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Earth's Atmosphere

Present Day Atmosphere

The atmosphere we breathe is a relatively stable mixture of several hundred types of gases from different origins. This gaseous envelope surrounds the planet and revolves with it. It has a mass of about 5.15 x 10E15 tons held to the planet by gravitational attraction. The proportions of gases, excluding water vapor, are nearly uniform up to approximately 80 kilometers (km) above Earth's surface. The major components of this region, by volume, are oxygen (21%), nitrogen (78%), and argon (0.93%). Small amounts of other gases are also present. These remaining trace gases exist in such small quantities that they are measured in terms of a mixing ratio. This ratio is defined as the number of molecules of the trace gas divided by the total number of molecules present in the volume sampled. For example, O3, CO2, and chlorofluorocarbons (CFCs) are measured in parts per million by volume (ppmv), parts per billion by volume (ppbv) or parts per trillion by volume (pptv).

Temerature Distribution
Detail of Temperature Distribution in the Atmosphere.(Click for the full and larger image.)

Atmospheric temperature and chemistry are believed to be controlled by the trace gases. There is increasing evidence that the percentages of environmentally significant trace gases are changing because of both natural and human factors. Examples of man-made gases are the chlorofluorocarbons CFC-11 and CFC-12 and halons. Carbon dioxide, nitrous oxide, and methane (CH4) are produced by the burning of fossil fuels, expelled from living and dead biomass, and released by the metabolic processes of microorganisms in the soil, wetlands, and oceans of our planet.

Atmosphere Structure

Atmospheric Strata
Detail of Atmospheric Strata. (Click for the full and larger image.)

The gaseous area surrounding the planet is divided into several concentric spherical strata separated by narrow transition zones. The upper boundary at which gases disperse into space lies at an altitude of approximately 1000 km above sea level. More than 99% of the total atmospheric mass is concentrated in the first 40 km from Earth's surface.

Atmospheric layers are characterized by differences in chemical composition that produce variations in temperature.


Innermost Layer of the Atmosphere
Detail of Innermost Layer of the Atmosphere. (Click for the full and larger image.)

The troposphere is the atmospheric layer closest to the planet and contains the largest percentage of the mass of the total atmosphere. It is characterized by the density of its air and an average vertical temperature change of 6 degrees Celsius (C) per kilometer.

Temperature and water vapor content in the troposphere decrease rapidly with altitude. Water vapor plays a major role in regulating air temperature because it absorbs solar energy and thermal radiation from the planet's surface. The troposphere contains 99% of the water vapor in the atmosphere. Water vapor concentrations vary with latitudinal position. They are greatest above the tropics, where they may be as high as 3%, and decrease toward the polar regions.

Weather in the Troposhere
Detail of Weather in the Troposphere. (Click for the full and larger image.)

All weather phenomena occur within the troposphere, although turbulence may extend into the lower portion of the stratosphere. Troposphere means "region of mixing" and is so named because of vigorous convective air currents within the layer.

Movement of the Tropopause
Detail of Movement of the Tropopause. (Click for the full and larger image.)

The upper boundary of the layer ranges in height from 8 km in high latitudes, to 18 km above the equator. Its height also varies with the seasons; highest in the summer and lowest in the winter. A narrow zone called the tropopause separates the troposphere from the next highest layer called the stratosphere. Air temperature within the tropopause remains constant with increasing altitude.


Location of the Stratosphere
Detail of Location of the Stratosphere. (Click for the full and larger image.)

The stratosphere is the second major strata of air in the atmosphere. It resides between 10 and 50 km above the planet's surface. The air temperature in the stratosphere remains relatively constant up to an altitude of 25 km. Then it increases gradually to 200-220 degrees Kelvin (K) at the lower boundary of the stratopause (~50 km), which is marked by a decrease in temperature. Because the air temperature in the stratosphere increases with altitude, it does not cause convection and has a stabilizing effect on atmospheric conditions in the region. Ozone plays the major role in regulating the thermal regime of the stratosphere, as water vapor content within the layer is very low. Temperature increases with ozone concentration. Solar energy is converted to kinetic energy when ozone molecules absorb ultraviolet radiation, resulting in heating of the stratosphere.

Ozone Distribution
Detail of Ozone Distribution in the Stratosphere. (Click for the full and larger image.)

The ozone layer is located at an altitude between 20-30 km. Approximately 90% of the ozone in the atmosphere resides in the stratosphere. Ozone concentration in this region is about 10 parts per million by volume as compared to approximately 0.04 parts per million by volume in the troposphere. Ozone absorbs the bulk of solar ultraviolet radiation in wavelengths from 290 nm - 320 nm. These wavelengths are harmful to life because they can be absorbed by the nucleic acid in cells. Increased penetration of ultraviolet radiation to the planet's surface would damage plant life and have harmful environmental consequences. Appreciably large amounts of solar ultraviolet radiation would result in a host of biological effects, such as a dramatic increase in cancers.

Meteorological conditions strongly affect the distribution of ozone. Most ozone production and destruction occurs in the tropical upper stratosphere, where the largest amounts of ultraviolet radiation are present. Dissociation takes place in lower regions of the stratosphere and occurs at higher latitudes than does production.


Location of the Mesosphere
Detail of Location of the Mesosphere. (Click for the full and larger image.)
The mesosphere, a layer extending from approximately 50 km to 80 km, is characterized by decreasing temperatures, which reach 190-180 K at an altitude of 80 km. In this region, concentrations of ozone and water vapor are negligible. Hence the temperature is lower than that of the troposphere or stratosphere. With increasing distance from Earth's surface the chemical composition of air becomes strongly dependent on altitude and the atmosphere becomes enriched with lighter gases. At very high altitudes, the residual gases begin to stratify according to molecular mass, because of gravitational separation.


Location of the Thermosphere
Detail of Location of the Thermosphere. (Click for the full and larger image.)
The thermosphere is located above the mesosphere and is separated from it by the mesopause transition layer. The temperature in the thermosphere generally increases with altitude up to 1000-1500 K. This increase in temperature is due to the absorption of intense solar radiation by the limited amount of remaining molecular oxygen. At an altitude of 100-200 km, the major atmospheric components are still nitrogen and oxygen. At this extreme altitude gas molecules are widely separated.


Outermost Layer of the Atmosphere
Detail of the Outermost Layer of the Atmosphere. (Click for the full and larger image.)

The exosphere is the most distant atmospheric region from Earth's surface. The upper boundary of the layer extends to heights of perhaps 960 to 1000 km and is relatively undefined. The exosphere is a transitional zone between Earth's atmosphere and interplanetary space.

Text, images and videos courtesy of Distributed Active Archive Center at NASA's Goddard Space Flight Center.

Related to chapter 3 in the print guide.
Related Materials

See Scientific Ballooning for more on how scientists use balloons for research. Also, see Layers of the Atmosphere.

Glossary Terms

Click for the definitions of the following words that are used on this page: (Definitions appear in a pop-up window.)

ultraviolet radiation

View the full, printable version of the glossary.

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