Atmosphere of Earth
The atmosphere of Earth consists of a layer of mixed gas that is retained by gravity, surrounding the Earth's surface. It contains variable quantities of suspended aerosols and particulates that create weather features such as clouds and hazes. The atmosphere serves as a protective buffer between the Earth's surface and outer space. It shields the surface from most meteoroids and ultraviolet solar radiation, reduces diurnal temperature variation – the [|temperature] extremes between day and night, and keeps it warm through heat retention via the greenhouse effect. The atmosphere redistributes heat and moisture among different regions via air currents, and provides the chemical and climate conditions that allow life to exist and evolve on Earth.
By mole fraction, dry air contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other trace gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere.
Earth's primordial atmosphere consisted of gases accreted from the solar nebula, but the composition changed significantly over time, affected by many factors such as volcanism, outgassing, impact events, weathering and the evolution of life. In the present day, human activity has contributed to atmospheric changes, such as climate change, ozone depletion and acid deposition.
The atmosphere has a mass of about 5.15 kg, three quarters of which is within about of the surface. The atmosphere becomes thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line at is often used as a conventional definition of the edge of space. Several [|layers] can be distinguished in the atmosphere based on characteristics such as temperature and composition, namely the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Air composition, temperature and atmospheric pressure vary with altitude. Air suitable for use in photosynthesis by terrestrial plants and respiration of terrestrial animals is found within the troposphere.
The study of Earth's atmosphere and its processes is called atmospheric science, and includes multiple subfields, such as climatology and atmospheric physics. Early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann. The study of the historic atmosphere is called paleoclimatology.
Composition
The three major constituents of Earth's atmosphere are nitrogen, oxygen, and argon. Water vapor accounts for roughly 0.25% of the atmosphere by mass. In the lower atmosphere, the concentration of water vapor varies significantly from around 10 ppm by mole fraction in the coldest portions of the atmosphere to as much as 5% by mole fraction in hot, humid air masses, and concentrations of other atmospheric gases are typically quoted in terms of dry air. The remaining gases are often referred to as trace gases, among which are other greenhouse gases, principally carbon dioxide, methane, nitrous oxide, and ozone. Besides argon, other noble gases, neon, helium, krypton, and xenon are also present. Filtered air includes trace amounts of many other chemical compounds.Many substances of natural origin may be present in locally and seasonally variable small amounts as aerosols in an unfiltered air sample, including dust of mineral and organic composition, pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present as gases or aerosols, such as chlorine, fluorine compounds, and elemental mercury vapor. Sulfur compounds such as hydrogen sulfide and sulfur dioxide may be derived from natural sources or from industrial air pollution.
File:Msis_atmospheric_composition_by_height.svg|thumb|339x339px|The volume fraction of the main constituents of the Earth's atmosphere as a function of height, based on the . The model only works above 85 km.
The average molecular weight of dry air, which can be used to calculate densities or to convert between mole fraction and mass fraction, is about 28.946 or 28.964 g/mol. This is decreased when the air is humid.
Up to an altitude of around, atmospheric turbulence mixes the component gases so that their relative concentrations remain the same. There exists a transition zone from roughly where this turbulent mixing gradually yields to molecular diffusion. The latter process forms the heterosphere where the relative concentration of lighter gases increase with altitude.
Stratification
In general, air pressure and density decrease with altitude in the atmosphere. However, temperature has a more complicated profile with altitude and may remain relatively constant or even increase with altitude in some regions. Because the general pattern of the temperature/altitude profile, or lapse rate, is constant and measurable by means of instrumented balloon soundings, the temperature behavior provides a useful metric to distinguish atmospheric layers. This atmospheric stratification divides the Earth's atmosphere into five main layers with these typical altitude ranges:- Exosphere:
- Thermosphere:
- Mesosphere:
- Stratosphere:
- Troposphere:
Exosphere
The upper limit varies depending on the definition. Various authorities consider it to end at about or about —about halfway to the moon, where the influence of Earth's gravity is about the same as radiation pressure from sunlight. The geocorona visible in the far ultraviolet extends to at least.
This layer is mainly composed of extremely low densities of hydrogen, with limited amounts of helium, carbon dioxide, and nascent oxygen closer to the exobase. The atoms and molecules are so far apart that they can travel hundreds of kilometres 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 and may migrate in and out of the magnetosphere or the solar wind. Every second, the Earth loses about 3 kg of hydrogen, 50 g of helium, and much smaller amounts of other constituents.
The exosphere is too far above Earth for meteorological phenomena to be possible. The exosphere contains many of the artificial satellites that orbit Earth.
Thermosphere
The thermosphere is the second-highest layer of Earth's atmosphere. It extends from the mesopause at an altitude of about up to the thermopause at an altitude range of. The height of the thermopause varies considerably due to changes in solar activity. The passage of the dusk and dawn solar terminator creates background density perturbations up to a factor of two through this layer, forming a dominant feature in this region. Because the thermopause lies at the lower boundary of the exosphere, it is also referred to as the exobase. Overlapping the thermosphere, from above Earth's surface, is the ionosphere – a region of enhanced plasma density.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 temperature in the usual sense is not very meaningful. This temperature increase is caused by absorption of ionizing UV and X-ray emission from the Sun. The air is so rarefied that an individual molecule 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 and aurora australis are occasionally seen in the thermosphere at an altitude of around. The colors of the aurora are linked to the properties of the atmosphere at the altitude they occur. The most common is the green aurora, which comes from atomic oxygen in the 1S state, and occurs at altitudes from. The International Space Station orbits in the thermosphere, 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 that marks the top of this middle layer of the atmosphere. It is the coldest place on Earth and has an average temperature around. Because the atmosphere absorbs sound waves at a rate that is proportional to the square of the frequency, audible sounds from the ground do not reach the mesosphere. Infrasonic waves can reach this altitude, but they are difficult to emit at a high power level.Just below the mesopause, the air is so cold that even the very scarce water vapor at this altitude can condense into polar-mesospheric noctilucent clouds of ice particles. 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 events occasionally form in the mesosphere above tropospheric thunderclouds. The mesosphere is also the layer where most meteors and satellites 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 rockets and rocket-powered aircraft.