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A gaseous layer that envelops the Earth and most other planets in the solar system. Earth, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Titan (Saturn's largest satellite) are all known to possess substantial atmospheres that are held by the force of gravity. The structure and properties of the various atmospheres are determined by the interplay of physical and chemical processes. Structural features of Earth's atmosphere detailed below can often be identified in the atmospheres of other planetary bodies.

Composition of the atmosphere*


Fraction volume near surface

Vertical distribution

Major constituents


7.8084 × 10-1

Mixed in homosphere; photochemical dissociation high in thermosphere


2.0946 × 10-1

Mixed in homosphere; photochemically dissociated in thermosphere, with some dissociation in mesosphere and stratosphere


9.34 × 10-3

Mixed in homosphere with diffusive separation increasing above

Important radiative constituents


3.5 × 10-4

Mixed in homosphere; photochemical dissociation in thermosphere


Highly variable

Forms clouds in stratosphere; photochemical dissociation above mesosphere



Small amounts, 10-8, in troposphere; important layer, 10-6 to 10-5, in stratosphere; dissociated above

Other constituents


1.82 × 10-5



5.24 × 10-6

Mixed in homosphere with diffusive separation increasing above


1.14 × 10-6



1.15 × 10-6

Mixed in troposphere; dissociated in upper stratosphere and above


5 × 10-7

Mixed in homosphere; product of H2O photochemical reactions in lower thermosphere, and dissociated above



Photochemically produced in stratosphere and mesosphere

*Other gases, for example, CO, N2O, NO2, and many by-products of atmospheric pollution also exist in small amounts.

The composition of the Earth's atmosphere is primarily nitrogen (N2), oxygen (O2), and argon (Ar) [see table]. The concentration of water vapor (H2O) is highly variable, especially near the surface, where volume fractions can vary from nearly 0% to as high as 4% in the tropics. There are many minor constituents or trace gases, such as neon (Ne), helium (He), krypton (Kr), and xenon (Xe), that are inert, and active species such as carbon dioxide (CO2), methane (CH4), hydrogen (H2), nitrous oxide (NO), carbon monoxide (CO), ozone (O3), and sulfur dioxide (SO2), that play an important role in radiative and biological processes.

In addition to the gaseous component, the atmosphere suspends many solid and liquid particles. Aerosols are particulates usually less than 1 micrometer in diameter that are created by gas-to-particle reactions or are lifted from the surface by the wind. A portion of these aerosols can become centers of condensation or deposition in the growth of water and ice clouds. Cloud droplets and ice crystals are made primarily of water with some trace amounts of particles and dissolved gases. Their diameters range from a few micrometers to about 100 ?m. Water or ice particles larger than about 100 ?m begin to fall because of gravity and may result in precipitation at the surface.Cloud physics Precipitation (meteorology)

One of the remarkable properties of the Earth's atmosphere is the large amount of free molecular oxygen in the presence of gases such as nitrogen, methane, water vapor, hydrogen, and others that are capable of being oxidized. The atmosphere is in a highly oxidizing state that is far from chemical equilibrium. This is in sharp contrast to the atmospheres of Venus and Mars, the planets closest to the Earth, which are composed almost entirely of the more oxidized state, carbon dioxide. The chemical disequilibrium on the Earth is maintained by a continuous source of reactive gases derived from biological processes. Life plays a vital role in maintaining the present atmospheric composition.Atmospheric chemistry

The total mass of the Earth's atmosphere is about 5.8 × 1015 tons (5.3 × 1015 metric tons). The vertical distribution of gaseous mass is maintained by a balance between the downward force of gravity and the upward pressure gradient force. The balance is known as the hydrostatic balance or the barometric law. Hence, the declining atmospheric pressure that is measured while ascending in the atmosphere is a result of gravity. The globally averaged pressure at mean sea level is 1013.25 millibars (101,325 pascals).

Below about 60 mi (100 km) in altitude, the atmosphere's composition of major constituents is very uniform. This region is known as the homosphere to distinguish it from the heterosphere above 60 mi (100 km), where the relative amounts of the major constituents change with height. In the homosphere there are sufficient atmospheric motions and a short enough molecular free path to maintain uniformity in composition. Above the boundary between the homosphere and the heterosphere, known as the homopause or turbopause, the mean free path of the individual molecules becomes long enough that gravity is able to partially separate the lighter molecules from the heavier ones. The mean free path is the average distance that a particle will travel before encountering a collision. Hence the average molecular weight of the heterosphere decreases with height as the lighter atoms dominate the composition.

The vertical structure of the atmosphere is in large part determined by the transfer properties of the solar and terrestrial radiation streams. The energy of the smallest unit of radiation, the photon, is directly proportional to its frequency. The type of interaction that occurs between photons and the atmosphere depends on the energy of the photons.

The most energetic of the photons are x-rays and extreme ultraviolet radiation of the eletromagnetic spectrum, which are capable of dissociating and ionizing the gaseous molecules. The less energetic near-ultraviolet photons are able to excite molecules and atoms into higher electronic levels. As a result, most of the ultraviolet and x-ray radiation is attenuated by the upper atmosphere. A cloudless atmosphere, however, is relatively transparent to visible light, where most of the solar energy resides. At the opposite end of the spectrum toward the lower frequencies of radiation is the infrared part, which is capable of inducing various vibrational and rotational motions in triatomic and polyatomic molecules.

