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2. Atmosphere: Dry air Primordial atmosphere – Volcanic activity, rock outgassing – H2O vapor, CO 2, N 2, S… no oxygen Present composition of dry air – 78% N 2 – 21% O 2 – 1% Ar “Minor” consitutents – CO 2 0.039%, CH 4 0.00018%, O 3 < 0.00005% Origin of oxygen: dissociation of water vapor by absorption of UV (minor), and photosynthesis (major) 2

3. Time series of CO 2 3

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) 4

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 5

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 6

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 7

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 8

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 9

10. Pressure Pressure = force per unit area p = N/m 2 = 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 10

11. Pressure 11 g ~ 9.81 m/s 2 at sea-level

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

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

14. Pressure vs. height 14 P 0 = reference (surface) pressure H = scale height

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

16. p and  vs. height 16

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 17

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

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 19