ATMOSPHERE
By volume , dry dry air contains 78.09% nitrogen,20.95% oxygen, 0.93% argon, 0.04% carbondioxide, and small amounts of other gases .Air is also contains a variable amount of watervapour, on average around 1% at sea level,and 0.4% over the entire atmosphere. Air content and atmospheric pressure vary at different layers , and air suitable for use in photosynthesis by terrestrial animals is found only in Earth’s troposphere and in artificial atmospheres
The study of Earth’s
atmosphere and its processes is called atmospheric science (aerology). Early
pioneers in the field include Leon Teisserence dc Bort
and Richard Assmann.
LAYERS
OF ATMOSPHERE
The atmosphere is comprised of layers based on temperature. These layers
are the troposphere, stratosphere, mesosphere and thermosphere. A further
region at about 500 km above the Earth's surface is called the exosphere.
The different layers of the
atmosphere
The atmosphere can be divided
into layers based on its temperature, as shown in the figure below. These
layers are the troposphere, the stratosphere, the mesosphere and the thermosphere.
A further region, beginning about 500 km above the Earth's surface, is called
the exosphere.
The red line on
the figure below shows how temperature varies with height (the temperature
scale is given along the bottom of the diagram). The scale on the right shows
the pressure. For example, at a height of 50 km, the pressure is only about one
thousandth of the pressure at the ground.
The Troposphere
This is the
lowest part of the atmosphere - the part we live in. It contains most of our
weather - clouds, rain, snow. In this part of the atmosphere the temperature
gets colder as the distance above the earth increases, by about 6.5°C per
kilometre. The actual change of temperature with height varies from day to
day, depending on the weather.
The troposphere
contains about 75% of all of the air in the atmosphere, and almost all of the
water vapour (which forms clouds and rain). The decrease in temperature with
height is a result of the decreasing pressure. If a parcel of air moves upwards
it expands (because of the lower pressure). When air expands it cools. So air
higher up is cooler than air lower down.
The lowest part
of the troposphere is called the boundary layer. This is where
the air motion is determined by the properties of the Earth's
surface. Turbulence is generated as the wind blows over the Earth's
surface, and by thermals rising from the land as it is heated by the sun.
This turbulence redistributes heat and moisture within the boundary layer, as
well as pollutants and other constituents of the atmosphere.
The top of the
troposphere is called the tropopause. This is lowest at the poles, where it is
about 7 - 10 km above the Earth's surface. It is highest (about 17 -
18 km) near the equator.
The Stratosphere
This extends
upwards from the tropopause to about 50 km. It contains much of the ozone in
the atmosphere. The increase in temperature with height occurs because of
absorption of ultraviolet (UV) radiation from the sun by this ozone.
Temperatures in the stratosphere are highest over the summer pole, and lowest
over the winter pole.
By absorbing
dangerous UV radiation, the ozone in the stratosphere protects us from skin
cancer and other health damage. However chemicals (called CFCs or freons, and
halons) which were once used in refrigerators, spray cans and fire
extinguishers have reduced the amount of ozone in the stratosphere,
particularly at polar latitudes, leading to the so-called "Antarctic ozone
hole".
Now humans have stopped making
most of the harmful CFCs we expect the ozone hole will eventually recover over
the 21st century, but this is a slow process.
The Mesosphere
The region above
the stratosphere is called the mesosphere. Here the temperature again decreases
with height, reaching a minimum of about -90°C at the "mesopause".
The Thermosphere
and Ionosphere
The thermosphere
lies above the mesopause, and is a region in which temperatures again increase
with height. This temperature increase is caused by the absorption of energetic
ultraviolet and X-Ray radiation from the sun.
The region of
the atmosphere above about 80 km is also caused the "ionosphere",
since the energetic solar radiation knocks electrons off molecules and atoms,
turning them into "ions" with
a positive charge. The temperature of the thermosphere varies between night and
day and between the seasons, as do the numbers of ions and electrons which are
present. The ionosphere reflects and absorbs radio waves, allowing us to
receive shortwave radio broadcasts in New Zealand from other parts of the
world.
The Exosphere
The region above
about 500 km is called the exosphere. It contains mainly oxygen and hydrogen
atoms, but there are so few of them that they rarely collide - they follow "ballistic" trajectories under
the influence of gravity, and some of them escape right out into space.
The
Magnetosphere
The earth
behaves like a huge magnet. It traps electrons (negative charge) and protons
(positive), concentrating them in two bands about 3,000 and 16,000 km above the
globe - the Van Allen "radiation" belts. This outer region
surrounding the earth, where charged particles
spiral along the magnetic field lines, is called the magnetosphere.