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Thermodynamics of the Atmosphere

Physics. Meterology. R varies with gas. Thermodynamics of the Atmosphere. Atmosphere contains N 2 + O 2 + Ar + CO 2 ± H 2 O well mixed in turbosphere ideal gas (unless saturated). Ideal Gas. p. Ideal Gas. dz. p + dp. z=0. Hydrostatic Equation for an Isothermal Atmosphere.

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Thermodynamics of the Atmosphere

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  1. Physics Meterology R varies with gas Thermodynamics of the Atmosphere • Atmosphere contains N2 + O2 + Ar + CO2 ± H2O • well mixed in turbosphere • ideal gas (unless saturated) Ideal Gas

  2. p Ideal Gas dz p+dp z=0 Hydrostatic Equation for an Isothermal Atmosphere

  3. p0/e p0/10 H10 H Scale Height H = RT/gis the (exponential) scale height Decade scale height H10= ln10 RT/g at 250 K H10=16.8 km at 280 K H10=18.9 km Scale height ~ thickness of troposphere  thicker troposphere at low latitudes, where air is warmer

  4. Atmospheric pressure profile

  5. Atmospheric layers • Troposphere • pressure drops from 1000 mbar to ~200 mbar,contains 80% mass • thickness 10 – 15 km • temperature drops with height by ~6.5 ºC/km,unstable as hot air rises! • well mixed: molecules travel up and down in few days • most weather in this layer, i.e. most heat transport • Tropopause • region of stable, isothermal air ~ 200 mbar • division between troposphere and stratosphere • top of “weather”

  6. Stratosphere • pressure from ~200 mb to ~1 mb, height 10 – 50 km • temperature increases with height (due to heating in ozone layer) •  stable air, isothermal strata with little mixing • stops weather extending upwards, so keeps water in atmosphere • temperature reaches maximum at Stratopause • Mesosphere • “middle atmosphere”, pressure 1 mb to 10-2 mb • temperature drops to lowest values ~ -90ºC • Mesopause is top of well mixed region, 99.9% of atmosphere below • Thermosphere • temperature increases rapidly (due to molecular dissociation) • v.large KE/molecule, but few molecules!  Feels cold • mean free path so large that no longer well mixed

  7. 1st law of thermodynamics dq=0for adiabatic (fast) processes Dry Adiabatic Lapse Rate (DALR) Saturated Adiabatic Lapse Rate (SALR) SALR < DALR since latent heat is released by water vapour

  8. Atmospheric stability Unstable if Conditionally stable if Stable if

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