Download Understanding the stratosphere

Upper Atmosphere
Basics
Unit 1
Understanding and observation of the mid
atmosphere
The region of the atmosphere above the tropopause is called the stratosphere. In
this unit, we look at how the stratosphere differs from the troposphere. We also
investigate why there are other distinct layers in the atmosphere and how these
layers are defined.
We look at how the physical and
meteorological parameters of the
atmosphere change with altitude and
investigate how the chemical
composition changes with
height. We also look at how modern
measuring techniques, using satellites
and lasers, have been used to provide
us with this infomation.
LIDAR in Davis / Antarctica with aurora in the
background
Photo: David Correll - Australian Antarctic
Division - http://www.antdiv.gov.au
Part 1: Layers
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Part 1: Players
The layers of the atmosphere
The different layers we see in the atmosphere have different physical
properties. As the altitude increases, atmospheric pressure decreases.
This is because the density of the air decreases - the higher we go, the
less air molecules we find in the same volume of space. Temperature,
humidity and wind speed also change with altitude.
If we look up into the sky from
the ground we can't see the
layers of the atmosphere, we
either see a clear blue sky or
clouds. However we get an idea
that the properties of the
atmosphere change with altitude
if we travel by aeroplane.
Regardless of the weather on the
ground, we see blue sky with no
clouds above us once we reach an
altitude of 10 - 11 km. At this
height we are in the tropopause
or even the lower stratosphere.
There are no clouds this high up
simply because there isn't enough
water in the air to allow them to
form.
1. Blue sky above the clouds.
source:www.freefoto.com
Why does the temperature
change?
Small scale temperature changes
are seen in the atmosphere which
occur as a result of local changes
in conditions, for example, the
land cools down and heats up
more quickly than the sea.
There are two main reasons why
large scale changes in
temperature are seen in the
atmosphere:
a) the surface of the Earth
absorbs sunlight and heats up.
As we move away from the warm
surface of the Earth, the cooler
the air becomes. This leads to a
decrease in temperature with
altitude.
2. Profiles of temperature, air pressure and air density
with increasing altitude. adapted from: Schirmer Wetter und Klima - Wie funktioniert das? Please click
to enlarge! (120 K)
Why does the temperature change?
Small scale temperature changes are seen in the atmosphere which occur as a
result of local changes in conditions, for example, the land cools down and heats
up more quickly than the sea.
There are two main reasons why large scale changes in temperature are seen in
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the atmosphere:
a) the surface of the Earth absorbs sunlight and heats up. As we move away from
the warm surface of the Earth, the cooler the air becomes. This leads to a
decrease in temperature with altitude.
b) the temperature of the atmosphere is also governed by the chemicals the air
contains. Some chemicals are able to absorb sunlight themselves and heat up the
air around them. Ozone (O3) molecules in the stratosphere are able to absorb
ultra-violet radiation from the Sun and warm the surrounding air. This leads to an
increase in the temperature in the stratosphere. The temperature increases with
altitude until a local maximum is reached. This temperature maximum defines the
border between the stratosphere and the next layer of the atmosphere above.
This border is known as the stratopause. The layer above the stratosphere is
known as the mesosphere and here temperature decreases with altitude.
Another temperature increase takes place in the thermosphere, where nitrogen
and oxygen absorb extremely energetic short wavelength ultra-violet radiation
from the Sun and are partially converted into charged ions. This layer is,
therefore, also known as the ionosphere.
2. Profiles of temperature, air pressure and air density with increasing altitude. adapted from:
Schirmer - Wetter und Klima - Wie funktioniert das?
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Why does the pressure decrease?
The difference between air and water is that air is
compressible and water is not. If you are diving in the
sea and have 10 metres of water above you, the
pressure is 1 bar, if you have 20 metres of water above
you it's 2 bar simply because the amount of water is
doubled. However, air is different. Just imagine you
have a tower of very light pillows. As the height of the
tower increases, the pillows on the bottom of the tower
become flatter due to the weight of the ones above.
They can be compressed because they have a lot of free
space in them. So at the end, you may have 10 pillows
in the first 30 cm layer of your tower and only one in
the 8th layer even though each pillow weighs the same.
