upper atmosphere basics unit 1
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Uppe r A tm osphe r e
Basics
Unit 1Understanding and observation of the midatmosphere
The region of the atmosphere above the tropopause is called the stratosphere. Inthis unit, we look at how the stratosphere differs from the troposphere. We alsoinvestigate 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 theatmosphere change with altitude andinvestigate how the chemicalcomposition changes withheight. We also look at how modern
measuring techniques, using satellitesand lasers, have been used to provideus with this infomation.
LIDAR in Davis / Antarctica with aurora in the
background
Photo: David Correll - Australian Antarcticivision - http://www.antdiv.gov.auD
Part 1: Layers
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Part 1: Players
The laye rs o f the a tm osphere
The d if fe ren t l aye rs w e see i n th e a tm osphe re have d i f fe ren t ph ys i ca l
p roper t ies . As th e a l t i t ude increases, a tm ospher ic p ressure decreases.Th is is because the dens i ty o f th e a i r decreases - th e h igher w e go , theless a ir m o lecu les w e f i nd i n the sam e vo lume o f space. Tem pera tu re ,hum id i t y and w ind speed al so change w i th a l t i t ude .
1. Blue sky above the clouds.
ource:www.freefoto.coms
If we look up into the sky fromthe ground we can't see thelayers of the atmosphere, weeither see a clear blue sky orclouds. However we get an idea
that the properties of theatmosphere change with altitude
if we travel by aeroplane.Regardless of the weather on theground, we see blue sky with noclouds above us once we reach analtitude of 10 - 11 km. At thisheight we are in the tropopauseor even the lower stratosphere.
There are no clouds this high upsimply because there isn't enoughwater in the air to allow them toform.
W h y d o es t h e t e m p e r atu r e
change?
Small scale temperature changesare seen in the atmosphere whichoccur as a result of local changesin conditions, for example, theland cools down and heats upmore quickly than the sea.
There are two main reasons whylarge scale changes intemperature are seen in theatmosphere:
a) the surface of the Earthabsorbs sunlight and heats up.As we move away from the warmsurface of the Earth, the coolerthe air becomes. This leads to a
decrease in temperature withaltitude.
2. Profiles of temperature, air pressure and air density
with increasing altitude. adapted from: Schirmer -
Wetter und Klima - Wie funktioniert das? Please clickto enlarge! (120 K)
Why does the tem pera tu re 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 fromthe warm surface of the Earth, the cooler the air becomes. This leads to adecrease in temperature with altitude.
b) the temperature of the atmosphere is also governed by the chemicals the aircontains. Some chemicals are able to absorb sunlight themselves and heat up theair around them. Ozone (O3) molecules in the s t ra tosphe re are able to absorbultra-violet radiation from the Sun and warm the surrounding air. This leads to anincrease in the temperature in the stratosphere. The temperature increases withaltitude until a local maximum is reached. This temperature maximum defines theborder between the stratosphere and the next layer of the atmosphere above.
This border is known as the s t ra topause. The layer above the stratosphere isknown as the mesosphe re and here temperature decreases with altitude.Another temperature increase takes place in the the rmosphe re , where nitrogenand oxygen absorb extremely energetic short wavelength ultra-violet radiationfrom the Sun and are partially converted into charged ions. This layer is,
therefore, also known as the i onosphe re.
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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3. Like a pillow tower:
How air is compressed ...
by Elmar Uherek
Wh y does th e p ressure decrease?
The difference between air and water is that air iscompressible and water is not. If you are diving in thesea and have 10 metres of water above you, the
pressure is 1 bar, if you have 20 metres of water aboveyou it's 2 bar simply because the amount of water isdoubled. However, air is different. Just imagine youhave a tower of very light pillows. As the height of thetower increases, the pillows on the bottom of the towerbecome flatter due to the weight of the ones above.They can be compressed because they have a lot of freespace in them. So at the end, you may have 10 pillows
in the first 30 cm layer of your tower and only one inthe 8th layer even though each pillow weighs the same.This is the same in the atmosphere. Therefore,meteorologists very often use pressure rather thanheight in metres to define the altitude of the
atmosphere. The amount the air compresses depends abit on the temperature but roughly we can divide thepressure by a factor of 2 for every 5.5 km increase inheight.
