advertisement

Thermodynamics of the Atmosphere Atmosphere contains N2 + O2 + Ar + CO2 ± H2O - well mixed in turbosphere - ideal gas (unless saturated) Ideal Gas R* 8.31 Jmol-1K -1 * R 8.31 Meterology p RT R M 28.97 103 =287 Jkg -1K -1 R varies with gas Physics pV NR*T Hydrostatic Equation for an Isothermal Atmosphere Ideal Gas p RT dp weight of air column per unit area p p+dp dz =- g dz dp g dz p RT p z p0e z=0 gz RT Pressure Scale Height p z p0 exp gz / RT p0 exp z / H H = RT/g p0/e is the (exponential) scale height Decade scale height H10= ln10 RT/g p0/10 H H10 Height at 250 K at 280 K H10=16.8 km H10=18.9 km Scale height ~ thickness of troposphere thicker troposphere at low latitudes, where air is warmer 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 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 1st law of thermodynamics 1 dq c p dT dp dq=0 for adiabatic (fast) processes dT 1 dp g dz c p dz cp dT g 9.81 ~ 10 C km1 dz c p 1005 Dry Adiabatic Lapse Rate (DALR) D Saturated Adiabatic Lapse Rate (SALR) S ~ 6 C km-1 SALR < DALR since latent heat is released by water vapour Atmospheric stability Unstable if dT dz D Conditionally dT S D stable if dz Stable if dT dz S