This is the same in the atmosphere. Therefore,
meteorologists very often use pressure rather than
height in metres to define the altitude of the
atmosphere. The amount the air compresses depends a
bit on the temperature but roughly we can divide the
pressure by a factor of 2 for every 5.5 km increase in
height.
Click here for more detailed information on how
atmospheric pressure is calculated.
Is the thermosphere really that hot?
3. Like a pillow tower:
How air is compressed ...
by Elmar Uherek
Temperatures recorded in the thermosphere, 200 - 500
km up in the atmosphere, reach 500 - 1000 oC. Is it
really that hot? The problem here is our definition of
temperature. In the thermosphere the molecules have
a huge amount of energy so the temperatures are
correct. However, the number of molecules per volume
of space is about one millionth of the number of
molecules near the surface of the Earth. This means
that the probability that the molecules will collide,
transfer their energy and cause heating is extremely
low. Therefore, the temperatures recorded in the
thermosphere are good measures of molecular energy
but not compable to temperatures measured with a
thermometer on the ground.
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4. a) Weather map at ground level. From:
Schirmer - Wetter und Klima - Wie
funktioniert das?
4. b) The same weather map at 300 hPa
(about 9 km in altitude). Please note the
wind speed symbols! From: Schirmer Wetter und Klima - Wie funktioniert das?
4. c) Have a
look at the
figure on the right
and compare the
wind speeds at the
ground (dark blue,
below) and at 9 km
altitude (light blue,
above) at the
same places. What
is the wind speed
in km h-1 at the
three marked
locations?
How does the wind change?
5. Wind speed is often measured in knots where
knot = kn = nautical mile h-1 or in km h-1.
The correct unit is m s-1.
1 m s-1 = 3.6 km h-1
1 knot = 1.852 km h-1
The symbols in the weather map tell us the wind
direction (where the wind comes from) and the wind
speed in knots. As the example shows, a full sized tick
mark represents a wind speed of 10 knots, a half sized
tick mark represents a wind speed of 5 knots.
The figure above shows that wind
speeds are much greater in the
upper troposphere than they are
lower in the atmosphere. So a
normal wind speed at the
tropopause is equivalent to a
severe storm at ground level. As
a result, air traffic uses a
special weather forecasting
system to take these changes in
wind speed into account. Once
we reach the stratosphere,
however, wind speed decreases
significantly.
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6. Wind speed vertical profile. Data from a balloon experiment of the US national weather service.
Published at Exploring Earth.
7. Comparisons of wind speed and temperature.
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Part 2: Composition
Composition of the stratosphere
Most of the compounds released at the Earth's surface do not reach the
stratosphere, instead they are:
•
•
•
•
decomposed by the main tropospheric oxidants (hydroxyl radicals
- OH, nitrate radicals - NO3, ozone - O3)
broken down by sunlight
deposited back to the surface of the Earth in rain or as particles
trapped in the cold tropopause.
Because the temperature trend between the troposphere and the
stratosphere reverses, there is almost no air exchange between these
two layers. Mixing of air in the troposphere takes hours to days whereas
mixing in the stratosphere takes months to years.
One of the consequences of this
lack of mixing between the
troposphere and the stratosphere
is that the water vapour content
of the stratosphere is very low.
Typical mixing ratios (see below
for definition) are in the range of
2 - 6 ppm (parts per million)
compared to 100 ppm in the
upper troposphere and 1,000 40,000 ppm in the lower
troposphere, close to the surface
of the Earth. This means that
stratospheric clouds form very
rarely and only if temperatures
are so low that ice crystals grow.
These conditions generally only
occur in the polar regions.
However, increasing water vapour
concentrations due to emissions
from aeroplanes and higher
temperatures due to tropospheric
warming below may lead to more
polar stratospheric clouds being
formed in the future.
1. Polar stratospheric clouds over Kiruna / Sweden.
source: MPI Heidelberg.
Inorganic compounds in the stratosphere
Stratospheric chemistry is dominated by the chemistry of ozone. Between 85 and
90% of all the ozone in the atmosphere is found in the stratosphere. Ozone is
formed when sunlight breaks down molecular oxygen (O2) in the
stratosphere into oxygen atoms (O). The highly reactive oxygen atoms then
react with more molecular oxygen to form ozone (O3). Most of the other gases
in the stratosphere are either really long lived compounds emitted originally into
the troposphere (such as the chlorofluorocarbons - CFC's) or are brought in by
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severe volcanic eruptions (generally sulphur containing compounds and
aerosols). So inorganic compounds such as ozone, nitrogen oxides, nitric acid,
sulphuric acid, halogens and halogen oxides from CFC's are the dominant
chemicals in the stratosphere.
