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The atmosphere of Earth is the layer of
gas Gas is one of the four fundamental states of matter (the others being solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and c ...

gas
es, commonly known as air, retained by
Earth's gravity The gravity of Earth, denoted by , is the that is imparted to objects due to the combined effect of (from within ) and the (from the ). In this acceleration is measured in (in symbols, /2 or m·s−2) or equivalently in per (N/kg or N· ...
, surrounding the planet
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

Earth
and forming its planetary
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
. The atmosphere of Earth protects
life Life is a characteristic that distinguishes physical entities that have biological processes, such as Cell signaling, signaling and self-sustaining processes, from those that do not, either because such functions have ceased (they have Death ...

life
on Earth by creating
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...
allowing for
liquid water Water (chemical formula H2O) is an inorganic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, struc ...
to exist on the Earth's
surface File:Water droplet lying on a damask.jpg, Water droplet lying on a damask. Surface tension is high enough to prevent floating below the textile. A surface, as the term is most generally used, is the outermost or uppermost layer of a physical obje ...
, absorbing
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that stud ...

ultraviolet
solar radiation Solar irradiance is the power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one j ...
, warming the surface through heat retention (
greenhouse effect The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without this atmosphere. Radiatively active gases (i.e., es) in a planet's atmosphere radi ...

greenhouse effect
), and reducing temperature extremes between
day A day is approximately the period during which the Earth completes one rotation around its axis, which takes around 24 hours. A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive t ...

day
and
night Night (also described as night time, night-time, or nighttime, unconventionally spelled as ''nite'') is the period of ambient Ambient or Ambiance or Ambience may refer to: Music and sound * Ambience (sound recording), also known as atmospher ...

night
(the
diurnal temperature variation In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progr ...
). By volume, dry air contains 78.09%
nitrogen Nitrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

nitrogen
, 20.95%
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
, 0.93%
argon Argon is a with the  Ar and  18. It is in group 18 of the and is a . Argon is the third-most abundant in the , at 0.934% (9340 ). It is more than twice as abundant as (which averages about 4000 ppmv, but varies greatly), 23 time ...

argon
, 0.04%
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is ...
, and small amounts of other gases. Air also contains a variable amount of
water vapor (99.9839 °C) , - , , , - , , 461.5 /(·K) , - , , 2.27 /kg , - , , 1.864 /(kg·K) Water vapor, water vapour or aqueous vapor is the eous phase of . It is one of water within the . Water can be produced from the or of li ...
, on average around 1% at sea level, and 0.4% over the entire atmosphere. Air composition, temperature, and
atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. ...
vary with altitude, and air suitable for use in
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
by
terrestrial plant A terrestrial plant is a plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; ho ...
s and
breathing Breathing (or ventilation) is the process of moving out and in the s to facilitate with the , mostly to flush out and bring in . All aerobic creatures need oxygen for , which uses the oxygen to break down foods for energy and produces ca ...

breathing
of
terrestrial animal Terrestrial animals are animals that live predominantly or entirely on land (e.g., cats, dogs, ants, spiders), as compared with aquatic animals, which live predominantly or entirely in the water (e.g., fish, lobsters, octopuses), or amphibians, w ...
s is found only in Earth's
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere Planetary means relating to a planet A planet is an astronomical body orbiting a star or Stellar evo ...
and in artificial atmospheres. Earth's atmosphere has changed much since its formation as primarily a hydrogen atmosphere, and has changed dramatically on several occasions—for example, the
Great Oxidation Event The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen catastrophe, and the Oxygen Crisis, was a time interval when the Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere ...
2.4 billion years ago, greatly increased oxygen in the atmosphere from practically no oxygen to levels closer to present day. Humans have also contributed to significant changes in atmospheric composition through air pollution, especially since
industrialisation Factories, refineries, mines, and agribusiness are all elements of industrialisation Industrialisation ( alternatively spelled industrialization) is the period of social and economic change that transforms a human group from an agrarian societ ...

industrialisation
, leading to rapid environmental change such as
ozone depletion Ozone depletion consists of two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth, Earth's atmosphere (the ozone layer), and a much larger springtime decrease in stratosphe ...

ozone depletion
and
global warming Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been , but the current changes are more rapid than any known events in Earth's history. The main cau ...

