The
Earth's atmosphere is a layer of gases surrounding the planet
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
that is retained by Earth's gravity. The
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
protects
life on EarthIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
by absorbing
ultravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
solar radiation, warming the surface through heat retention (
greenhouse effectThe greenhouse effect is the heating of the surface of a planet or moon due to the presence of an atmosphere containing gases that absorb and emit infrared radiation. Thus, greenhouse gases trap heat within the surface-troposphere system...
), and reducing
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
extremes between
dayOn Earth, daytime is roughly the period on any given point of the planet's surface during which it experiences natural illumination from indirect or direct sunlight....
and
nightNight or nighttime is the period of time when the sun is below the horizon. The opposite of night is day . Time of day varies based on factors such as season, latitude, longitude and timezone.-Duration and geography:Nights are shorter than days on average due to two factors...
. Dry air contains roughly (by volume) 78.08%
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
, 20.95%
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
, 0.93%
argonArgon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is present in the Earth's atmosphere at 0.94%. Terrestrially, it is the most abundant and most frequently used of the noble gases...
, 0.038%
carbon dioxideCarbon dioxide forms approximately 0.04% of the Earth's atmosphere. It is essential to photosynthesis in plants and other photoautotrophs, and is also a prominent greenhouse gas.-Concentration:...
, and trace amounts of other gases. Air also contains a variable amount of
water vaporWater vapor or water vapour , also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice...
, on average around 1%.
The atmosphere has a mass of about five quadrillion (5x10
15)
tonneA tonne or metric ton , also referred to as a metric tonne, is a measurement of mass equal to , or approximately the mass of one cubic metre of water. It is not an SI unit but is accepted for use with the SI...
s, three quarters of which is within about of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and
outer spaceOuter space comprises the relatively empty regions of the universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace and terrestrial locations....
. An altitude of is where atmospheric effects become noticeable during
atmospheric reentryAtmospheric reentry refers to the movement of human-made or natural objects as they enter the atmosphere of a planet from outer space, in the case of Earth from an altitude above the "edge of space." This article primarily addresses the process of controlled reentry of vehicles which are intended...
of spacecraft. The
Kármán lineThe Kármán line lies at an altitude of 100 km above the Earth's sea level, and is commonly used to define the boundary between the Earth's atmosphere and outer space...
, at , also is often regarded as the boundary between atmosphere and outer space.
Composition
Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the "major gases" of the atmosphere. The remaining gases often are referred to as "trace gases," among which are the
greenhouse gasGreenhouse gases are gases in an atmosphere that absorb and emit radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The main greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
es such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s. Many natural substances may be present in tiny amounts in an unfiltered air sample, including
dustDust is a general name for minute solid particles with diameters less than 20 thou . Particles in the atmosphere arise from various sources such as soil dust lifted up by wind, volcanic eruptions, and pollution...
,
pollenPollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement between the stamens to the pistil of flowering plants or from the male cone to the...
and spores,
sea spraySea spray is a spray of water that forms when ocean waves crash.- Make up :As a result, salt spray contains a high concentration of mineral salts, particularly chloride anions.- Effects :...
,
volcanic ashVolcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
, and
meteoroidA meteoroid is a sand to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor. If a meteoroid reaches the ground, it is then called a meteorite. Many meteors are part of a meteor shower...
s. Various industrial
pollutantA pollutant is a waste material that pollutes air, water or soil.Three factors determine the severity of a pollutant: its chemical nature, the concentration and the persistence...
s also may be present, such as
chlorineChlorine Chlorine Chlorine ( , from the Greek word 'χλωρóς' (khlôros, meaning 'pale green'), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VII, VIIa, or VIIb). As the chloride ion, which is part of common salt and...
(elementary or in compounds),
fluorineFluorine is the chemical element with atomic number 9, represented by the symbol F. Fluorine forms a single bond with itself in elemental form, resulting in the diatomic F
2 molecule. F
2 is a supremely reactive, poisonous, pale, yellowish brown gas. Elemental fluorine is the...
(in compounds), elemental
mercuryMercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80...
, and
sulfurSulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
(in compounds such as
sulfur dioxideSulfur dioxide is the chemical compound with the formula SO
2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide...
[SO
2]).
