Troposphere
The troposphere is the lowest atmospheric layer reaching an altitude of about 20 km. Density, pressure and temperature decline with altitude. The troposphere is largely convective, which translates into a uniform chemical composition. Critical component is H2O vapor from evaporation and transpiration processes. Water vapor decreases with altitude because of condensation and cloud formation. Temperature profile of the Troposphere Q T W Heat transfer in air: k A kair 0.026 thermalconductivity t z mK Heat transfer takes place by energy transfer through cross sectional area A! Vertical convective motion takes place by moving volume of air in a reversible adiabatic process (no heat exchange with surrounding environment Q=0!) This is subject to energy conservation according to the first law of thermodynamics: dU Q W Q PdV Heat exchange between air packet and surrounding is: kJ dQ C dT P dV C 1.010 v v Specific heat or heat capacity of K kg an air volume element at constant kJ volume or constant pressure. CP 1.005 K kg
Adiabatic conditions: Q 0 Cv dT P dV For adiabatic conditions Q 0 Cv dT P dV
dU Internal energy for V const : dU C dT C v v dT dU dQ specific heat for V const : Cv for P const :CP dT V dT P
Gas Cp J/mole·K Cv J/mole·K He 20.79 12.52 Ar 20.79 12.45 Universal gas constant N 29.12 20.8 2 R = 8.314 J/mole·K O2 29.37 20.98 CO 29.04 20.74 R CP Cv CO2 36.62 28.17
H2O vapor 37.47 28.03 Conversion: 1 mole ≡ A g air 29.07 20.95 To derive the temperature profile along the vertical atmosphere axis as function of altitude T(z) requires differentiating the ideal gas equation P·V=R·T for Q=0:
PdV V dP RdT CP Cv dT R CP Cv
R dT CP dT Cv dT CP dT P dV P dV V dP
CP dT V dP for adiabatic element
dP Reformulating the hydrostatic equation: g g 9.81m / s2 dz z V dP V g dz m g dz C dT m g dz z P Lapse rate defines the rate at dT m g which temperature T decreases T dz C with increasing altitude z. P (For the troposphere a negative value) T Adiabatic lapse rate: =-9.7 K/km for dry air. The adiabatic lapse rate describes the temperature change with altitude for absolutely dry air, it serves as a normalization parameter since air is never dry.
CP increases with humidity, CP=1.88 KJ/km K for H2O vapor. An increase in humidity decreases the temperature gradient. The actual lapse rate L depends on humidity and is around 6-7 K/km! dT g V L dz CP K 6 7 km
Higher humidity increases CP and changes the temperature gradient that -L < -Γ or L > Γ.
Flight height of 10 km, T=-50oC=223K If the density ρ in an air volume element V is smaller than the density ρs of surrounding environment, the buoyancy causes acceleration upward, if density of air volume is larger, element will stabilize. dT V g smaller L dz CP Unstable situation, convection dT V g larger L dz CP
stable situation, no convection wet g dTair V dz L dz Convective equilibrium CP z dT dry g dT V dz dz dz dry CP convection m 0.828 0.232 29 amu dry 3 radiation V V V m m 0.7228 0.1832 0.118 27.7 amu wet 3 T V V V m Clouds Rising Cloud physics and weather
see next chapter The stratosphere
The stratosphere is the second layer of the atmosphere above the troposphere ranging from about 20 km to 50 km altitude. The density declines in this layer from 100 g/m3 to 3 g/m3, the pressure drops exponentially from 100mb to 1mb.
The temperature remains constant at the lower layer of the stratosphere and then rises with increasing altitude. At the top of the stratosphere the thin air may reach temperatures close to 0°C (273 K). Temperature profile of Stratosphere ~20-50km altitude f Temperature is dominated by radiative energy balance f Incoming flux must be balanced by outgoing flux. The factor f is the fraction of transmitted radiation. F T 4 f T 4 f in E S 4 4 Fout 1 f TE f TS f 4 4 4 4 TE f TS 1 f TE f TS 4 4 4 TE 1 f TE 2 f TS T T 4 2 T 4 T E E S S 21/ 4
TE = 255 K TS = 215 K This suggests that temperature in stratosphere remains constant. This is only partially correct since ozone layer in stratosphere causes further absorption and an increase of the stratosphere temperature up to 270K. Chemical composition of Stratosphere
Temperature is low and constant for ~10 km, that region is frequently called tropopause. Towards higher altitude the temperature increases due to the absorption of cosmic radiation, reaching a maximum of about 0oC (273 K) at top of stratosphere. This is associated with the absorption of UV radiation by Oxygen and Ozone causing stratosphere heating in its upper layers.
UV range Nuclear test program deposition
A total of 2057 nuclear weapon explosions since 1945
51017 Bq=500 PBq Mesosphere
Mesosphere is the atmospheric layer above stratosphere. It is characterized by steadily decreasing density. High altitude (~80km) location for rare kind of clouds (noctilucent clouds) in polar zone, frequency of occurrence seems to increase (signature for climate change, consequence of space shuttle exhaust?) NLC: first observed 1885 (two years after the Krakatau eruption), increasing appearance of noctilucent clouds is interpreted as a consequence of a decrease in temperature at the altitude where the clouds form parallel with an increase in water vapor by complex NLC formation requires a combination of photochemical processes. The warming very low temperatures, a source of water in the in the lower atmospheric layers vapor, and condensation points (meteoritic causes the cloud layer to cool dust, volcano ashes, aerosols?) increasing the probability of formation. Mesosphere is too high to be reached by balloon or aircraft and is therefore poorly understood! New satellites such as AIM mission are launched for more detailed exploration. Temperature declines towards its lowest value in the atmosphere of about T=-80 oC (193K). Limited absorption of the solar radiation flux and possible
cooling by de-excitation of CO2 vibrational excitation modes by collision with oxygen atoms, which are in equilibrium at higher density in lower atmospheric regions. 960 Q 2 e T
collision rate This leads to the discussion of possible enhanced cooling with
increasing CO2 budget! Aerosol impact on stratosphere temperature Example: Mount Pinatubo eruption in June 13, 1991 causing emission of particles into higher atmosphere with direct impact on optical depth (·d) and temperature T.
Optical depth for 1020 nm range
F d e d 40km; 0 d 0.001 v d 0.06 F0 F F e0 d 99% v ev d 94% F0 F0 Impact on high altitude temperature Heavy metal enrichment Large numbers of meteorites are absorbed by During the early phase of the planet atmosphere, they typically evaporate in the formation, the accretion rate was in mesosphere, enriching the layer with about 40 the range of more than millions /sec tons/day of heavy metal containing dust! from meteors to planetesimals.
FREQUENCY OF IMPACTORS:
Pea-size meteoroids - 10 per hour 2.7 Walnut-size - 1 per hour N 37 D Grapefruit-size - 1 every 10 hours Basketball-size - 1 per month N: number of meteorites 50-m rock - 1 per 100 years D: diameter of meteorites 1-km asteroid - 1 per 100,000 years 2-km asteroid - 1 per 500,000 years
Meteorites fragment in higher atmosphere layers and the fragments evaporate in the mesosphere. Meteoritic material is enriched in heavy elements (iron meteorites) which form dust particles in the high altitude layers.