Atmosphere” and are representative of mid-latitude
conditions. The extension shown in the insert is
speculative. These divisions are for reference of
thermal structure (lapse rates) or other significant
features and are not intended to imply that these layers
or zones are independent domains. Earth is surrounded
by
one
atmosphere,
not
by
a
number
of
sub-atmospheres.
The layers and zones are discussed under two
separate
classifications.
One
is
the
METEOROLOGICAL classification that defines zones
according to their significance for the weather. The
other is the ELECTRICAL classification that defines
zones according to electrical characteristics of gases of
the atmosphere.
Meteorological Classification
In the meteorological classification (commencing
with Earth’s surface and proceeding upward) we have
the troposphere, tropopause, stratosphere, stratopause,
mesosphere,
mesopause,
thermosphere,
and
the
exosphere.
These
classifications
are
based
on
temperature characteristics. (See fig. 1-9 for some
examples.)
TROPOSPHERE.—The troposphere is the layer
of air enveloping Earth immediately above Earth’s
surface. It is approximately 5 1/2 miles (29,000 ft or 9
kin) thick over the poles, about 7 1/2 miles (40,000 ft or
12.5 kin) thick in the mid-latitudes, and about 11 1/2
miles (61,000 ft or 19 kin) thick over the Equator. The
figures for thickness are average figures; they change
somewhat from day to day and from season to season.
The troposphere is thicker in summer than in winter and
is thicker during the day than during the night. Almost
all weather occurs in the troposphere. However, some
phenomena such as turbulence, cloudiness (caused by
ice crystals), and the occasional severe thunderstorm
top occur within the tropopause or stratosphere.
The troposphere is composed of a mixture of
several different gases. By volume, the composition of
dry air in the troposphere is as follows: 78 percent
nitrogen, 21 percent oxygen, nearly 1-percent argon,
and about 0.03 percent carbon dioxide. In addition, it
contains minute traces of other gases, such as helium,
hydrogen, neon, krypton, and others.
The air in the troposphere also contains a variable
amount of water vapor. The maximum amount of water
vapor that the air can hold depends on the temperature
of the air and the pressure. The higher the temperature,
the more water vapor it can hold at a given pressure.
The
air
also
contains
variable
amounts
of
impurities, such as dust, salt particles, soot, and
chemicals. These impurities in the air are important
because of their effect on visibility and the part they
play in the condensation of water vapor. If the air were
absolutely pure, there would be little condensation.
These
minute
particles
act
as
nuclei
for
the
condensation of water vapor. Nuclei, which have an
affinity for water vapor, are called HYGROSCOPIC
NUCLEI.
The
temperature
in
the
troposphere
usually
decreases with height, but there may be inversions for
relatively thin layers at any level.
TROPOPAUSE.—The tropopause is a transition
layer between the troposphere and the stratosphere. It is
not uniformly thick, and it is not continuous from the
equator to the poles. In each hemisphere the existence
of three distinct tropopauses is generally agreed
upon—one in the subtropical latitudes, one in middle
latitudes, and one in subpolar latitudes. They overlap
each other where they meet.
The tropopause is characterized by little or no
change in temperature with increasing altitude. The
composition of gases is about the same as that for the
troposphere. However, water vapor is found only in
very minute quantities at the tropopause and above it.
STRATOSPHERE.—The stratosphere directly
overlies the tropopause and extends to about 30 miles
(160,000 ft or 48 kilometers). Temperature varies little
with height in the stratosphere through the first 30,000
feet (9,000 meters); however, in the upper portion the
temperature increases approximately linearly to values
nearly equal to surface temperatures. This increase in
temperature through this zone is attributed to the
presence of ozone that absorbs incoming ultraviolet
radiation.
STRATOPAUSE.—The stratopause is the top of
the stratosphere. It is the zone marking another reversal
with
increasing
altitude
(temperature
begins
to
decrease with height).
MESOSPHERE.—The mesosphere is a layer
approximately 20 miles (100,000 ft or 32 kilometers)
thick
directly
overlaying
the
stratopause.
The
temperature decreases with height.
MESOPAUSE.—The mesopause is the thin
boundary zone between the mesosphere and the
thermosphere.
It
is
marked
by
a
reversal
of
temperatures; i.e., temperature again increases with
altitude.
1-16
