EES215

Lecture 16

Composition of atmosphere - major constituents; trace gases (CO₂, O₃); reference to element distribution - lack of light gases:  Table 1
Origin of gases - degassing of earth; history of oxygen content
Variations of composition with height - presence of ozone:  Fig. 1

Layering of atmosphere:

• Troposphere, lowest layer, contains 75% of mass, all of water and aerosols; decrease with height by 6.25^oC/km; capped by temperature inversion level - tropopause which makes troposphere internally contained; height of tropopause between 16 km (equator) and 8 km (poles)
• Stratosphere: above tropopause to about 50 km; contains most of atmospheric ozone; formation of ozone at stratopause
• Upper atmosphere: mesosphere - mesopause - thermosphere
• Exosphere and magnetosphere, not really part of regular atmosphere but show influence of earth on cosmic rays and flux of particles

Atmospheric pressure and mass:

Boyle's Law:     P=k₁/V (T=const)
Charles's Law: V=k₂T (P=const)

PV = RmT
or
P= rRT
[gas constant R = 287 J/kg-^oK]

Air is compressible - lower layers are much more dense than higher layers: 1.2kg/m³ at sealevel; 0.7 at 5,000m
Pressure (force/area)
Units: mbar = 100N/m² = 100 Pa
Mercury barometer measures height of mercury column in equilibrium with air at given altitude
Mean sealevel pressure 1,000 mbar (760 mm mercury)

P = g(M/4pR_E²)

At sealevel M = 5.14x10¹⁸ kg; g=9.8m/s²; R= 6370 km   =>  P = 10⁵kg/m-s² = 10⁵ Nm^-2 = 10⁵ pascals (Pa) (1mbar = 10²Pa)
Average at sealevel: 1010.25mb; 760mb from N₂; 240mb from O₂; 10mb from water vapor
Atmospheric pressure decreases with height:  Fig. 2

Solar radiation: electromagnetic spectrum -  Fig.3
Spectral distribution of solar and terrestrial radiation:  Fig. 4
Shift to infrared; from short wavelength to long wavelength; energy flux diagram

Gases in atmosphere absorb/emit radiation at specific wavelength: Examples – CO₂ and H₂O absorb in infrared; O₃ in ultraviolet:  Fig. 5
Atmosphere (and oceans) act to dampen differences between equator and poles
General effect of atmosphere: difference in transmissivity between long-wave and short wavelength radiation - energy flux diagram:  Fig. 6
Direct reflection
Conversion from short to long-wavelength radiation
Cloud cover - importance of water and trace gases; albedo (ratio of reflected to total radiation):  Table 2

Presence of water in atmosphere is strongly temperature dependent:  Table 3

Example: Air masses moving over mountain range (Föhn winds): Moisture laden air forced upward – cooling and precipitation; on other side increase in temperature, but low relative humidity (rain shadow):  Fig. 7