global warming
. The atmosphere has a mass of about 5.15 kg, 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 space Outer space, commonly shortened to space, is the expanse that exists beyond Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting ...
. The
Kármán line The Kármán line is an attempt to define a boundary between Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose ab ...
, at , or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during
atmospheric reentry An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other solid, material body, that is held ...
of spacecraft at an altitude of around . Several
layers Layer or layered may refer to: Arts, entertainment, and media Music Albums * ''Layers'' (Kungs album) * ''Layers'' (Les McCann album) * ''Layers'' (Royce da 5'9" album) Songs *"Layers", the title track of Royce da 5'9"'s sixth studio album Othe ...
can be distinguished in the atmosphere, based on characteristics such as temperature and composition. The study of Earth's atmosphere and its processes is called
atmospheric science Atmospheric science is the study of the Atmosphere of Earth, Earth's atmosphere and its various inner-working physical processes. Meteorology includes atmospheric chemistry and atmospheric physics with a major focus on weather forecasting. Climat ...
(aerology), and includes multiple subfields, such as
climatology Climatology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appr ...
and
atmospheric physics Within the atmospheric sciences, atmospheric physics is the application of physics to the study of the Earth's atmosphere, atmosphere. Atmospheric physicists attempt to model Earth's atmosphere and the atmospheres of the other planets using fluid d ...
. Early pioneers in the field include
Léon Teisserenc de Bort
Léon Teisserenc de Bort
and
Richard Assmann Richard Assmann (Anglicized spelling of the German name Richard Aßmann) (13 April 1845 in Magdeburg Magdeburg (; nds, label= Low Saxon, Meideborg ) is the capital and second-largest city of the German States of Germany, state of Saxony-Anha ...
. The study of historic atmosphere is called
paleoclimatology Paleoclimatology (, palaeoclimatology) is the study of s for which direct measurements were not taken. As instrumental records only span a tiny part of , the reconstruction of ancient climate is important to understand natural variation and the e ...
.


Composition

The three major constituents of Earth's atmosphere are
nitrogen Nitrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

nitrogen
,
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
, and
argon Argon is a with the  Ar and  18. It is in group 18 of the and is a . Argon is the third-most abundant in the , at 0.934% (9340 ). It is more than twice as abundant as (which averages about 4000 ppmv, but varies greatly), 23 time ...

argon
. Water vapor accounts for roughly 0.25% of the atmosphere by mass. The concentration of water vapor (a greenhouse gas) varies significantly from around 10 ppm by volume in the coldest portions of the atmosphere to as much as 5% by volume in hot, humid air masses, and concentrations of other atmospheric gases are typically quoted in terms of dry air (without water vapor).Wallace, John M. and Peter V. Hobbs
''Atmospheric Science: An Introductory Survey''
. Elsevier. Second Edition, 2006. . Chapter 1
The remaining gases are often referred to as trace gases, among which are the
greenhouse gas A greenhouse gas (GHG or GhG) is a gas Gas is one of the four fundamental states of matter (the others being solid, liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformat ...
es, principally carbon dioxide, methane, nitrous oxide, and ozone. Besides argon, already mentioned, other
noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of s with similar properties; under , they are all odorless, colorless, gases with very low . The six naturally occurring noble gases are ( ...
es, neon, helium, krypton, and xenon are also present. Filtered air includes trace amounts of many other
chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be ...
s. Many substances of natural origin may be present in locally and seasonally variable small amounts as
aerosol An aerosol (abbreviation of "aero-solution") is a suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematics * Suspension of a ring, in mathe ...

aerosol
s in an unfiltered air sample, including
dust Dust is made of s of solid . On Earth, it generally consists of particles in the that come from various sources such as lifted by wind (an ), , and . Dust in homes is composed of about 20–50% dead . The rest, and in offices, and other ...
of mineral and organic composition,
pollen Pollen Tube Diagram Pollen is a powdery substance consisting of pollen grains which are microsporophytes of seed plants The spermatophytes, also known as phanerogams (taxon Phanerogamae) or phaenogams (taxon Phaenogamae), comprise those plan ...

