Unit 7: The atmosphere and human activities
7.1 The structure and composition of the atmosphere
● The atmosphere is a layer of gases held down to Earth by gravitational force. When
the Earth was first formed, it’s believed that it had no atmosphere, as the planet
cooled gases were released and the atmosphere began to form.
● It’s composed mainly of nitrogen, oxygen, argon, carbon dioxide and ozone, it also
contains aerosols and water vapour. Carbon dioxide and ozone are variable gases,
because their quantity can change as a result of processes like evaporation and
transpiration, in the case of ozone, because of varying rates of formation, pollution
and seasonal change.
● The natural balance of gases in the atmosphere is maintained by various cycles,
such as the nitrogen cycle and the carbon cycle. However human activities can alter
their composition.
Structure
Atmospheric pressure drops with height in the atmosphere, but temperatures varies.
● Troposphere: Temperature decreases with height because the warming effect of the
Earth’s surface through conduction and convection decreases as altitude increases.
The strength of the earth’s gravitational pull declines with altitude, and pressure
declines too. Wind speeds increase with height. The top of this layer is marked by the
tropopause, where temperatures remain constant.
● Stratosphere: 50 km above the Earth’s surface. Pressure falls but temperatures
increase with height (temperature inversion) and is caused by the concentration of
ozone which absorbs the incoming ultraviolet radiation from the sun, this also shield
against incoming meteorites which burn out when they enter the earth’s gravitational
field. The upper limit is marked by the stratopause.
● Mesosphere: 50-80 km in height. Pressure decreases and temperatures fall rapidly
to below -80°c because there is no water vapour, dust or ozone to absorb the
incoming shortwave radiation.Winds can reach speeds up to 3000 km −1 The
mesopause marks the upper limit of this layer.
● Thermosphere: 80-1000 km in height and temperature rise rapidly to as high
1500°c because of the absorption of ultraviolet radiation by atomic oxygen. The
thermopause marks the upper limit of this layer.
The natural greenhouse effect
It’s the process that helps keep the Earth’s surface and atmosphere warm.
● The Earth receives incoming short-wave radiation from the Sun, half of this radiation
is absorbed by the Earth’s surface. As the surface warms, outgoing long-wave
radiation is emitted back into the atmosphere. Greenhouse gases absorb some of
this radiation and deflect it back to the Earth’s surface. The greater the concentration
of greenhouse gases, the more effectively they return radiation back to the earth’s
surface, this process maintains the earth’s temperature at around 33°c warmer
allowing life to exist.
● The main gases are carbon dioxide, methane and nitrous oxides. The CFCs
(chlorofluorocarbons; fluorine, chlorine and carbon) are also greenhouse gases but
are artificial.
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7.1 The structure and composition of the atmosphere
● The atmosphere is a layer of gases held down to Earth by gravitational force. When
the Earth was first formed, it’s believed that it had no atmosphere, as the planet
cooled gases were released and the atmosphere began to form.
● It’s composed mainly of nitrogen, oxygen, argon, carbon dioxide and ozone, it also
contains aerosols and water vapour. Carbon dioxide and ozone are variable gases,
because their quantity can change as a result of processes like evaporation and
transpiration, in the case of ozone, because of varying rates of formation, pollution
and seasonal change.
● The natural balance of gases in the atmosphere is maintained by various cycles,
such as the nitrogen cycle and the carbon cycle. However human activities can alter
their composition.
Structure
Atmospheric pressure drops with height in the atmosphere, but temperatures varies.
● Troposphere: Temperature decreases with height because the warming effect of the
Earth’s surface through conduction and convection decreases as altitude increases.
The strength of the earth’s gravitational pull declines with altitude, and pressure
declines too. Wind speeds increase with height. The top of this layer is marked by the
tropopause, where temperatures remain constant.
● Stratosphere: 50 km above the Earth’s surface. Pressure falls but temperatures
increase with height (temperature inversion) and is caused by the concentration of
ozone which absorbs the incoming ultraviolet radiation from the sun, this also shield
against incoming meteorites which burn out when they enter the earth’s gravitational
field. The upper limit is marked by the stratopause.
● Mesosphere: 50-80 km in height. Pressure decreases and temperatures fall rapidly
to below -80°c because there is no water vapour, dust or ozone to absorb the
incoming shortwave radiation.Winds can reach speeds up to 3000 km −1 The
mesopause marks the upper limit of this layer.
● Thermosphere: 80-1000 km in height and temperature rise rapidly to as high
1500°c because of the absorption of ultraviolet radiation by atomic oxygen. The
thermopause marks the upper limit of this layer.
The natural greenhouse effect
It’s the process that helps keep the Earth’s surface and atmosphere warm.
● The Earth receives incoming short-wave radiation from the Sun, half of this radiation
is absorbed by the Earth’s surface. As the surface warms, outgoing long-wave
radiation is emitted back into the atmosphere. Greenhouse gases absorb some of
this radiation and deflect it back to the Earth’s surface. The greater the concentration
of greenhouse gases, the more effectively they return radiation back to the earth’s
surface, this process maintains the earth’s temperature at around 33°c warmer
allowing life to exist.
● The main gases are carbon dioxide, methane and nitrous oxides. The CFCs
(chlorofluorocarbons; fluorine, chlorine and carbon) are also greenhouse gases but
are artificial.
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