Composition of dry atmosphere, by volume
| ppmv: parts per million by volume (note: volume fraction Volume fractions are useful alternatives to mole fractions when dealing with mixtures in which there is a large disparity between the sizes of the various kinds of molecules; e.g., polymer solutions...
is equal to mole fractionIn chemistry, mole fraction x is a way of expressing the composition of a mixture...
for ideal gas only, see Gas volume#Partial volume) |
| Gas |
Volume |
NitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
(N2) |
780,840 ppmv (78.084%) |
OxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
(O2) |
209,460 ppmv (20.946%) |
ArgonArgon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is present in the Earth's atmosphere at 0.94%. Terrestrially, it is the most abundant and most frequently used of the noble gases...
(Ar) |
9,340 ppmv (0.9340%) |
| Carbon dioxide Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
(CO2) |
383 ppmv (0.0383%) |
NeonNeon is the chemical element that has the symbol Ne and atomic number 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in discharge tubes and neon lamps...
(Ne) |
18.18 ppmv (0.001818%) |
HeliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
(He) |
5.24 ppmv (0.000524%) |
| Methane Methane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
(CH4) |
1.745 ppmv (0.0001745%) |
KryptonKrypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...
(Kr) |
1.14 ppmv (0.000114%) |
HydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H 2...
(H2) |
0.55 ppmv (0.000055%) |
Nitrous oxideNitrous oxide, commonly known as happy gas or laughing gas, is a chemical compound with the chemical formula N 2O. At room temperature, it is a colorless non-flammable gas, with a pleasant, slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...
(N2O) |
0.3 ppmv (0.00003%) |
XenonXenon is a chemical element represented by the symbol Xe. Its atomic number is 54. A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
(Xe) |
0.09 ppmv (9x10−6%) |
OzoneOzone or trioxygen is a simple triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O 2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals...
(O3) |
0.0 to 0.07 ppmv (0% to 7x10−6%) |
Nitrogen dioxideNitrogen dioxide is the chemical compound with the formula . It exists as a radical in nature. One of several nitrogen oxides, is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp,...
(NO2) |
0.02 ppmv (2x10−6%) |
IodineIodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
(I) |
0.01 ppmv (1x10−6%) |
Carbon monoxideCarbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
(CO) |
0.1 ppmv |
AmmoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH 3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
(NH3) |
trace |
| Not included in above dry atmosphere: |
Water vaporWater vapor or water vapour , also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice...
(H2O) |
~0.40% over full atmosphere, typically 1%-4% at surface |
Principal layers
Earth's atmosphere can be divided into five main layers. These layers are mainly determined by whether temperature increases or decreases with altitude. From lowest to highest, these layers are:
TroposphereThe troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75 percent of the atmosphere's mass and 99 percent of its water vapor and aerosols....
: The troposphere begins at the surface and extends to between at the poles and at the equator, with some variation due to weather. The troposphere is mostly heated by transfer of energy from the surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude. This promotes vertical mixing (hence the origin of its name in the Greek word "τροπή",
trope, meaning turn or overturn). The troposphere contains roughly 80% of the mass of the atmosphere. The
tropopauseThe tropopause is the atmospheric boundary between the troposphere and the stratosphere. Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry. More formally, it is the region of the atmosphere where the lapse rate changes from...
is the boundary between the troposphere and stratosphere.
StratosphereThe stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
: The stratosphere extends from the tropopause to about . Temperature increases with height, which restricts turbulence and mixing. The
stratopauseThe stratopause is the level of the atmosphere which is the boundary between two layers, stratosphere and the mesosphere...
, which is the boundary between the stratosphere and mesosphere, typically is at . The pressure here is 1/1000th sea level.
MesosphereThe mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. The mesosphere is located about 50 to 85 kilometers above the Earth's surface.The stratosphere and mesosphere are referred to as the middle atmosphere...
: The mesosphere extends from the stratopause to . It is the layer where most
meteorMETEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...
s burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The
mesopauseThe mesopause is the temperature minimum at the boundary between the mesosphere and the thermosphere atmospheric regions. Due to the lack of solar heating and very strong cooling by radiative emission from CO2, the mesopause is the coldest place on Earth with temperatures as low as -100°C...
, the temperature minimum that marks the top of the mesosphere, is the coldest place on Earth and has an average temperature around .
ThermosphereThe thermosphere is biggest of all the layers of the earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the upper part of the thermosphere, between...