Click here for more detailed information on howatmospheric pressure is calculated.
I s t h e t h e r m o sp h e r e r ea l ly t h a t h o t?
Temperatures recorded in the thermosphere, 200 - 500km up in the atmosphere, reach 500 - 1000 oC. Is itreally that hot? The problem here is our definition of
temperature. In the thermosphere the molecules havea huge amount of energy so the temperatures are
correct. However, the number of molecules per volumeof space is about one millionth of the number ofmolecules near the surface of the Earth. This meansthat the probability that the molecules will collide,transfer their energy and cause heating is extremelylow. Therefore, the temperatures recorded in thethermosphere are good measures of molecular energy
but not compable to temperatures measured with athermometer on the ground.
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http://www.atmosphere.mpg.de/enid/50164413c1f8d088621ec6ca26951418,0/1__Understanding_the_stratosphere/pressure___altitude_1z4.htmlhttp://www.atmosphere.mpg.de/enid/50164413c1f8d088621ec6ca26951418,0/1__Understanding_the_stratosphere/pressure___altitude_1z4.htmlhttp://www.atmosphere.mpg.de/enid/50164413c1f8d088621ec6ca26951418,0/1__Understanding_the_stratosphere/pressure___altitude_1z4.htmlhttp://www.atmosphere.mpg.de/enid/50164413c1f8d088621ec6ca26951418,0/1__Understanding_the_stratosphere/pressure___altitude_1z4.html -
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4. a) Weather map at ground level. From:
Schirmer - Wetter und Klima - Wieunktioniert das?f
4. b) The same weather map at 300 hPa
(about 9 km in altitude). Please note thewind speed symbols! From: Schirmer -
Wetter und Klima - Wie funktioniert das?
4. c) Have alook at thefigure on the rightand compare thewind speeds at theground (dark blue,
below) and at 9 kmaltitude (light blue,above) at the
same places. Whatis the wind speedin km h-1 at thethree marked
locations?
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 winddirection (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 sizedtick mark represents a wind speed of 5 knots.
How does the w ind change?
The figure above shows that wind
speeds are much greater in theupper troposphere than they are
lower in the atmosphere. So anormal wind speed at thetropopause is equivalent to asevere storm at ground level. Asa result, air traffic uses a
special weather forecastingsystem to take these changes in
wind speed into account. Oncewe reach the stratosphere,however, wind speed decreasessignificantly.
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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
Compos i t i on o f t he s t r a tosphe re
Most o f t he compounds r e leased a t the Ear t h ' s su r face do no t r each t hes t ra tosphe re , i nstead t hey a re :
decomposed by t he ma in t r oposphe r i c ox idan ts ( hyd rox y l r ad i cal s- OH , n i t r a t e r a di ca ls - NO , ozon e - O )3 3
b r o ke n d o wn b y su n l ig h t
deposi ted back to t he sur face o f the Ear t h in ra in o r as par t ic les
t r apped in t he co ld t r opopause.
Because the tem pera tu re t r end be tw een th e t r oposphe re and thes t ra tosphe re reve rses , the re i s a lmos t n o a i r exchange be tw een thesetw o laye rs . M ix ing o f ai r i n the t r oposphe re takes hou rs to days w he reas
m ix ing i n the s t r a tosphe re takes m on ths to yea rs .
One of the consequences of thislack of mixing between the
troposphere and the stratosphereis that the water vapour contentof the stratosphere is very low.Typical mixing ratios (see belowfor definition) are in the range of2 - 6 ppm (parts per million)compared to 100 ppm in the
upper troposphere and 1,000 -
40,000 ppm in the lowertroposphere, close to the surfaceof the Earth. This means thatstratospheric clouds form veryrarely and only if temperaturesare so low that ice crystals grow.