pollen
and
spores )'', growing on a thinned hybrid black poplar ''(Populus x canadensis)''. The last stage of the moss lifecycle is shown, where the sporophyte 350px, Sporophytes of moss during spring A sporophyte () is the diploid Ploidy () is the number ...
,
sea spray Sea spray refers to aerosol particles that are formed directly from the ocean, mostly by ejection into the atmosphere by bursting bubbles at the air-sea interface. Sea spray contains both organic matter and inorganic salts that form sea salt aer ...
, and
volcanic ash , Chile Chile, officially the Republic of Chile, is a country in the western part of South America South America is a continent A continent is one of several large landmasses. Generally identified by convention (norm), con ...
. Various industrial
pollutant A pollutant is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. A pollutant may cause long- or short-term damage by changing the growth rate of plant or animal spec ...
s also may be present as gases or aerosols, such as
chlorine Chlorine is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the s ...

chlorine
(elemental or in compounds),
fluorine Fluorine is a chemical element with the Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard conditions for temperature and pressure, standard conditions as a highly toxic, pale yellow Diatomic molecule, ...

fluorine
compounds and elemental
mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

mercury
vapor. Sulfur compounds such as
hydrogen sulfide Hydrogen sulfide is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havi ...

hydrogen sulfide
and
sulfur dioxide Sulfur dioxide (-recommended spelling) or sulphur dioxide (traditional ) is the with the formula . It is a responsible for the smell of burnt es. It is released naturally by and is produced as a by-product of extraction and the burning of ...
(SO2) may be derived from natural sources or from industrial air pollution. The average
molecular weight A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion I ...
of dry air, which can be used to calculate densities or to convert between mole fraction and mass fraction, is about 28.946Detlev Möller: ''Luft: Chemie, Physik, Biologie, Reinhaltung, Recht.'' Walter de Gruyter, 2003, , S. 173
(View in Google Books)
or 28.96 g/mol. This is decreased when the air is humid. The relative concentration of gases remains constant until about .


Stratification

In general, air pressure and density decrease with altitude in the atmosphere. However, the temperature has a more complicated profile with altitude, and may remain relatively constant or even increase with altitude in some regions (see the
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...
section, below). Because the general pattern of the temperature/altitude profile, or
lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is 9 ...
, is constant and measurable by means of instrumented balloon soundings, the temperature behavior provides a useful metric to distinguish atmospheric layers. In this way, Earth's atmosphere can be divided (called atmospheric stratification) into five main layers. Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere. From highest to lowest, the five main layers are: * Exosphere: 700 to 10,000 km (440 to 6,200 miles) * Thermosphere: 80 to 700 km (50 to 440 miles) * Mesosphere: 50 to 80 km (31 to 50 miles) * Stratosphere: 12 to 50 km (7 to 31 miles) * Troposphere: 0 to 12 km (0 to 7 miles)


Exosphere

The exosphere is the outermost layer of Earth's atmosphere (i.e. the upper limit of the atmosphere). It extends from the
exobase The thermopause is the Earth's atmosphere, atmospheric boundary of Earth's energy system, located at the top of the thermosphere. The temperature of the thermopause could range from nearly absolute zero to . Below this, the atmosphere is defined to ...
, which is located at the top of the thermosphere at an altitude of about 700 km above sea level, to about 10,000 km (6,200 mi; 33,000,000 ft) where it merges into the
solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the solar corona, corona. This plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . ...

solar wind
. This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase. The atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another. Thus, the exosphere no longer behaves like a gas, and the particles constantly escape into space. These free-moving particles follow
ballistic trajectories Ballistics is the field of mechanics concerned with the launching, flight behavior and impact effects of projectiles, especially ranged weapon munitions such as bullets, unguided bombs, rockets or the like; the science or art of designing and acc ...
and may migrate in and out of the
magnetosphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses m ...

magnetosphere
or the solar wind. The exosphere is located too far above Earth for any meteorological phenomena to be possible. However, the
aurora borealis An aurora (plural: auroras or aurorae), sometimes referred to as polar lights (aurora polaris), northern lights (aurora borealis), or southern lights (aurora australis), is a natural light display in the Earth's sky, predominantly seen in high-l ...

aurora borealis
and
aurora australis An aurora (plural: auroras or aurorae), sometimes referred to as polar lights (aurora polaris), northern lights (aurora borealis), or southern lights (aurora australis), is a natural light display in the Earth's sky, predominantly seen in high-l ...

aurora australis
sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere. The exosphere contains many of the satellites orbiting Earth.