: Temperature increases with height in the thermosphere from the mesopause up to the
thermopauseThe thermopause is the atmospheric boundary of Earth's energy system, located at the top of the thermosphere.Below this, the atmosphere is defined to be active on the insolation received, due to the increased presence of heavier gases such as monoatomic oxygen. The solar constant is thus expressed...
, then is constant with height. The temperature of this layer can rise to , though the gas molecules are so far apart that
temperature in the usual senseKinetic theory is the theory that gases are made up of a large number of small particles , all of which are in constant, random motion. The rapidly moving particles constantly collide with each other and with the walls of the container...
is not well defined. The
International Space StationThe International Space Station is an internationally developed research facility currently being assembled in Low Earth Orbit. On-orbit construction of the station began in 1998 and is scheduled to be completed by 2011, with operations continuing until at least 2015...
orbits in this layer, between . The top of the thermosphere is the bottom of the exosphere, called the exobase. Its height varies with solar activity and ranges from about .
ExosphereThe exosphere is the uppermost layer of the atmosphere. In the exosphere, an upward travelling molecule can escape to space if it is moving fast enough to attain escape velocity; otherwise it will be pulled back to the celestial body by gravity...
: The outermost layer of Earth's atmosphere extends from the exobase upward. Here the particles are so far apart that they can travel hundreds of km without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the
magnetosphereA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
or the
solar windThe solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It consists mostly of electrons and protons with energies of about 1 keV. The stream of particles varies in temperature and speed with the passage of time...
. The exosphere is mainly composed of hydrogen and helium.
Other layers
Within the five principal layers determined by temperature are several layers determined by other properties.
- The ozone layer
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 93-99% of the sun's high frequency ultraviolet light, which is potentially damaging to life on earth. Over 91% of the ozone in Earth's atmosphere is present here...
is contained within the stratosphere. In this layer ozoneOzone or trioxygen is a simple triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O
2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals...
concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from about , though the thickness varies seasonally and geographically. About 90% of the ozone in our atmosphere is contained in the stratosphere.
- The ionosphere
The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
, the part of the atmosphere that is ionized by solar radiation, stretches from and typically overlaps both the exosphere and the thermosphere. It forms the inner edge of the magnetosphere. It has practical importance because it influences, for example, radioRadio is the transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
propagation on the Earth. It is responsible for auroraAuroras, sometimes called the northern and southern lights or aurorae , are natural light displays in the sky, usually observed at night, particularly in the polar regions. They typically occur in the ionosphere. They are also referred to as polar auroras...
s.
- The homosphere and heterosphere are defined by whether the atmospheric gases are well mixed. In the homosphere the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence. The homosphere includes the troposphere, stratosphere, and mesosphere. Above the turbopause
The turbopause marks the altitude in the Earth's atmosphere below which turbulent mixing dominates. The region below the turbopause is known as the homosphere, where the chemical constituents are well mixed and display identical height distributions; in other words, the chemical composition of the...
at about (essentially corresponding to the mesopause), the composition varies with altitude. This is because the distance that particles can move without colliding with one anotherIn physics the mean free path of a particle is the average distance covered by a particle between successive impacts.. Alternatively, it is the distance at which the intensity of particles drops by 1/e.-Derivation:...
is large compared with the size of motions that cause mixing. This allows the gases to stratify by molecular weight, with the heavier ones such as oxygen and nitrogen present only near the bottom of the heterosphere. The upper part of the heterosphere is composed almost completely of hydrogen, the lightest element.
- The planetary boundary layer
The planetary boundary layer , also known as the atmospheric boundary layer , is the lowest part of the atmosphere and its behavior is directly influenced by its contact with a planetary surface. On Earth it usually responds to changes in surface forcing in an hour or less...
is the part of the troposphere that is nearest the Earth's surface and is directly affected by it, mainly through turbulent diffusionIn fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time. Flow that is not turbulent is called laminar flow...
. During the day the planetary boundary layer usually is well-mixed, while at night it becomes stably stratified with weak or intermittent mixing. The depth of the planetary boundary layer ranges from as little as about 100 m on clear, cold nights to 3000 m or more during the afternoon in dry regions.
The average temperature of the atmosphere at the surface of Earth is or , depending on the reference.
Pressure and thickness
The average atmospheric pressure at
sea levelMean sea level is the average height of the ocean's surface ; used as a standard in reckoning land elevation.- Measurement :...
