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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