Volcanic eruptions
2. Eruption of Mt. Pinatubo Philippines in June 2001.
source: Cascades Volcano Observatory USGS Photo by
Rick Hoblitt.
Severe volcanic eruptions can
inject large quantities of gases
and particles directly into the
stratosphere. These gases include
the halogen containting acids,
hydrochloric acid (HCl) and
hydrofluoric acid (HF) and sulphur
dioxide (SO2) which is converted
to sulphuric acid (H2SO4), one of
the compounds responsible for
cloud formation. The particles
emitted include silicates and
sulphates and these absorb
sunlight in the
stratosphere. Volcanic eruptions
can, therefore, lead to a
temporary warming in the
stratosphere and a temporary
cooling in the troposphere. These
effects on temperature can last
around 1 - 2 years. If the
eruption is large enough, such the
eruption of Mt. Pinatubo in the
Philippines in June 1991, the
effect can be seen over the whole
hemisphere.
Understanding concentrations and mixing ratios
We can express the amount of a compound in the atmosphere in two ways,
relative and absolute:
a) mixing ratio = the fraction of the compound as a proportion of all the air
molecules present. If there are 40 ozone molecules in 1 million air molecules the
mixing ratio is 40 ppm (parts per million). This is relative.
b) concentration = the concentration of the molecules of the compound in a
certain volume of air. If there are 100 molecules of ozone in one cubic meter of
air, the concentration is 100 molecules m-3. This is absolute.
If you know the air pressure, it is possible to convert between the two units.
Pressure decreases with altitude, i.e. the higher we go in the stratosphere,
the fewer molecules there are in each unit volume of air. This means that if the
absolute amount of ozone remains the same as the altitude increases, the mixing
ratio for ozone also increases.
We can explain this general principle very simply. In a certain volume (light blue
box) there is a certain number of air molecules (blue) and a certain number of
ozone molecules (red). The number of air molecules decreases with altitude.
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3. Here the number of ozone molecules
remains constant with altitude. As the total
number of air molecules decreases with
altitude, the ozone mxing ratio increases with
altitude (see below).
3. b) Here the absolute number of ozone
molecules decreases in parallel with the
decrease in the number of air molecules. As
a result, the mixing ratio remains constant as
the altitude increases.
In reality, there is only around 1 molecule of ozone for every million molecules of
air!
3. a) Simple ozone profile for the example above. The total concentration of air is given in blue, the
ozone concentration in red and the ozone mixing ratio (% ozone) is shown in green. Since the
number of ozone molecules stays constant but the total air concentration decreases, the mixing ratio
increases with altitude.
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3. Simple ozone profile for the example above. The total concentration of air molecules is given in
blue, the ozone concentration in red and the ozone mixing ratio (% ozone) in green. As the ozone
concentration decreases in parallel with the decrease in the total concentration of air molecules, the
ozone mixing ratio is constant with altitude.
Between the ground and the lower stratosphere, ozone mixing ratios tend to
increase with altitude as ozone concentrations remain nearly constant but air
becomes thinner. In the lower stratosphere, ozone concentrations increase with
altitude (the example below shows an increase of a factor of eight) increasing
ozone mixing ratios further. It is only above the ozone layer that mixing ratios
are approximately constant with altitude.
4. Figure showing how the ozone mixing
ratio and ozone concentration changes
with altitude.
source: adapted from IUP Bremen.
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Part 3: Observation
Measurements in the Stratosphere
The stratosphere begins at an altitude of between 8 and 15 km and the
interesting regions are higher than normal planes can fly. So how do we
know about the chemistry of the stratosphere?
In order to study the chemistry of the stratosphere we can either:
1. send measurement instruments into the stratosphere on special aircraft or
on balloons.
2. use the characteristic way in which a specific chemical compound interacts
with light to study the stratosphere from the ground or from space using
satellites.
Aeroplanes
Unique
measurements
have been made
possible with
special
aeroplanes, such
as the former
Russian high
altitude spy plane.