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 5.1480×10
18 kg (1.135×10
19 lb). Atmospheric pressure is the total weight of the air above the point at which the pressure is measured. Thus air pressure varies with location and time, because the amount of air above the Earth's surface varies with location and time.
Were atmospheric density to remain constant with height the atmosphere would terminate abruptly at . Instead, density decreases with height, dropping by 50% at an altitude of about . As a result the pressure decrease is approximately exponential with height, so that pressure decreases by approximately half every and by 63.2% every , the average
scale heightA scale height is a term often used in scientific contexts for a distance over which a quantity decreases by a factor of e...
of Earth's atmosphere below . 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
satelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s.
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 .
- 90% of the atmosphere by mass is below an altitude of . The common altitude of commercial airliners is about and Mt. Everest's summit is above sea level.
- 99.99997% of the atmosphere by mass is below , although in the rarefied region above this there are aurora
Auroras, sometimes called the northern and southern lights or aurorae , are natural light displays in the sky, usually observed at night, particularly in the polar regions. They typically occur in the ionosphere. They are also referred to as polar auroras...
s and other atmospheric effects. The highest X-15The North American X-15 rocket-powered aircraft was part of the X-series of experimental aircraft, initiated with the Bell X-1, that were made for the USAF, NASA, and the USN. The X-15 set speed and altitude records in the early 1960s, reaching the edge of outer space and returning with valuable...
plane flight in 1963 reached an altitude of .
Density and mass
The density of air at sea level is about 1.2 kg/m
3 (1.2 g/L). Density is not measured directly but is calculated from measurements of temperature, pressure and humidity using the equation of state for air (a form of the
ideal gas lawThe Ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behaviour of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's law...
). Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the
barometric formulaThe barometric formula, sometimes called the exponential atmosphere or isothermal atmosphere, is a formula used to model how the pressure of the air changes with altitude.-Pressure equations:...
. More sophisticated models are used by
meteorologistsMeteorology is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century...
and space agencies to predict weather and orbital decay of satellites.
The average mass of the atmosphere is about 5 quadrillion (5x10
15)
tonneA tonne or metric ton , also referred to as a metric tonne, is a measurement of mass equal to , or approximately the mass of one cubic metre of water. It is not an SI unit but is accepted for use with the SI...
s or 1/1,200,000 the mass of Earth. According to the
National Center for Atmospheric ResearchThe National Center for Atmospheric Research conducts collaborative research in atmospheric and Earth system science, encompassing meteorology, climate science, atmospheric chemistry, solar-terrestrial interactions, environmental and societal impacts, and more...
, "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."
Optical properties
Solar
radiationIn physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body...
(or sunlight) is the energy the Earth receives from the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
. 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
cloudA cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae...
s and the
greenhouse effectThe greenhouse effect is the heating of the surface of a planet or moon due to the presence of an atmosphere containing gases that absorb and emit infrared radiation. Thus, greenhouse gases trap heat within the surface-troposphere system...
.
Scattering
When light passes through our atmosphere,
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s 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
overcastOvercast or overcast weather is the meteorological condition of clouds obscuring 95% or more of the sky.Overcast happens when the entire sky becomes covered with clouds; hence the word to describe it. Sometimes clouds can be different colors such as black or white, but the word usually refers to...
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 scatteringRayleigh scattering is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light, which may be individual atoms or molecules. It can occur when light travels in transparent solids and liquids, but is most prominently seen in gases...
, 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
sunsetSunset is the daily disappearance of the sun below the horizon as a result of the Earth's rotation. The atmospheric conditions created by the setting of the sun, occurring before and after it disappears below the horizon, are also commonly referred to as "sunset".In astronomy the time of sunset is...
s 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
Different molecules absorb different wavelengths of radiation. For example, O
2 and O
3 absorb almost all wavelengths shorter than 300 nanometers. Water (H
2O) absorbs many wavelengths above 700 nm. 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.
The combined absorption spectra of the gasses in the atmosphere leave "windows" of low
opacityOpacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
, allowing the transmission of only certain bands of light. The
optical windowThe meaning of this term depends on the context:* In astronomy, the optical window is the optical portion of the electromagnetic spectrum that passes through the atmosphere all the way to the ground...
runs from around 300 nm (
ultravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
-C) up into the range humans can see, the
visible spectrumThe visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 380 to 750 nm...