Thermosphere

The thermosphere is the second-highest layer of Earth's atmosphere. It extends from the mesopause (which separates it from the mesosphere) at an altitude of about up to the
thermopause The thermopause is the Earth's atmosphere, atmospheric boundary of Earth's energy system, located at the top of the thermosphere. The temperature of the thermopause could range from nearly absolute zero to . Below this, the atmosphere is defined to ...
at an altitude range of . The height of the thermopause varies considerably due to changes in solar activity. Because the thermopause lies at the lower boundary of the exosphere, it is also referred to as the
exobase The thermopause is the Earth's atmosphere, atmospheric boundary of Earth's energy system, located at the top of the thermosphere. The temperature of the thermopause could range from nearly absolute zero to . Below this, the atmosphere is defined to ...
. The lower part of the thermosphere, from above Earth's surface, contains the
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
. The temperature of the thermosphere gradually increases with height and can rise as high as , though the gas molecules are so far apart that its is not very meaningful. The air is so rarefied that an individual molecule (of
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
, for example) travels an average of between collisions with other molecules. Although the thermosphere has a high proportion of molecules with high energy, it would not feel hot to a human in direct contact, because its density is too low to conduct a significant amount of energy to or from the skin. This layer is completely cloudless and free of water vapor. However, non-hydrometeorological phenomena such as the
aurora borealis An aurora (plural: auroras or aurorae), sometimes referred to as polar lights (aurora polaris), northern lights (aurora borealis), or southern lights (aurora australis), is a natural light display in the Earth's sky, predominantly seen in high-l ...

aurora borealis
and
aurora australis An aurora (plural: auroras or aurorae), sometimes referred to as polar lights (aurora polaris), northern lights (aurora borealis), or southern lights (aurora australis), is a natural light display in the Earth's sky, predominantly seen in high-l ...

aurora australis
are occasionally seen in the thermosphere. The
International Space Station The International Space Station (ISS) is a Modular design, modular space station (habitable satellite, artificial satellite) in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (Uni ...

International Space Station
orbits in this layer, between. . It is this layer where many of the satellites orbiting the earth are present.


Mesosphere

The mesosphere is the third highest layer of Earth's atmosphere, occupying the region above the stratosphere and below the thermosphere. It extends from the stratopause at an altitude of about to the mesopause at above sea level. Temperatures drop with increasing altitude to the
mesopause The mesopause is the point of minimum temperature at the boundary between the mesosphere and the thermosphere Atmosphere of Earth, atmospheric regions. Due to the lack of solar heating and very strong radiative cooling from carbon dioxide, the me ...
that marks the top of this middle layer of the atmosphere. It is the coldest place on Earth and has an average temperature around . Just below the mesopause, the air is so cold that even the very scarce water vapor at this altitude can be sublimated into polar-mesospheric
noctilucent cloud Noctilucent clouds, or night shining clouds, are tenuous cloud In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. ...
s. These are the highest clouds in the atmosphere and may be visible to the naked eye if sunlight reflects off them about an hour or two after sunset or similarly before sunrise. They are most readily visible when the Sun is around 4 to 16 degrees below the horizon. Lightning-induced discharges known as
transient luminous event Upper-atmospheric lightning or ionospheric lightning are terms sometimes used by researchers to refer to a family of short-lived electrical-breakdown phenomena that occur well above the altitudes of normal lightning and storm clouds. Upper-atmosp ...
s (TLEs) occasionally form in the mesosphere above tropospheric thunderclouds. The mesosphere is also the layer where most
meteor A meteoroid () is a small rocky or metallic body in . Meteoroids are significantly smaller than s, and range in size from small grains to one-meter-wide objects. Objects smaller than this are classified as or . Most are fragments from s or as ...

meteor
s burn up upon atmospheric entrance. It is too high above Earth to be accessible to jet-powered aircraft and balloons, and too low to permit orbital spacecraft. The mesosphere is mainly accessed by
sounding rocket A sounding rocket or rocketsonde, sometimes called a research rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight. The rockets are used to carry instruments from ...
s and
rocket-powered aircraft A rocket-powered aircraft or rocket plane is an aircraft An aircraft is a vehicle or machine that is able to fly Flies are insect Insects or Insecta (from Latin Latin (, or , ) is a classical language belonging to the Ita ...
.


