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Atmosphere: Dry air. Primordial atmosphere Volcanic activity, rock outgassing H2O vapor, CO 2 , N 2 , S… no oxygen Present composition of dry air 78% N 2 21% O 2 1% Ar “Minor” consitutents CO 2 0.039%, CH 4 0.00018%, O 3 < 0.00005% - PowerPoint PPT Presentation
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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 variation15˚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 areap = 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 = millibarhPa = hectopascal1 mb = 100 Pa
For surface p = 1000 mb:50% of mass below 500 mb80% of mass below 200 mb99.9% 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) pressureH = 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, well-mixed 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