Lecture 11 Atmosphere

Lecture 11 Atmosphere - 3/1/09
 Outline:


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Unformatted text preview: 3/1/09
 Outline:
 Introduc/on
to
the
atmosphere
 Electromagne/c
radia/on
and
Earth’s
heat
budget
 Lecture 11: The Atmosphere The
greenhouse
effect
 Suggested Readings: KKC Chapter 3 (on Blackboard) Atmospheric
composi/on
and
structure
 Earth’s
atmosphere
 Major
gases
in
the
atmosphere
(concentra1on
by
%
volume)
 1.  • Mixture
of
gas
molecules
that
surrounds
the
planet
 • Held
close
to
the
planet
by
gravity
 • Some
gases
are
chemically
inert
(inac1ve)
 • Others
are
chemically
reacAve
 • Some
contribute
to
the
greenhouse
effect
(absorb
 infrared
radiaAon)
 • Others
absorb
ultraviolet
radiaAon
 Nitrogen,
N2 
 
78%
 2.  Oxygen,
O2 
 
21%
 3.  Argon,
Ar 
 
 4.  Water
vapor,
H2O 
 
range
0.00001%
(polar)
to
4%
(tropics)
 5.  Carbon
dioxide,
CO2 
0.0385%
(385
ppm)
 
0.9%
 Increased
from
280
ppm
 since
the
19th
century
 These
five
account
for
>
99.9%
of
all
gases
molecules
in
the
atmosphere

 1
 3/1/09
 Terms:
 Electromagne/c
radia/on:
Self
propagaAng
electric
and
magneAc
wave,
similar
to
waves
on
the
 surface
of
a
pond
 1.  Wavelength
 2.  Frequency
 3.  ElectromagneAc
spectrum
 Blackbody:
Something
that
emits
(or
absorbs)
electromagneAc
radiaAon
at
100%
efficiency
at
all
 wavelengths
 Pressure
decreases
with
increasing
alAtude
(exponenAally)
 
The
amount
of
energy
emiZed
by
a
blackbody
is
a
funcAon
of
its
temperature
 Temperature
profile
is
more
complex
due
to
changes
in
atmospheric
composiAon
 Hydrogen
fusion
in
the
sun
creates
energy
(sun
is
a
blackbody)
 When
this
energy
reaches
Earth:
 • 70%
is
absorbed
 • 30%
is
reflected
away
by
oceans,
ice,
other
‘reflec/ve’
surfaces
 Solar Radiation Reflection and Absorption Reflected Radiation Solar Radiation Absorbed radiation heats the Earth • Earth’s
reflec/vity
is
called
it’s
albedo
(Earth’s
albedo
is
0.3)
 The
absorbed
energy
warms
the
surface
of
Earth
 Reflected Radiation 2
 3/1/09
 How
Warm
Does
It
Get?
 Blackbody emission & the laws of thermodynamics Blackbody
Emission
 Reflected Radiation Absorbed radiation heats the Earth Solar Radiation Heat energy is emitted back to space as blackbody radiation Reflected Radiation How
warm
does
Earth
get?
 “Blackbody”
Emission
 If
we
treat
the
Earth’s
energy
budget
this
way,
what
temperature
do
 All objects emit electromagnetic radiation Intensity of emission vs. wavelength is a function of temperature we
predict
today? 
 We
know
this
is
not
true,
liquid
water
has
been
present
at
Earth’s
 surface
for
billions
of
years
(evidence
from
sedimentary
rocks
 3
 3/1/09
 The
Greenhouse
effect
 Not
to
scale
this
 peak
should
be
 much
higher
 Earth’s
emission
 Sun’s
emission
 wavelength
in
µm
 Ultraviolet
 visible
 wavelength
in
µm
 Large
temperature
difference
between
the
Earth
and
the
sun
 Differences
between
blackbody
emission
spectra
 Short
λ Important
greenhouse
gases
(ppm
by
volume)
 *Greenhouse
gases
are
molecules
in
Earth’s
atmosphere
that
 absorb
electromagne/c
radia/on
in
the
infrared
range
 1.  Water
vapor,
H2O 
 
range
0.1
(polar)
to
40,000
(tropics)
 2.  Carbon
dioxide,
CO2 Methane,
CH4 
 
 Nitrous
oxide,
N2O 
 
0.3
 5.  Ozone,
O3 
.01
(at
Earth’s
surface)
 Long
λ
 Earth’s
atmosphere
interacts
 differentlywith
short
and
long
 wave
radia1on
producing
the

