Atmospheric Layers
Although air is well mixed throughout the atmosphere,
the atmosphere itself is not physically uniform but has significant
variations in temperature and pressure with altitude, which define a
number of atmospheric layers. These include the troposphere (0 to 16
km), stratosphere (16 to 50 km), mesosphere (50 to 80km) and
thermosphere (80 to 640km). The boundaries between these four layers are
defined by abrupt changes in temperature, and include respectively the
tropopause, stratopause and mesopause. In the troposphere and
mesosphere, temperature generally falls with increasing altitude, whilst
in the stratosphere and thermosphere, temperature rises with increasing
altitude.
In addition to temperature, other criteria can be
used to define different layers in the atmosphere. The ionosphere, for
example, which occupies the same region of the atmosphere as the
thermosphere, is defined by the presence of ions, a physico-chemical
criterion. The region beyond the ionosphere is known as the exosphere.
The ionosphere and the exosphere together make up the upper atmosphere
(or thermosphere). The magnetosphere is the region above the Earth's
surface in which charged particles are affected by the Earth's magnetic
field.
Another well-known layer of the atmosphere is the
ozone layer, occupying much of the stratosphere. This layer is defined
by its chemical composition - where ozone is especially abundant.
Layers of the Atmosphere
Troposphere
The lowest layer of the atmosphere is called the
troposphere. It ranges in thickness from 8km at the poles to 16km over
the equator. The troposphere is bounded above by the tropopause, a
boundary marked by stable temperatures. Above the troposphere is the
stratosphere. Although variations do occur, temperature usually declines
with increasing altitude in the troposphere. Hill walkers know that it
will be several degrees cooler on the top of a mountain than in the
valley below.
The troposphere is denser than the layers of the
atmosphere above it (because of the weight compressing it), and it
contains up to 75% of the mass of the atmosphere. It is primarily
composed of nitrogen (78%) and oxygen (21%) with only small
concentrations of other trace gases. Nearly all atmospheric water vapour
or moisture is found in the troposphere.
The troposphere is the layer where most of the
world's weather takes place. Since temperature decreases with altitude
in the troposphere, warm air near the surface of the Earth can readily
rise, being less dense than the colder air above it. In fact air
molecules can travel to the top of the troposphere and back down again
in a just a few days. Such vertical movement or convection of air
generates clouds and ultimately rain from the moisture within the air,
and gives rise to much of the weather which we experience. The
troposphere is capped by the tropopause, a region of stable temperature.
Air temperature then begins to rise in the stratosphere. Such a
temperature increase prevents much air convection beyond the tropopause,
and consequently most weather phenomena, including towering
cumulonimbus thunderclouds, are confined to the troposphere.
Sometimes the temperature does not decrease with
height in the troposphere, but increases. Such a situation is known as a
temperature inversion. Temperature inversions limit or prevent the
vertical mixing of air. Such atmospheric stability can lead to air
pollution episodes with air pollutants emitted at ground level becoming
trapped underneath the temperature inversion.
Stratosphere
The stratosphere is the second major layer of the
atmosphere. It lies above the troposphere and is separated from it by
the tropopause. It occupies the region of atmosphere from about 12 to 50
km, although its lower boundary tends to be higher nearer the equator
and lower nearer the poles.
The stratosphere defines a layer in which
temperatures rises with increasing altitude. At the top of the
stratosphere the thin air may attain temperatures close to 0°C. This
rise in temperature is caused by the absorption of ultraviolet (UV)
radiation from the Sun by the ozone layer. Such a temperature profile
creates very stable atmospheric conditions, and the stratosphere lacks
the air turbulence that is so prevalent in the troposphere.
Consequently, the stratosphere is almost completely free of clouds or
other forms of weather.
The stratosphere provides some advantages for
long-distant flight because it is above stormy weather and has strong,
steady, horizontal winds.
The stratosphere is separated from the mesosphere above by the stratopause.
Mesosphere
The mesosphere (literally middle sphere) is the third
highest layer in our atmosphere, occupying the region 50 km to 80 km
above the surface of the Earth, above the troposphere and stratosphere,
and below the thermosphere. It is separated from the stratosphere by the
stratopause and from the thermosphere by the mesopause.
Temperatures in the mesosphere drop with increasing
altitude to about -100°C. The mesosphere is the coldest of the
atmospheric layers. In fact it is colder then Antarctica's lowest
recorded temperature. It is cold enough to freeze water vapor into ice
clouds. You can see these clouds if sunlight hits them after sunset.