(commonly called
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
), at roughly 400–700 nm and continues to the
infraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
to around 1100 nm. There are also infrared and
radio windowThe radio window is the range of frequencies of electromagnetic radiation that the earth's atmosphere lets through. The wavelengths in the radio window run from about one centimeter to about eleven-meter waves.-See also:*Atmospheric window...
s that transmit some infrared and
radio wavesRadio waves are that part of the electromagnetic spectrum with wavelengths longer than infrared light. Naturally-occuring radio waves are produced by lightning, or by astronomical objects. Artificially-generated radio waves are used for broadcasting, mobile and fixed communications, navigation,...
at longer wavelengths. For example, the radio window runs from about one centimeter to about eleven-meter waves.
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 bodyIn physics, a black body is an idealized object that absorbs all electromagnetic radiation that falls on it. No electromagnetic radiation passes through it and none is reflected. Because no light is reflected or transmitted, the object appears black when it is cold. However, a black body emits a...
" 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 , its radiation peaks near 500 nm, and is visible to the human eye. The Earth is approximately , 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 (H
2O) 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 CO
2 and H
2O. If there are too much of these
greenhouse gasses, sunlight heats the Earth's surface, but the gases block the infrared radiation from exiting back to space. This imbalance causes the Earth to warm, and thus
climate changeClimate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average...
.
Refractive index
The
refractive indexThe refractive index of a medium is a measure of how much the speed of light is reduced inside the medium. For example, typical soda-lime glass has a refractive index close to 1.5, which means that in glass, light travels at 1 / 1.5 = 2/3 the speed of light in a vacuum...
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
horizonThe horizon is the apparent line that separates earth from sky.It is the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not...
because light is refracted in the same direction as the
curvatureIn mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line, but this is defined in different ways depending on the context...
of the Earth's surface.
- See also: Scintillation (astronomy)
Scintillation or twinkling are generic terms for rapid variations in apparent brightness or color of a distant luminous object viewed through a medium, most commonly the atmosphere ....
Circulation
Atmospheric circulation is the large-scale movement of air, and the means (with ocean circulation) by which
heatIn physics and thermodynamics, heat is the process of energy transfer from one body or system due to thermal contact, which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body....
is distributed around the Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant as it is determined by the Earth's rotation rate and the difference in solar radiation between the equator and poles.
Earliest atmosphere
The outgassings of the Earth were stripped away by
solar windThe solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It consists mostly of electrons and protons with energies of about 1 keV. The stream of particles varies in temperature and speed with the passage of time...
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 sulfide, 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 paradoxThe faint young Sun paradox or problem describes the apparent contradiction between observations of liquid water early in the Earth's history and the astrophysical expectation that the Sun's output would be only 70% as intense during that epoch as it is during the modern epoch...
".
The geological record however shows a continually relatively warm surface during the complete early
temperature recordThe temperature record shows the fluctuations of the temperature of the atmosphere and the oceans through various spans of time. The most detailed information exists since 1850, when methodical thermometer-based records began. There are numerous estimates of temperatures since the end of the...
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
stromatoliteStromatolites are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria...
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 tectonicsPlate tectonics is a theory which describes the large scale motions of Earth's lithosphere...
, 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 oxidising atmosphere. O
2 showed major ups and downs until reaching a steady state of more than 15%. The following time span was the
PhanerozoicThe Phanerozoic eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 545 million years and goes back to the time when diverse hard-shelled animals first appeared...
era, during which oxygen-breathing metazoan life forms began to appear.
Currently,
anthropogenicAnthropogenic effects, processes or materials are those that are derived from human activities, as opposed to those occurring in biophysical environments without human influence....
greenhouse gases are increasing in the atmosphere. According to the Intergovernmental Panel on Climate Change, this increase is the main cause of
global warmingGlobal warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C during the last century...
.
Air pollution
Air pollution is the human introduction of chemicals, particulate matter, or
biological materialBiological material may refer to:* Tissue , or just tissue* Biomass, living or dead biological matter, often plants grown as fuel* Biomass , the total mass of living biological matter* Biomaterials...
s that cause harm or discomfort to organisms, into the atmosphere.
StratosphericThe stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
ozone depletionOzone depletion describes two distinct, but related observations: a slow, steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere since the late 1970s, and a much larger, but seasonal, decrease in stratospheric ozone over Earth's polar regions during the same...
is believed to be caused by air pollution (chiefly from chlorofluorocarbons).