Stratosphere

The stratosphere is the second-lowest layer of Earth's atmosphere. It lies above the troposphere and is separated from it by the
tropopause The tropopause is the atmospheric boundary that demarcates the troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of w ...
. This layer extends from the top of the troposphere at roughly above Earth's surface to the
stratopause The stratopause (formerly mesopeak) is the level of the Earth's atmosphere, atmosphere which is the boundary between two layers: the stratosphere and the mesosphere. In the stratosphere the temperature increases with altitude, and the stratopause i ...
at an altitude of about . The atmospheric pressure at the top of the stratosphere is roughly 1/1000 the pressure at sea level. It contains the , which is the part of Earth's atmosphere that contains relatively high concentrations of that gas. The stratosphere defines a layer in which temperatures rise with increasing altitude. This rise in temperature is caused by the absorption of
ultraviolet radiation Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...
(UV) radiation from the Sun by the ozone layer, which restricts turbulence and mixing. Although the temperature may be at the tropopause, the top of the stratosphere is much warmer, and may be near 0 °C. The stratospheric temperature profile creates very stable atmospheric conditions, so the stratosphere lacks the weather-producing air turbulence that is so prevalent in the troposphere. Consequently, the stratosphere is almost completely free of clouds and other forms of weather. However, polar stratospheric or nacreous clouds are occasionally seen in the lower part of this layer of the atmosphere where the air is coldest. The stratosphere is the highest layer that can be accessed by jet-powered aircraft.


Troposphere

The troposphere is the lowest layer of Earth's atmosphere. It extends from Earth's surface to an average height of about , although this
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference and a point or object. The exact definition and reference datum varies according to the context (e.g. ...

altitude
varies from about at the
geographic pole A geographical pole or geographic pole is either of the two points on Earth where its axis of rotation intersects its surface. The North Pole lies in the Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five m ...
s to at the
Equator The Equator is a , about in circumference, that divides into the and hemispheres. It is an located at 0 degrees , halfway between the and poles. In , as applied in , the equator of a rotating (such as a ) is the parallel (circle of l ...

Equator
, with some variation due to weather. The troposphere is bounded above by the
tropopause The tropopause is the atmospheric boundary that demarcates the troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of w ...
, a boundary marked in most places by a
temperature inversion , Scotland Scotland ( sco, Scotland, gd, Alba ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a 96-mile (154 km) An ...
(i.e. a layer of relatively warm air above a colder one), and in others by a zone which is
isothermal In thermodynamics, an isothermal process is a type of thermodynamic process in which the temperature ''T'' of a Thermodynamic system, system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside the ...
with height. Although variations do occur, the temperature usually declines with increasing altitude in the troposphere because the troposphere is mostly heated through energy transfer from the surface. Thus, the lowest part of the troposphere (i.e. Earth's surface) is typically the warmest section of the troposphere. This promotes vertical mixing (hence, the origin of its name in the Greek word τρόπος, ''tropos'', meaning "turn"). The troposphere contains roughly 80% of the
mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value ...
of Earth's atmosphere. The troposphere is denser than all its overlying atmospheric layers because a larger atmospheric weight sits on top of the troposphere and causes it to be most severely compressed. Fifty percent of the total mass of the atmosphere is located in the lower of the troposphere. Nearly all atmospheric water vapor or moisture is found in the troposphere, so it is the layer where most of Earth's weather takes place. It has basically all the weather-associated cloud genus types generated by active wind circulation, although very tall cumulonimbus thunder clouds can penetrate the tropopause from below and rise into the lower part of the stratosphere. Most conventional
aviation Aviation is the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes airplane, fixed-wing and helicopter, rotary-wing types, morphable wings, wing-less lifting bodies, as well as aerostat, lighter-than-air ...
activity takes place in the troposphere, and it is the only layer that can be accessed by propeller-driven aircraft.


