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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.