These conditions generally onlyoccur in the polar regions.
However, increasing water vapourconcentrations due to emissionsfrom aeroplanes and highertemperatures due to tropospheric
warming below may lead to morepolar stratospheric clouds beingformed in the future.
1. Polar stratospheric clouds over Kiruna / Sweden.
source: MPI Heidelberg.
I no rgan ic compounds i n the s t r a tosphe re
Stratospheric chemistry is dominated by the chemistry of ozone. Between 85 and90% of all the ozone in the atmosphere is found in the stratosphere. Ozone isformed when sunlight breaks down molecular oxygen (O2) in the
stratosphere into oxygen atoms (O). The highly reactive oxygen atoms thenreact with more molecular oxygen to form ozone (O3). Most of the other gases
in the stratosphere are either really long lived compounds emitted originally intothe troposphere (such as the chlorofluorocarbons - CFC's) or are brought in by
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severe volcanic eruptions (generally sulphur containing compounds andaerosols). So inorganic compounds such as ozone, nitrogen oxides, nitric acid,sulphuric acid, halogens and halogen oxides from CFC's are the dominantchemicals in the stratosphere.
2. Eruption of Mt. Pinatubo Philippines in June 2001.
source: Cascades Volcano Observatory USGS Photo by
ick Hoblitt.R
Volcan ic erup t ion s
Severe volcanic eruptions caninject large quantities of gases
and particles directly into thestratosphere. These gases include
the halogen containting acids,hydrochloric acid (HCl) andhydrofluoric acid (HF) and sulphurdioxide (SO2) which is convertedto sulphuric acid (H2SO4), one ofthe compounds responsible forcloud formation. The particles
emitted include silicates andsulphates and these absorb
sunlight in thestratosphere. Volcanic eruptionscan, therefore, lead to atemporary warming in thestratosphere and a temporary
cooling in the troposphere. Theseeffects on temperature can last
around 1 - 2 years. If theeruption is large enough, such theeruption of Mt. Pinatubo in the
Philippines in June 1991, theeffect can be seen over the wholehemisphere.
Understand ing concen t r a t i ons and m ix ing ra t i os
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 airmolecules 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 ofair, 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 mixingratio for ozone also increases.
We can explain this general principle very simply. In a certain volume (light bluebox) there is a certain number of air molecules (blue) and a certain number ofozone 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 withaltitude, the ozone mxing ratio increases with
ltitude (see below).a
3. b) Here the absolute number of ozone
molecules decreases in parallel with the
decrease in the number of air molecules. Asa result, the mixing ratio remains constant as
he altitude increases.tIn reality, there is only around 1 molecule of ozone for every million molecules ofair!
3. a) Simple ozone profile for the example above. The total concentration of air is given in blue, theozone 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
i creases with altitude.n
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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, theozone mixing ratio is constant with altitude.
Between the ground and the lower stratosphere, ozone mixing ratios tend toincrease with altitude as ozone concentrations remain nearly constant but air
becomes thinner. In the lower stratosphere, ozone concentrations increase withaltitude (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 ratiosare approximately constant with altitude.
4. Figure showing how the ozone mixingratio and ozone concentration changes
with altitude.
source: adapted from IUP Bremen.
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Part 3: Observation
Measu remen ts i n the S t ra tosphe re
The s t ra tosphe re beg ins at an a l t i t ude o f be tw een 8 and 15 km and thein te res t i ng reg ions a re h igher than no rm a l p lanes can f l y . So how do w eknow abou t the chem is t r y o f the s t r a tosphe re?
In order to study the chemistry of the stratosphere we can either:
1. send measurement instruments into the stratosphere on special aircraft oron balloons.
2. use the characteristic way in which a specific chemical compound interactswith light to study the stratosphere from the ground or from space usingsatellites.
