel on Climate Change, this increase is a causative factor in global warming.

 

Air pollution

 

Air pollution is the human introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to organisms, into the atmosphere.  Stratospheric ozone depletion is believed to be caused by air pollution (chiefly from chlorofluorocarbons).

 

Worldwide, air pollution is responsible for large numbers of deaths and 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 is actually mobile sources, principally the automobile.

 

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