Other layers

Within the five principal layers above, which are largely determined by temperature, several secondary layers may be distinguished by other properties: * The is contained within the stratosphere. In this layer
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lower ...

ozone
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 Earth's atmosphere is contained in the stratosphere. * The
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
is a region of the atmosphere that is ionized by solar radiation. It is responsible for auroras. During daytime hours, it stretches from and includes the mesosphere, thermosphere, and parts of the exosphere. However, ionization in the mesosphere largely ceases during the night, so auroras are normally seen only in the thermosphere and lower exosphere. The ionosphere forms the inner edge of the
magnetosphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses m ...

magnetosphere
. It has practical importance because it influences, for example, radio propagation on Earth. * The homosphere and heterosphere are defined by whether the atmospheric gases are well mixed. The surface-based homosphere includes the troposphere, stratosphere, mesosphere, and the lowest part of the thermosphere, where the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence. This relatively homogeneous layer ends at the ''
turbopause The turbopause, also known as the homopause, marks the altitude in an atmosphere below which turbulent mixing dominates. Mathematically, it is defined as the point where the coefficient of eddy diffusion is equal to the coefficient of molecular dif ...
'' found at about , the very edge of space itself as accepted by the FAI, which places it about above the mesopause. :Above this altitude lies the heterosphere, which includes the exosphere and most of the thermosphere. Here, the chemical composition varies with altitude. This is because the 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 Depiction of where the planetary boundary layer lies on a sunny day. In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. ...
is the part of the troposphere that is closest to Earth's surface and is directly affected by it, mainly through . During the day the planetary boundary layer usually is well-mixed, whereas at night it becomes stably stratified with weak or intermittent mixing. The depth of the planetary boundary layer ranges from as little as about on clear, calm nights to or more during the afternoon in dry regions. The average temperature of the atmosphere at Earth's surface is or , depending on the reference.


Physical properties


Pressure and thickness

The average atmospheric pressure at sea level is defined by the
International Standard Atmosphere The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗ ...

International Standard Atmosphere
as . This is sometimes referred to as a unit of standard atmospheres (atm). Total atmospheric mass is 5.1480×1018 kg (1.135×1019 lb), about 2.5% less than would be inferred from the average sea level pressure and Earth's area of 51007.2 megahectares, this portion being displaced by Earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and
weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a p ...
. If the entire mass of the atmosphere had a uniform density equal to sea level density (about 1.2 kg per m3) from sea level upwards, it would terminate abruptly at an altitude of . It actually decreases exponentially with altitude, dropping by half every or by a factor of 1/ e every , the average
scale height In various scientific contexts, a scale height, usually denoted by the capital letter ''H'', is a distance over which a quantity decreases by a factor of e (the base of natural logarithms The natural logarithm of a number is its logarithm to ...
of the atmosphere below . However, the atmosphere is more accurately modeled with a customized equation for each layer that takes gradients of temperature, molecular composition, solar radiation and gravity into account. In summary, the mass of Earth's atmosphere is distributed approximately as follows: * 50% is below . * 90% is below . * 99.99997% is below , the
Kármán line The Kármán line is an attempt to define a boundary between Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose ab ...
. By international convention, this marks the beginning of space where human travelers are considered
astronaut An astronaut (from the Greek "astron" (ἄστρον), meaning "star", and "nautes" (ναύτης), meaning "sailor") is a person trained, equipped, and deployed by a List of human spaceflight programs, human spaceflight program to serve as a ...

astronaut
s. By comparison, the summit of is at ; commercial
airliners An airliner is a type of aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or by using the Lift (force) ...
typically cruise between where the thinner air improves fuel economy; weather balloons reach and above; and the highest North American X-15, X-15 flight in 1963 reached . Even above the Kármán line, significant atmospheric effects such as aurora (astronomy), auroras still occur. Meteors begin to glow in this region, though the larger ones may not burn up until they penetrate more deeply. The various layers of Earth's
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
, important to HF radio propagation, begin below 100 km and extend beyond 500 km. By comparison, the
International Space Station The International Space Station (ISS) is a Modular design, modular space station (habitable satellite, artificial satellite) in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (Uni ...

International Space Station
and Space Shuttle typically orbit at 350–400 km, within the F-layer of the ionosphere where they encounter enough atmospheric drag to require reboosts every few months, otherwise, orbital decay will occur resulting in a return to earth. Depending on solar activity, satellites can experience noticeable atmospheric drag at altitudes as high as 700–800 km.


