Earth's atmosphere
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, with trace amounts of other gases. 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.
Atmospheric layers
The properties of the Earth's atmosphere vary with altitude. Based on these properties, the atmosphere is thought to consist of different layers or zones. According to one system of nomenclature, there are five layers: the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The boundaries between these regions are called the tropopause, stratopause, and mesopause.
The pressure of the atmosphere is highest at the surface and decreases with height. This is because air at the surface is compressed by the weight of all the air above it.
Troposphere
The troposphere (a name derived from the Greek word tropos, meaning to turn or mix) is the atmosphere's lowest and densest layer. It starts from the Earth's surface and reaches up to about 7 km at the poles and 17 km at the equator, with some variation caused by weather factors. The troposphere has a great deal of vertical mixing due to solar heating at the Earth's surface. This heating warms the air masses, which rise to release heat, and that in turn further buoys the air masses. This process continues until all water vapor is removed.
In the troposphere, temperature decreases with height. At middle latitudes, the temperature drops from about +17°C at sea level to about -52°C at the beginning of the tropopause. At the poles, the troposphere is thinner and the temperature falls to only -45°C, while at the equator, the temperature at the top of the troposphere can reach -75°C.
Stratosphere
The stratosphere sits directly above the troposphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down—the reverse of the temperature variation in the troposphere. The stratosphere is situated between about 10 km and 50 km altitude above the surface at moderate latitudes, while at the poles it starts at about 8 km altitude.
The stratosphere is layered in temperature because it is heated from above by absorption of ultraviolet radiation from the Sun. The stratopause (at the top of the stratosphere) has a temperature of about 270 K—fairly close to the temperature at ground level. The stratosphere is dynamically stable, with no regular mixing of air and associated turbulence. The upper layers of the stratosphere are heated by the presence of an ozone layer that absorbs solar ultraviolet radiation. The base of the stratosphere occurs where heating by conduction from above and heating by convection from below (through the troposphere) balance out; hence, the stratosphere begins at lower altitudes near the poles due to the lower ground temperature there.
Commercial airliners typically cruise at an altitude near 10 km in temperate latitudes, in the lower reaches of the stratosphere. In this manner, they avoid atmospheric turbulence in the troposphere.
Mesosphere
The mesosphere (from the Greek words mesos = middle and sphaira = ball) is the layer between about 50 km and about 80–85 km above the Earth's surface. It is sandwiched between the stratosphere and the thermosphere. The temperature in this layer decreases with increasing altitude and can be as low as 200K (≈-99°F), varying according to latitude and season.
Given that it lies between the maximum altitude for most aircraft and the minimum altitude for most spacecraft, this region of the atmosphere is directly accessible only through the use of sounding rockets. As a result, it is one of the most poorly understood regions of the atmosphere.
Millions of meteors burn up daily in the mesosphere, as a result of collisions with the gas particles contained there, leading to a high concentration of iron and other metal atoms. The collisions almost always create enough heat to burn the falling objects long before they reach the ground.
The stratosphere and mesosphere are referred to as the middle atmosphere. The mesopause, at an altitude of about 80 km, separates the mesosphere from the thermosphere. This is also about the same altitude as the turbopause, below which different chemical species are well mixed due to turbulent eddies.
Thermosphere
The thermosphere (from the Greek word thermos for heat) extends from 80–85 km to 640+ km. It lies directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization.
At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass (see turbosphere). The temperature in this layer increases with altitude, due to the absorption of highly energetic solar radiation by the small amount of oxygen present. Temperatures are highly dependent on solar activity and can rise to 2,000°C. Radiation causes the air particles in this layer to become electrically charged (see ionosphere), enabling radio waves to bounce off and be received beyond the horizon.
Exosphere
The exosphere (from the Greek words exo = out(side) and sphaira = ball) is the uppermost layer of the atmosphere. Its lower boundary at the edge of the thermosphere is estimated to be 500 km to 1,000 km above the Earth's surface, and its upper boundary at about 10,000 km.
It is only from the exosphere that atmospheric gases, atoms, and molecules can, to any appreciable extent, escape into outer space. The main gases in the exosphere are the lightest ones, mainly hydrogen and helium, with some atomic oxygen near the exobase (the lowest altitude of the exosphere). The few particles of gas here can reach 2,500°C (4,500°F) during the day.
The atmosphere in this layer is sufficiently rarified for satellites to orbit the Earth, although they still receive some atmospheric drag. The exact altitude at which the exosphere ends and space begins is not well-defined, and attempting to attach a specific value to it is not particularly useful.
Various atmospheric regions
Atmospheric regions are also named in other ways:
- ionosphere – the region containing ions: approximately the mesosphere and thermosphere up to 550 km.
- exosphere – above the ionosphere, where the atmosphere thins out into space. This is the last major atmosphere.
- magnetosphere – 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 – 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 – the region of the atmosphere above the mesopause.
- Van Allen radiation belts – regions where particles from the Sun become concentrated.
Pressure
- Barometric Formula: (used for airplane flight) barometric formula
- Main article: Atmospheric pressure
- Nasa 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 (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
- 57.8% of the atmosphere is below the summit of Mount Everest.
- 72% of the atmosphere is below the common cruising altitude of commercial airliners (about 10000 m or 32800 ft).
- 99.99999% of the atmosphere is below the highest flight altitude of the aircraft X-15, which reached 354,300 ft (108 km) on August 22, 1963.
Therefore, most of the atmosphere (99.9999%) is below 100 km, although in the rarified region above this there are auroras and other atmospheric effects. Although the atmosphere exists at altitudes of 1,000 km and higher, it is so thin as to be considered nonexistent.
Composition
| 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) | 350 ppmv |
| 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 (highly variable) | typically 1% |
Source for figures above: NASA. Carbon dioxide and 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:
| 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 molecular 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 molecular 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]
Density and mass
- Main article: Density of air
The density of air at sea level is about 1.2 kg/m3. 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 1018 kg with an annual range due to water vapor of 1.2 or 1.5 x 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 x 1016 kg and the dry air mass as 5.1352 ±0.0003 x 1018 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.
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 (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".
ReferencesISBN links support NWE through referral fees
- The thermosphere: a part of the heterosphere, by J. Vercheval.
See also
- Air glow
- Atmospheric electricity
- Atmospheric dispersion modeling
- Compressed air
- Global warming
- Greenhouse effect
- Historical temperature record
- Intergovernmental Panel on Climate Change (IPCC)
External links
- NASA atmosphere models
- NASA's Earth Fact Sheet
- American Geophysical Union: Atmospheric Sciences
- Layers of the Atmosphere
- The AMS Glossary of Meteorology
|
Troposphere | Stratosphere | Mesosphere | Thermosphere | Exosphere |
|
Tropopause | Stratopause | Mesopause | Exobase |
|
Ozone layer | Turbopause | Ionosphere |
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