 GREENHOUSE
EFFECT
 What
makes
a
greenhouse
gas
 able
to
absorb
IR
radiaiton?
 Molecular
modes
of
mo5on
 • VibraAonal
(stretching)
 • TranslaAonal
(bending)
 • RotaAon
 
1.7
 4.  Infrared
 
370
 3.  Earth’s
emission
 Sun’s
emission
 
 
 6.  Chlorofluorocarbons
(CFC’s) 
.00082
(0.82
ppb)
 The
greenhouse
effect
is
the
result
of
less
than
0.1%
of
the
atmosphere
 Frequencies
of
these
modes
are
‘pre‐set’
like
 the
speed
of
a
ceiling
fan
 • Only
absorb
specific
λ • CO2,
H2O
absorb
in
the
infrared
(IR)
 • CO2
absorbs
strongly
in
the
15
μm
band
 Small
changes
in
these
make
large
differences
 4
 3/1/09
 The
Greenhouse
effect
 Sun’s emission The
Greenhouse
effect
 Earth’s emission Sun’s emission Earth’s emission There
is
a
very
narrow
 window
of
λ’s
where
 outgoing
energy
from
the
 Earth
can
escape.
 Called
“windows
of
 minimum
absorp1on”
 Lunine Fig. 14.5 a & b The
Greenhouse
Effect
 Qualita1ve
explana1on
 The outgoing radiation absorbed by the Earth’s atmosphere must ultimately escape to space to maintain energy balance. It escapes in the following manner: Atmospheric
pressure:
The
force
per
unit
area
exerted
by
a
column
 of
atmosphere
on
a
surface
 Pressure
is
highest
at
sea
level,
at
the
boXom
of
the
full
 atmospheric
column
 The absorbed outgoing radiation heats the atmosphere. This increases the intensity of blackbody emission by the atmosphere. This allows more energy out, via “windows” of minimal absorption. The atmosphere warms until a new balance is achieved between the rates of energy in and out. Hence, despite atmospheric absorption, the planet emits energy to space at the same rate as it receives energy from the Sun. Surface temperature ends up being ~ 33 °C warmer than it would be w/out IRabsorbing gases in the atmosphere. With
increasing
al/tude,
pressure
decreases
10x
for
every
16km
 
Called
the
pressure
lapse
rate
 5
 3/1/09
 Pressure
decreases
with
increasing
alAtude
(negaAve
lapse
rate)
 Pressure
lapse
rate
=
Pressure
decrease
10x
for
every
16
km
 Increasing
alAtude
(km)
 64
 Decrease
in
pressure
is
 exponen1al
 48
 32
 16
 sealevel
 Swiss
alps
(4000m)
 0
 .1
 1
 10
 100
 1000
 1000
mbar
=
1
bar
(~
1atm)
 50%
<
5km
 Over
50%
of
the
air
in
the
 atmosphere
is
found
within
the
 first
5
km
 Temperature
lapse
rate
in
the
first
12
km
looks
negaAve
 80%
<
~12km
 • Most
infrared
absorpAon
 occurs
close
to
the
surface
 • The
lowest
part
of
the
 atmosphere
should
be
the
 warmest
 • Would
expect
that
 cooling
occurs
with
 increasing
alAtude
 Lapse
rate
is
linear
 Look
at
the
scale
 6
 3/1/09
 Why
are
the
temperature
profiles
of
the
troposphere
and
stratosphere
so
different?
 • 50%
of
the
enAre
atmosphere
is
squashed
into
the
lowest
5
km
of
the
troposphere
 • Most
infrared
absorpAon
goes
on
at
the
boZom
of
the
troposphere
 • Result

Troposphere
is
heated
from
below
(heat
source
at
the
boZom)
 • Air
is
a
fluid
 • The
troposphere

 cools
by
convecAon
 Why
are
the
temperature
profiles
of
the
troposphere
and
stratosphere
so
different?
 Ozone
absorbs
the
sun’s
highest
energy
 wavelengths
 • For
reasons
we
will
discuss
much
later,
molecules
of
ozone
are
confined
to
the
 stratosphere
 • Ozone
molecules
are
most
concentrated
around
30
km
 ozone
 ozone
 This
energy
is
 converted
to
heat
and
 warms
the
 stratosphere
 7
 ...
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This note was uploaded on 06/15/2011 for the course GEOL 103 taught by Professor Lakshmi during the Spring '10 term at South Carolina.

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