Temperature and speed of sound

The division of the atmosphere into layers mostly by reference to temperature is discussed above. Temperature decreases with altitude starting at sea level, but variations in this trend begin above 11 km, where the temperature stabilizes through a large vertical distance through the rest of the troposphere. In the stratosphere, starting above about 20 km, the temperature increases with height, due to heating within the ozone layer caused by the capture of significant
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that stud ...

ultraviolet
radiation from the Sun by the dioxygen and ozone gas in this region. Still another region of increasing temperature with altitude occurs at very high altitudes, in the aptly-named thermosphere above 90 km. Because in an ideal gas of constant composition the speed of sound depends only on temperature and not on the gas pressure or density, the speed of sound in the atmosphere with altitude takes on the form of the complicated temperature profile (see illustration to the right), and does not mirror altitudinal changes in density or pressure.


Density and mass

The density of air at sea level is about 1.2 kg/m3 (1.2 g/L, 0.0012 g/cm3). 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 law). Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula. More sophisticated models are used to predict the orbital decay of satellites. The average mass of the atmosphere is about 5 quadrillion (5) tonnes or 1/1,200,000 the mass of Earth. According to the American 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."


Optical properties

Solar radiation (or sunlight) is the energy Earth receives from the Sun. Earth also emits radiation back into space, but at longer wavelengths that we cannot see. Part of the incoming and emitted radiation is absorbed or reflected by the atmosphere. In May 2017, glints of light, seen as twinkling from an orbiting satellite a million miles away, were found to be Reflection (physics), reflected light from ice crystals in the atmosphere.


Scattering

When light passes through Earth's 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 cannot 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's 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, O2 and O3 absorb almost all wavelengths shorter than 300 nanometers. Water (H2O) absorbs many wavelengths above 700 nm. When a molecule absorbs a photon, it increases the energy of the molecule. This heats the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below. The combined absorption spectra of the gases in the atmosphere leave "windows" of low Opacity (optics), opacity, allowing the transmission of only certain bands of light. The optical window runs from around 300 nm (
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that stud ...

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 window, 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.


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 , its radiation peaks near 500 nm, and is visible to the human eye. 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 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
greenhouse effect The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without this atmosphere. Radiatively active gases (i.e., es) in a planet's atmosphere radi ...

greenhouse effect
is directly related to this absorption and emission effect. Some gases in the atmosphere absorb and emit infrared radiation, but do not interact with sunlight in the visible spectrum. Common examples of these are and H2O.


Refractive index

The refractive index of air is close to, but just greater than 1. Systematic variations in the refractive index can lead to the bending of light rays over long optical paths. One example is that, under some circumstances, observers onboard ships can see other vessels just over the horizon because light is refracted in the same direction as the curvature of Earth's surface. The refractive index of air depends on temperature, giving rise to refraction effects when the temperature gradient is large. An example of such effects is the mirage.


Circulation

''Atmospheric circulation'' is the large-scale movement of air through the troposphere, and the means (with ocean circulation) by which heat is distributed around Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant because it is determined by Earth's rotation rate and the difference in solar radiation between the equator and poles.


Evolution of Earth's atmosphere


Earliest atmosphere

The paleoatmosphere, first atmosphere consisted of gases in the Solar nebula#Formation of planets, solar nebula, primarily hydrogen. There were probably simple hydrides such as those now found in the gas giants (Jupiter and Saturn), notably water vapor, methane and ammonia.


Second atmosphere

Outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids, produced the next atmosphere, consisting largely of
nitrogen Nitrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