Aerop lanes
Uniquemeasurementshave been madepossible withspecial
aeroplanes, suchas the former
Russian highaltitude spy plane.This plane, nowcalled"Geophysica", hasbeen convertedinto an airborne
laboratory andsuch planes canreach altitudes ofaround 20 km.The flights are veryexpensive and, asa result, this
method is not usedoften.
1. Geophysica - high altitude research aircraft.source:MDB Design Bureau
Bal loons
A more common alternative is to take measurements using
meteorological balloons. Weather balloons can reach altitudes of between 30 and35 km before they burst. They carry sensors to measure, for example, ozone andsend the information back to Earth via a radio signal. As the balloon travels upthrough 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 atHohenpeissenberg Observatory, Germany.
hoto courtesy of Ulf Khler.P
2. b) Ozone probe for balloonmeasurements. Photo courtesy of
lf Khler, DWD Hohenpeissenberg.UI n te rac t i on o f mo lecu les w i th l i gh t
The way in which different chemicals interact with light is really complicated. Invery simple terms, something happens when light and matter interact. The lightcan be absorbed completely by the compound. It can be reflected or scattereddirectly 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 lightabsorption by clouds, water andlarge particles- direct sunlight isblocked by clouds, as we dive intothe sea it becomes darker as
more light is lost and a duststorm makes the sun look pale.Smaller molecules do the same.They can also absorb orreflect light, they can scatter the
light back to Earth or absorb thelight and re-emit less energeticlight of a different wavelength.Examples of this are
phosphorescence andfluorescence. These effectshappen when chemicals take updaylight and emit different energylight which we can see in thedark. The sort of light re-emittedtells us something about the typeof chemical and the intensity ofthe 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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4. How does a LIDAR work?Please press reload in order
to restart the animation!by EU
Interaction of light
with molecules in
the stratosphere canbe observed fromthe groundor measured from
space usinginstruments
mounted onsatellites.
LI DAR
Lidar (LI ght
Detection AndRanging) is one
technique which canbe used from theground. A shortpulse of veryintensive laserlight is sent into thesky. After a while,
light returns toEarth and is
measured. This lightgives us informationabout thecompounds in theatmosphere (fromthe wavelength ofthe returning light)
and at whatconcentration theyoccur (the intensityof the returninglight). But how dowe know how highup in the
atmosphere thesecompounds are?
Light has a certainvelocity and thelonger the lighttakes to come backto Earth, the higherthe compounds are.
5. LIDAR measurements. Imagesource: University of Western Ontario.
The animation on the left showsa laser pulse (light blue) whoselight is scattered back to Earthat three different altitudes by airmolecules (green) and arrives atthe detector (light green) atthree different times.
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6. SODAR - wind speed measurements.
icture source: Meteotestp
RADAR and SODAR
Different variations of the wave detectionand ranging technique can also beused. The best known is RADAR (RAdio
Detection And Ranging), which is used tomeasure particles in the air and theproperties of clouds. RADAR allows us totrack thunderstorms over several hundredkilometers. If sound is used instead oflight, the technique is known asSODAR (SOund Detection And Ranging)and this gives us a powerful tool for the
measurements of wind speed anddirection.
Sate l l i tes
Satellites observe our planet from space.Some of them observe just one area of theEarth and are known as geostationarysatellites whereas others orbit the Earth atan altitude of between 500 and 1000 km
and can circle the Earth in about 1.5 to 2hours. Some of these satellites haveinstruments known as spectrometersaboard and these can detect differentwavelengths of light and give usinformation on the chemical compositionof the atmosphere.
1. Using satellites wecan measure the amountof sunlight scattered byclouds or air molecules.
2. Satellites can carryspectrometers which workin the infra-red region ofthe spectrum and measurelong wave radiationcoming directly from theEarth.
3. For certain positions of theSun and
the Earth, sunbeamspass through just air tothe detector on thesatellite. This can give usinformation on howconcentrations of differentmolecules changethroughout the
atmosphere.
7. Different techniques of satellite measurements.
scheme by Elmar Uherek.
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