Thermodynamics of the Atmosphere Ideal Gas Atmosphere contains N 2 + O 2 + Ar + CO 2 ± H 2 O - well mixed in turbosphere - ideal gas (unless saturated) Meterology Physics R varies with gas

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

H = RT/g is the (exponential) scale height H p 0 /e Decade scale height H 10 = ln10 RT/g at 250 KH 10 =16.8 km at 280 KH 10 =18.9 km Scale height ~ thickness of troposphere  thicker troposphere at low latitudes, where air is warmer Scale Height H 10 p 0 /10

Atmospheric pressure profile

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”

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 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

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

Atmospheric stability Unstable if Stable if Conditionally stable if