nitrogen
plus carbon dioxide and inert gases. A major part of carbon-dioxide emissions dissolved in water and reacted with metals such as calcium and magnesium during weathering of crustal rocks to form carbonates that were deposited as sediments. Water-related sediments have been found that date from as early as 3.8 billion years ago. About 3.4 billion years ago, nitrogen formed the major part of the then stable "second atmosphere". The influence of life has to be taken into account rather soon in the history of the atmosphere because hints of early life-forms appear as early as 3.5 billion years ago. How Earth at that time maintained a climate warm enough for liquid water and life, if the early Sun put out 30% lower solar radiance than today, is a puzzle known as the "faint young Sun paradox". The geological record however shows a continuous relatively warm surface during the complete early temperature record of Earth – with the exception of one cold glacial phase about 2.4 billion years ago. In the late Archean Eon (geology), Eon an oxygen-containing atmosphere began to develop, apparently produced by photosynthesizing cyanobacteria (see Great Oxygenation Event), which have been found as stromatolite fossils from 2.7 billion years ago. The early basic carbon isotopy (isotope ratio proportions) strongly suggests conditions similar to the current, and that the fundamental features of the carbon cycle became established as early as 4 billion years ago. Proxy (climate)#Lake and ocean sediments, Ancient sediments in the Gabon dating from between about 2.15 and 2.08 billion years ago provide a record of Earth's dynamic oxygenation evolution. These fluctuations in oxygenation were likely driven by the Lomagundi carbon isotope excursion.


Third atmosphere

The constant re-arrangement of continents by plate tectonics influences the long-term evolution of the atmosphere by transferring carbon dioxide to and from large continental carbonate stores. Free oxygen did not exist in the atmosphere until about 2.4 billion years ago during the Great Oxygenation Event and its appearance is indicated by the end of the banded iron formations. Before this time, any oxygen produced by photosynthesis was consumed by the oxidation of reduced materials, notably iron. Molecules of free oxygen did not start to accumulate in the atmosphere until the rate of production of oxygen began to exceed the availability of reducing materials that removed oxygen. This point signifies a shift from a Redox, reducing atmosphere to an oxidizing atmosphere. O2 showed major variations until reaching a steady state of more than 15% by the end of the Precambrian. The following time span from 541 million years ago to the present day is the Phanerozoic Eon, during the earliest period of which, the Cambrian, oxygen-requiring metazoan life forms began to appear. The amount of oxygen in the atmosphere has fluctuated over the last 600 million years, reaching a peak of about 30% around 280 million years ago, significantly higher than today's 21%. Two main processes govern changes in the atmosphere: Plants photosynthesis, using carbon dioxide from the atmosphere and releasing oxygen, and then plants using some oxygen at night by the process of photorespiration with the remainder of the oxygen being used to breakdown adjacent organic material. Breakdown of pyrite and volcanic eruptions release sulfur into the atmosphere, which oxidizes and hence reduces the amount of oxygen in the atmosphere. However, volcanic eruptions also release carbon dioxide, which plants can convert to oxygen. The exact cause of the variation of the amount of oxygen in the atmosphere is not known. Periods with much oxygen in the atmosphere are associated with the rapid development of animals. Today's atmosphere contains 21% oxygen, which is great enough for this rapid development of animals.


Air pollution

''Air pollution'' is the introduction into the atmosphere of chemicals, Atmospheric particulate matter, particulate matter or Organic matter, biological materials that cause harm or discomfort to organisms. Stratosphere, Stratospheric
ozone depletion Ozone depletion consists of two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth, Earth's atmosphere (the ozone layer), and a much larger springtime decrease in stratosphe ...

ozone depletion
is caused by air pollution, chiefly from chlorofluorocarbons and other ozone-depleting substances. The scientific consensus is that the human impact on the environment, anthropogenic greenhouse gases currently accumulating in the atmosphere are the main cause of climate change.


Images from space

On October 19, 2015, NASA started a website containing daily images of the full sunlit side of Earth on http://epic.gsfc.nasa.gov/. The images are taken from the Deep Space Climate Observatory (DSCOVR) and show Earth as it rotates during a day.


See also

* Aerial perspective * Air (classical element) * Airglow, Air glow * Airshed * Atmospheric dispersion modeling * Atmospheric electricity * Atmospheric Radiation Measurement Climate Research Facility (ARM) (in the U.S.) * #Principal layers, Atmospheric stratification * Biosphere * Climate system ** Earth's energy budget * COSPAR international reference atmosphere (CIRA) * Environmental impact of aviation * Global dimming * Instrumental temperature record, Historical temperature record * Hydrosphere * Hypermobility (travel) * Kyoto Protocol * Leaching (agriculture) * Lithosphere * Reference atmospheric model


References


External links


Interactive global map of current atmospheric and ocean surface conditions.
{{Authority control Atmosphere of Earth, Atmospheric thermodynamics Breathing gases, Air Environments Articles containing video clips Climate,