In order to maintain an energy balance, the Earth must emit about the same amount of radiation as it absorbs from the Sun. The terrestrial radiation occurs in the infrared part of the spectrum and hence is strongly affected by water vapor, clouds, carbon dioxide, and ozone and other trace gases. The ability of these gases to absorb and emit in the infrared allows them to effectively trap some of the outgoing radiation that is emitted by the surface, creating the so-called greenhouse effect.Greenhouse effect Insolation Terrestrial radiation

The atmospheric layer that extends from the surface to about 7 mi (11 km) is called the troposphere. The tropopause, which is the top of the troposphere, has an average altitude that varies from about 11 mi (18 km) near the Equator to about 5 mi (8 km) near the Poles. The actual tropopause height varies considerably on time scales from a few days to an entire year. The troposphere contains about 80% of the atmospheric mass and exhibits most of the day-to-day weather fluctuations that are observed from the ground. Temperatures generally decrease with increasing altitude at an average lapse rate of about 17°F/mi (6°C/km), although this rate varies considerably, depending on time and location.Tropopause Troposphere

The stratosphere is the atmospheric layer that extends from the tropopause up to the stratopause at about 30 mi (50 km) above the surface. It is characterized by a nearly isothermal layer in the first 6 mi (10 km) overlaid by a layer in which the temperature increases with height to a maximum of about 32°F (0°C) at the stratopause. The reversal in the temperature lapse rate is a result of direct absorption of solar radiation, mainly by ozone and oxygen at the ultraviolet frequencies.Stratosphere

The reversal of the temperature lapse rate makes the stratosphere vertically stable. This stability limits the amount of vertical mixing and results in molecular residence times of many months to years. Another consequence of a stable stratosphere is that it acts as a lid on the troposphere, confining the strong vertical overturning and hence most of the surface-based weather phenomena.Weather

The mesosphere is the atmospheric layer extending from the stratopause up to the mesopause at an altitude of about 53 mi (85 km). The mesosphere is characterized by temperatures decreasing with height at a rate of about 12°F/mi (4°C/km). Although the mesosphere has less vertical stability than the stratosphere, it is still more stable than the troposphere and does not experience rapid overturning. The coldest temperatures of the entire atmosphere are encountered at the mesopause, with values as low as ?150°F (?100°C). The temperature lapse rate found in the mesosphere is a result of the gradual weakening with height of the direct absorption of solar radiation by ozone. The radiative infrared cooling to space by the carbon dioxide molecules is responsible for the low temperatures near the mesopause.Mesosphere

The thermosphere is found above the mesopause. The thermosphere is characterized by rising temperatures with height up to an altitude of about 190 mi (300 km) and then is nearly isothermal above that. Although there is no clear upper limit to the thermosphere, it is convenient to consider it extending several thousand kilometers. Embedded within the thermosphere is the ionosphere, comprising those atmospheric layers in which the ionized molecules and atoms are dominating the processes.

Molecular species dominate the lower thermosphere, while atomic species are dominant above 190 mi (300 km). The distribution of the constituents is controlled by diffusive equilibrium in which the concentration of each constituent decreases exponentially with height according to its molecular weight. Hence the concentration of the heavier constituents such as nitrogen, oxygen, and carbon dioxide will decrease with height faster than the lighter constituents such as helium and hydrogen. At an altitude of 560 mi (900 km) helium becomes the dominant constituent while hydrogen dominates above 1900 mi (3000 km).

The ionosphere can be defined operationally as that part of the atmosphere that is sufficiently ionized to affect the propagation of radio waves. In the ionosphere, the dominant negative ion is the electron, and the main positive ions include O+, NO+, and O2+. The ionosphere is classified into four subregions. The D region extends from 40 to 60 mi (60 to 90 km) and contains complex ionic chemistry; most of the ionization is caused by ultraviolet ionization of NO and by galactic cosmic rays. This region is responsible for the daytime absorption of radio waves, which prevents distant propagation of certain frequencies. The E region extends from 60 to 90 mi (90 to 150 km) and is caused primarily by the x-rays from the Sun. The F1 region from 90 to 125 mi (150 to 200 km) is caused by the extreme ultraviolet radiation from the Sun and disappears at night. Finally, the F2 region includes all the ionized particles above 125 mi (200 km), with the peak ion concentrations occurring near 190 mi (300 km).

The exosphere is the atmosphere above 300 mi (500 km) where the probability of interatomic collisions is so low that some of the atoms traveling upward with sufficient velocity can escape the Earth's gravitational field. The dominant escaping atom is hydrogen since it is the lightest constituent. Calculations of the thermal escape of hydrogen (also known as the Jeans escape) yield a value of about 3 × 108 atoms · cm?2 · s?1. This is a very small amount since at this rate less than 0.5% of the oceans would disappear over the current age of the Earth.

The magnetosphere is the region surrounding the Earth where the movement of ionized gases is dominated by the geomagnetic field. The lower boundary of the magnetosphere, which occurs at an altitude of nearly 75 mi (120 km), can be roughly defined as the height where there are enough neutral atoms that the ion-neutral particle collisions dominate the ion motion. The dynamics of the magnetosphere is dictated in part by its interaction with the plasma of ionized gases that blows away from the Sun, the solar wind. The solar wind interacts with the Earth's magnetic field and severely deforms it, producing a magnetosphere around the Earth. It extends about 40,000 mi (60,000 km) toward the Sun but extends beyond the orbit of the Moon away from the Sun.

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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