Worldwide, air pollution is responsible for large numbers of deaths and
respiratory diseaseRespiratory Disease is the term for diseases of the respiratory system. These include diseases of the lung, pleural cavity, bronchial tubes, trachea, upper respiratory tract and of the nerves and muscles of breathing. Respiratory diseases range from mild and self-limiting such as the common cold to...
. Enforced air quality standards, like the
Clean Air ActA Clean Air Act is one of a number of pieces of legislation relating to the reduction of smog and air pollution in general. The use by governments to enforce clean air standards has contributed to an improvement in human health and longer life spans...
in the United States, have reduced the presence of some pollutants. While
major stationary sourceA major stationary source is a source that emits more than a certain amount of a pollutant as defined by the U.S. Environmental Protection Agency...
s are often identified with air pollution, the greatest
source of emissionsThe AP 42 Compilation of Air Pollutant Emission Factors, was first published by the U.S. Public Health Service in 1968. In 1972, it was revised and issued as the second edition by the U.S. Environmental Protection Agency . In 1985, the subsequent fourth edition was split into two volumes...
is actually mobile sources, principally the
automobileAn automobile, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
.
See also
- Aerial perspective
Aerial perspective or atmospheric perspective refers to the effect the atmosphere has on the appearance of an object as it is viewed from a distance. As the distance between an object and a viewer increases, the contrast between the object and its background decreases, and the contrast of any...
- Air glow
- Airshed
An airshed is a part of the atmosphere that behaves in a coherent way with respect to the dispersion of emissions. It typically forms an analytical or management unit.Also: A geographic boundary for air quality standards....
- Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
(for information on atmospheres in general)
- Atmospheric dispersion modeling
Atmospheric dispersion modeling is the mathematical simulation of how air pollutants disperse in the ambient atmosphere. It is performed with computer programs that solve the mathematical equations and algorithms which simulate the pollutant dispersion...
- Atmospheric electricity
Atmospheric electricity is the regular diurnal variations of the Earth's atmospheric electromagnetic network . The Earth’s surface, the ionosphere, and the atmosphere is known as the global atmospheric electrical circuit...
- Atmospheric models
Static atmospheric models describe how the ideal gas properties of an atmosphere change, primarily as a function of altitude....
- Atmospheric Radiation Measurement
The United States Department of Energy's Atmospheric Radiation Measurement Program was established in 1989 to study the fundamental physics related to interactions between clouds and radiative feedback processes in the atmosphere...
(ARM) (in the U.S.)
- Atmospheric stratification
Atmospheric stratification is the division of the atmosphere into distinct layers, each with specific characteristics such as temperature or composition....
- Aviation
Aviation is the activity involving man-made air-borne flying devices , including the people, organizations, and regulatory bodies involved with them.- History :...
- Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth. From the broadest biophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements...
- Carbon dioxide in the Earth's atmosphere
Carbon dioxide forms approximately 0.04% of the Earth's atmosphere. It is essential to photosynthesis in plants and other photoautotrophs, and is also a prominent greenhouse gas.-Concentration:...
- Compressed air
Compressed air is air which is kept under a certain pressure, usually greater than that of the atmosphere. In Europe 10 % of all electricity used by industry is used to produce compressed air...
- Global dimming
Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4%...
- Historical temperature record
- Hydrosphere
A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....
- Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change is a scientific intergovernmental body tasked to evaluate the risk of climate change caused by human activity. The panel was established in 1988 by the World Meteorological Organization and the United Nations Environment Programme , two organizations...
- Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet.- Earth's lithosphere :...
- Standard Dry Air
Standard Dry Air is a standard gas composition for dry air, intended to represent the bulk average air composition encountered at or around sea level. Despite its intent of being near sea level, the composition omits water vapour entirely, since the humidity and the temperature will determine the...
- US Standard Atmosphere
The U.S. Standard Atmosphere is a series of models that define values for atmospheric temperature, density, pressure and other properties over a wide range of altitudes. The first model, based on an existing international standard, was published in 1958 by the U.S...
- Warm period
A warm period is an epoch or era, usually lasting a century or more, during which the earth's average atmospheric temperature is higher than usual. As far back as geological proxy measurements go, each warm period has been followed by a cool period....
- Water vapor in Earth's atmosphere
External links