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Atmosphere: Dry air
• Primordial atmosphere
– Volcanic activity, rock outgassing
– H2O vapor, CO2, N2, S… no oxygen
• Present composition of dry air
– 78% N2
– 21% O2
– 1% Ar
• “Minor” consitutents
– CO2 0.039%, CH4 0.00018%, O3 < 0.00005%
• Origin of oxygen: dissociation of water vapor by
absorption of UV (minor), and photosynthesis (major)
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Time series of CO2
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Atmosphere: Dry and moist
• Dry air constituents are well-mixed and vary only
slowly over time and space
– Roughly constant over lowest 80 km (50 mi)
– Very convenient for thermodynamic calculations
• Water vapor (“wv”) 0-4% of total atmospheric
mass, but also concentrated near surface for
these reasons
– Surface source
– Efficient return mechanism (precipitation)
– Absolute humidity is a very strong function of
temperature (T)
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Standard atmosphere
• Averaged over time and
horizontal space
• Four layers:
–
–
–
–
Troposphere
Stratosphere
Mesosphere
Thermosphere
• “Lapse rate” = how T
decreases with height
Temperature vs. height for standard atmosphere
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Standard atmosphere
• Troposphere
– “turning sphere”
– Averages 12 km (7.5 mi)
deep
– Top = tropopause
– T range 15˚C @ sfc to 60˚C at tropopause
– Average tropospheric
lapse rate: 6.5˚C/km
(19˚F/mi)
Temperature vs. height for standard atmosphere
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Standard atmosphere
• Stratosphere
– “layered”… very stable
– Extends upward to 50
km
– Top = stratopause
– T increases with height
(lapse rate negative)
– UV interception by O2
and O3
– “lid” for troposphere… in
a sense
Temperature vs. height for standard atmosphere
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Standard atmosphere
• Mesosphere
– “middle sphere”
– T decreases with height
again
– Top = mesopause
• Thermosphere
– Very hot… and yet no
“heat” (very little mass)
– Freeze and fry
simultaneously
Temperature vs. height for standard atmosphere
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Standard atmosphere
• Tropospheric T variation
 15˚C at surface
 -60˚C at 12 km elevation
• If “warm air rises and
cold air sinks”, why
doesn’t the troposphere
turn over?
Temperature vs. height for standard atmosphere
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Pressure
• Pressure = force per unit area
p = N/m2 = Pascal (Pa)
• Air pressure largely due to weight of overlying
air
– Largest at the surface, zero at atmosphere top
– Decreases monotonically with height (z)
– Pressure linearly proportional to mass
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Pressure
g ~ 9.81 m/s2 at sea-level
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Sea-level pressure (SLP)
mb = millibar
hPa = hectopascal
1 mb = 100 Pa
For surface p = 1000 mb:
50% of mass below 500 mb
80% of mass below 200 mb
99.9% of mass below 1 mb
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Various p and z levels
Infer how pressure varies with height
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Pressure vs. height
P0 = reference (surface) pressure
H = scale height
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Density = r = mass/volume
Infer how density varies with height
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p and r vs. height
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Warm air rises and cold air sinks…
• NOT always true.
• True statement is: less
dense air rises, more
dense air sinks
• Note near-surface air,
although warm, is also
more dense
Temperature vs. height for standard atmosphere
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Warm air rises and cold air sinks…
Temperature vs. height for standard atmosphere
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Summary
• Dry air dominated by nitrogen & oxygen, wellmixed and relatively fixed
• wv variable, concentrated near surface
• T variation with z in standard atmosphere is
complex
• Average SLP ~ 1000 mb
• On average, 80% of mass below tropopause,
99.9% below stratopause
• We need to start thinking about density
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