Atmosphere: Dry air

2. Atmosphere: Dry air • Primordial atmosphere • Volcanic activity, rock outgassing • H2O vapor, CO2, N2, S… no oxygen • Present composition of dry air • 78% N2 • 21% O2 • 1% Ar • “Minor” consitutents • CO2 0.039%, CH4 0.00018%, O3 < 0.00005% • Origin of oxygen: dissociation of water vapor by absorption of UV (minor), and photosynthesis (major)

3. Time series of CO2

4. Atmosphere: Dry and moist • Dry air constituents are well-mixed and vary only slowly over time and space • Roughly constant over lowest 80 km (50 mi) • Very convenient for thermodynamic calculations • Water vapor (“wv”) 0-4% of total atmospheric mass, but also concentrated near surface for these reasons • Surface source • Efficient return mechanism (precipitation) • Absolute humidity is a very strong function of temperature (T)

5. Standard atmosphere • Averaged over time and horizontal space • Four layers: • Troposphere • Stratosphere • Mesosphere • Thermosphere • “Lapse rate” = how T decreases with height Temperature vs. height for standard atmosphere

6. Standard atmosphere • Troposphere • “turning sphere” • Averages 12 km (7.5 mi) deep • Top = tropopause • T range 15˚C @ sfc to -60˚C at tropopause • Average tropospheric lapse rate: 6.5˚C/km (19˚F/mi) Temperature vs. height for standard atmosphere

7. Standard atmosphere • Stratosphere • “layered”… very stable • Extends upward to 50 km • Top = stratopause • T increases with height (lapse rate negative) • UV interception by O2 and O3 • “lid” for troposphere… in a sense Temperature vs. height for standard atmosphere

8. Standard atmosphere • Mesosphere • “middle sphere” • T decreases with height again • Top = mesopause • Thermosphere • Very hot… and yet no “heat” (very little mass) • Freeze and fry simultaneously Temperature vs. height for standard atmosphere

9. Standard atmosphere • Tropospheric T variation • 15˚C at surface • -60˚C at 12 km elevation • If “warm air rises and cold air sinks”, why doesn’t the troposphere turn over? Temperature vs. height for standard atmosphere

10. Pressure • Pressure = force per unit area • p = N/m2 = Pascal (Pa) • Air pressure largely due to weight of overlying air • Largest at the surface, zero at atmosphere top • Decreases monotonically with height (z) • Pressure linearly proportional to mass

11. Pressure g ~ 9.81 m/s2 at sea-level

12. Sea-level pressure (SLP) For surface p = 1000 mb: 50% of mass below 500 mb 80% of mass below 200 mb 99.9% of mass below 1 mb mb = millibar hPa = hectopascal 1 mb = 100 Pa

13. Various p and z levels Infer how pressure varies with height

14. Pressure vs. height P0 = reference (surface) pressure H = scale height

15. Density = r = mass/volume Infer how density varies with height

16. pand r vs. height

17. Warm air rises and cold air sinks… • NOT always true. • True statement is: less dense air rises, more dense air sinks • Note near-surface air, although warm, is also more dense Temperature vs. height for standard atmosphere

18. Warm air rises and cold air sinks… Temperature vs. height for standard atmosphere

19. Summary • Dry air dominated by nitrogen & oxygen, well-mixed and relatively fixed • wv variable, concentrated near surface • T variation with z in standard atmosphere is complex • Average SLP ~ 1000 mb • On average, 80% of mass below tropopause, 99.9% below stratopause • We need to start thinking about density