This plane, now
called
"Geophysica", has
been converted
into an airborne
laboratory and
such planes can
reach altitudes of
around 20 km.
The flights are very
expensive and, as
a result, this
method is not used
often.
1. Geophysica - high altitude research aircraft.
source:MDB Design Bureau
Balloons
A more common alternative is to take measurements using
meteorological balloons. Weather balloons can reach altitudes of between 30 and
35 km before they burst. They carry sensors to measure, for example, ozone and
send the information back to Earth via a radio signal. As the balloon travels up
through the air it sends continuous information back to Earth. Balloons are,
therefore, a very useful and relatively inexpensive way of finding out about the
vertical structure of the atmosphere.
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2. a) Start of an ozone balloon ascent at
Hohenpeissenberg Observatory, Germany.
Photo courtesy of Ulf Köhler.
2. b) Ozone probe for balloon
measurements. Photo courtesy of
Ulf Köhler, DWD Hohenpeissenberg.
Interaction of molecules with light
The way in which different chemicals interact with light is really complicated. In
very simple terms, something happens when light and matter interact. The light
can be absorbed completely by the compound. It can be reflected or scattered
directly back into space or can be taken up and re-emitted at a different
energy (as a different wavelength).
Its easy to see the impact of light
absorption by clouds, water and
large particles- direct sunlight is
blocked by clouds, as we dive into
the sea it becomes darker as
more light is lost and a dust
storm makes the sun look pale.
Smaller molecules do the same.
They can also absorb or
reflect light, they can scatter the
light back to Earth or absorb the
light and re-emit less energetic
light of a different wavelength.
Examples of this are
phosphorescence and
fluorescence. These effects
happen when chemicals take up
daylight and emit different energy
light which we can see in the
dark. The sort of light re-emitted
tells us something about the type
of chemical and the intensity of
the light tells us something about
its concentration.
3. Phosphorescence takes place if light is absorbed and
reemitted again at an other wavelength. source:
composed from web-advertisements.
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Interaction of light
with molecules in
the stratosphere can
be observed from
the ground
or measured from
space using
instruments
mounted on
satellites.
LIDAR
4. How does a LIDAR work?
Please press reload in order
to restart the animation!
by EU
Lidar (LIght
Detection And
Ranging) is one
technique which can
be used from the
ground. A short
pulse of very
intensive laser
light is sent into the
sky. After a while,
light returns to
Earth and is
measured. This light
gives us information
about the
compounds in the
atmosphere (from
the wavelength of
the returning light)
and at what
concentration they
occur (the intensity
of the returning
light). But how do
we know how high
up in the
atmosphere these
compounds are?
Light has a certain
velocity and the
longer the light
takes to come back
to Earth, the higher
the compounds are.
5. LIDAR measurements. Image
source: University of Western Ontario.
The animation on the left shows
a laser pulse (light blue) whose
light is scattered back to Earth
at three different altitudes by air
molecules (green) and arrives at
the detector (light green) at
three different times.
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RADAR and SODAR
Different variations of the wave detection
and ranging technique can also be
used. The best known is RADAR (RAdio
Detection And Ranging), which is used to
measure particles in the air and the
properties of clouds. RADAR allows us to
track thunderstorms over several hundred
kilometers. If sound is used instead of
light, the technique is known as
SODAR (SOund Detection And Ranging)
and this gives us a powerful tool for the
measurements of wind speed and
direction.
Satellites
6. SODAR - wind speed measurements.
picture source: Meteotest
1. Using satellites we
can measure the amount
of sunlight scattered by
clouds or air molecules.
2. Satellites can carry
spectrometers which work
in the infra-red region of
the spectrum and measure
long wave radiation
coming directly from the
Earth.
3. For certain positions of the
Sun and
the Earth, sunbeams
pass through just air to
the detector on the
satellite. This can give us
information on how
concentrations of different
molecules change
throughout the
atmosphere.
Satellites observe our planet from space.
Some of them observe just one area of the
Earth and are known as geostationary
satellites whereas others orbit the Earth at
an altitude of between 500 and 1000 km
and can circle the Earth in about 1.5 to 2
hours. Some of these satellites have
instruments known as spectrometers
aboard and these can detect different
wavelengths of light and give us
information on the chemical composition
of the atmosphere.
7. Different techniques of satellite measurements.
scheme by Elmar Uherek.
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