They are called Noctilucent Clouds (NLC). NLCs are most readily visible
when the Sun is from 4 to 16 degrees below the horizon.
The mesosphere is also the layer in which a lot of meteors burn up while
entering the Earth's atmosphere. From the Earth they are seen as
shooting stars. The dark blue layer next to the blackness of space in
the image below is the mesosphere. In fact the upper atmosphere extends
much further out, into the thermosphere.
The thermosphere (literally "heat sphere") is the
outer layer of the atmosphere, separated from the mesosphere by the
mesopause. Within the thermosphere temperatures rise continually to well
beyond 1000°C. The few molecules that are present in the thermosphere
receive extraordinary amounts of energy from the Sun, causing the layer
to warm to such high temperatures. Air temperature, however, is a
measure of the kinetic energy of air molecules, not of the total energy
stored by the air. Therefore, since the air is so thin within the
thermosphere, such temperature values are not comparable to those of the
troposphere or stratosphere. Although the measured temperature is very
hot, the thermosphere would actually feel very cold to us because the
total energy of only a few air molecules residing there would not be
enough to transfer any appreciable heat to our skin.
The lower part of the thermosphere, from 80 to 550 km
above the Earth's surface, contains the ionosphere. Beyond the
ionosphere extending out to perhaps 10,000 km is the exosphere or outer
thermosphere, which gradually merges into space.
Ionosphere
The ionosphere is a layer of ionized air in the
atmosphere extending from almost 80 km above the Earth's surface
altitudes of 600 km and more. Technically, the ionosphere is not another
atmospheric layer. It occupies the same region of the upper atmosphere
as the thermosphere. In this region of the atmosphere the Sun's energy
is so strong that it breaks apart molecules and atoms of air, leaving
ions (atoms with missing electrons) and free-floating electrons. The
ionosphere is the region of the atmosphere where the aurorae occur.
Ionisation of air molecules in the ionosphere is
produced by ultraviolet radiation from the Sun, and to a lesser extent
by high-energy particles from the Sun and from cosmic rays.
Aurora
On certain nights at high latitudes shifting patterns
of light may been seen in the sky. These are the aurorae. The bright
lights are caused by high-energy particles streaming out from the Sun -
the solar wind - striking the Earth's upper atmosphere or ionosphere.
Energy from these electrically charged particles is converted into
light, forming visible glows, rays, arcs, bands and veils. This light is
usually greenish, but is sometimes red as well. The charged particles
are attracted by the Earth's magnetic field. Near the magnetic poles,
the Earth's magnetic field becomes much stronger. Consequently, it is
nearer the magnetic poles that the aurorae are most frequently
witnessed.
The power of aurorae depends mostly on the strength
of the solar wind. During an intense solar storm, the wind can intensify
very strongly, and aurorae may be seen at lower latitudes further from
the magnetic poles. Every 11 years, at the peak of the sunspot cycle
there is an increase in intensity of the solar wind, and with it, an
increase in frequency and intensity of auroral displays.
Aurorae occur in both the Northern and Southern
Hemispheres. In the Northern Hemisphere, the display is known as the
aurora borealis, or northern lights. In the Southern Hemisphere, it is
called the aurora australis, or southern lights. The term aurora
polaris, polar lights, is a general name for both. Aurorae are usually
visible from within the Arctic or Antarctic circles - Antarctica,
Greenland, Iceland and Northern regions of Canada, Alaska, Scandinavia
and Russia. During times of more intense activity on the Sun, auroral
storms can be viewed at lower latitudes such as northern Scotland and
most of Norway, Sweden and Finland. Very rarely displays can be seen
from northerly parts of Europe and the United States.
The large number of free electrons in the ionosphere
allows the propagation of electromagnetic waves. Radio signals - a form
of electromagnetic radiation - can be "bounced" off the ionosphere
allowing radio communication over long distances.
Exosphere
The exosphere is the highest layer of the atmosphere.
Together with the ionosphere, it makes up the thermosphere. The
exosphere extends to 10,000 km above the Earth's surface. This is the
upper limit of our atmosphere. The atmosphere here merges into space in
the extremely thin air. Air atoms and molecules are constantly escaping
to space from the exosphere. In this region of the atmosphere, hydrogen
and helium are the prime components and are only present at extremely
low densities. This is the area where many satellites orbit the Earth.
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