12.003 Introduction to Atmosphere,
Ocean, and Climate Dynamics
Topic 2
Characteristics of the Atmosphere
1
Topic 2 Outline
1. Geometry of the Earth
2. Chemical composition of the atmosphere
2
Components of the Climate System
1. Atmosphere
• fast timescales (days to weeks)
2. Oceans
• intermediate timescales (decades to millenia)
3. Cryosphere
• slow timescales (> 100 years)
4. Lithosphere
• very slow timescales (> 10,000 years)
5. Biosphere
• all timescales
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Geometry of lithosphere
• Earth is a rotating oblate spheroid
• 70% of the lithosphere is covered by ocean
• 70% of emerged land is in Northern Hemisphere
z/H
⌅(z) = ⌅0 e
⌅(z) = ⌅0 e
z/H
⌅0 = 1.35 kg/m3 ⌅0 = 1.35 kg/m3
H = 6.8 km
6356
⇥=
H = 6.8 km
2⇤
v 2⇤R/day v2⇤ 2⇤R/day
=
= = = 7.27 ⇤ 10
= 5 s 1= 7.2
⇥=
r
R
rday
R
day
⇤
⇤
Ve = 2GM/R Ve = 2GM/R
⇤
⇤
Vm = 2kT /m Vm = 2kT /m
6378
u e Te · ⇤T̄
4
u e Te · ⇤T̄
2
(⇧t + u g · ⇤) T + (⇧Tt v++uN
w T=
+ TDvT + NT2 w
g ·T⇤)
2
2
Geometry of lithosphere
• Emerged land rarely exceeds 2 km
• atmosphere flows above slightly corrugated surface • Oceans are on average 4 km deep
• oceans sit in deeply corrugated basins
5
Air density decreases approximately
exponentially with height
⌅(z) = ⌅0 e
z/H
⌅0 = 1.35 kg/m3
H = 6.8 km
Duu
⌅
= Pressure and frictional f
Dt
⇤|⇥T |2
u e Te · ⇥T̄
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Atmosphere & ocean are shallow
compared with radius of earth
10 km
6370 km
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Characteristics of the atmosphere
• Gravity is approx. constant throughout atmosphere
• g ≈ 9.81 m/s2
10 km
6370 km
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Chemical composition of atmosphere
The chemical composition of Earth’s atmosphere differs
from that of other planets in the solar system
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Chemical Evolution (Clarendon Press, 1992)
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Atmospheric loss
• Atmospheres can lose atoms to space, especially low
mass ones, if they can escape gravitational pull of planet
• Escape velocity Ve=(2GM/a)1/2
• A variety of escape mechanisms: Jeans escape,
hydrodynamic escape, impact erosion, interaction with
solar wind • Also loss to surface (sequestration)
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Jeans escape (molecular motions)
• Need sufficiently high thermal velocity and few
subsequent collisions (above exobase)
• Mean molecular velocity Vm=(3kT/m’)1/2 (exobase at
500-1000 km, T roughly 1000K) • Negligible (∼0.1%) number of molecules have V > 3Vm
(Boltzmann distribution)
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Jeans escape
• Molecular hydrogen has 3Vm = 11 km/s using T=1000K
• Jupiter has Ve = 60 km/s, Earth has Ve = 11 km/s
• H2 cannot escape from gas giants like Jupiter, but it is
relatively easily lost from lower-mass bodies like Earth or
Mars
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Chemical composition of atmosphere
Life also changed the chemical composition of Earth’s
atmosphere e.g., photosynthesis
See Gaidos and Yung, Evolution of Earth’s
Atmosphere, 2003 for general review
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Earth’s atmospheric composition
Atmospheric composition is relatively constant below 50 km
Atmospheric Composition
Gas Name
Nitrogen
Chemical Formula
Percent Volume
N2
78.08%
Oxygen
O2
20.95%
*Water
H2O
0 to 4%
Argon
Ar
0.93%
CO2
0.0360%
Neon
Ne
0.0018%
Helium
He
0.0005%
*Methane
CH4
0.00017%
Hydrogen
H2
0.00005%
N2O
0.00003%
O3
0.000004%
*Carbon Dioxide
*Nitrous Oxide
*Ozone
* variable gases
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Saturation vapor pressure of water
• Saturation vapor pressure increases exponentially
with temperature (Clausius-Clapeyron law)
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Earth’s atmospheric composition
• Water vapor and CO2 are strong absorbers in infrared
(low brightness temperature -> emitted higher in atmosphere)
•
Atmospheric Composition
Gas Name
Nitrogen
Chemical Formula
N2
Percent Volume
78.08%
Oxygen
O2
20.95%
*Water
H2O
0 to 4%
Argon
Ar
0.93%
CO2
0.0360%
Neon
Ne
0.0018%
Helium
He
0.0005%
*Methane
CH4
0.00017%
Hydrogen
H2
0.00005%
N2O
0.00003%
O3
0.000004%
*Carbon Dioxide
*Nitrous Oxide
*Ozone
* variable gases
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CO2 concentration and climate
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