1. NATS 101 Lecture 2 Atmospheric Composition and Vertical Structure

2. Lecture 2-Nats 1012 Atmospheric Composition Permanent Gases N 2 and O 2 are most abundant gases Percentages hold constant up to 80 km Ar, Ne, He, and Xe are chemically inert N 2 and O 2 are chemically active, removed & returned Ahrens, Table 1.1, 3 rd Ed.

3. Lecture 2-Nats 1013 Atmospheric Composition Important Trace Gases Ahrens, Table 1.1, 3 rd ed.

4. Lecture 2-Nats 1014 CO 2 Trend Ahrens, Fig. 1.3, 3th Ed. Keeler Curve from Hawaii Obs Some gases can vary by season and can vary over many years CO 2 increases in spring decreases in fall

5. Lecture 2-Nats 1015 H 2 O Vapor Variability Precipitable Water (mm) Some gases can vary spatially and daily

6. Lecture 2-Nats 1016 Two Important Concepts Let’s introduce two new concepts... Density Pressure

7. Lecture 2-Nats 1017 What is Density? Density (  ) = Mass (M) per unit Volume (V)  = M/V  = Greek letter “rho” Typical Units: kg/m 3, gm/cm 3 Mass = # molecules  molecular weight (gm/mole) Avogadro number (6.023x10 23 molecules/mole)

8. Lecture 2-Nats 1018 Density Change Density (  ) changes by altering either a) # molecules in a constant volume b) volume occupied by the same # molecules a b

9. Lecture 2-Nats 1019 What is Pressure? Pressure (p) = Force (F) per unit Area (A) Typical Units: pounds per square inch (psi), millibars (mb), inches Hg Average pressure at sea-level: 14.7 psi 1013 mb 29.92 in. Hg

10. Lecture 2-Nats 10110 Pressure Can be thought of as weight of air above you. (Note that pressure acts in all directions!) So as elevation increases, pressure decreases. Higher elevation Less air above Lower pressure Lower elevation More air above Higher pressure Bottom Top

11. Lecture 2-Nats 10111 Density and Pressure Variation Key Points 1.Both decrease rapidly with height 2.Air is compressible, i.e. its density varies Ahrens, Fig. 1.5

12. Lecture 2-Nats 10112 Why rapid change with height? Consider a spring with 10 kg bricks on top of it compressible The spring compresses a little more with each addition of a brick. The spring is compressible. 10 kg

13. Lecture 2-Nats 10113 Why rapid change with height? Now consider several 10 kg springs piled on top of each other. Topmost spring compresses the least! Bottom spring compresses the most! The total mass above you decreases rapidly w/height.  mass

14. Lecture 2-Nats 10114 Why rapid change with height? Finally, consider piled-up parcels of air, each with the same # molecules. The bottom parcel is squished the most. Its density is the highest. Density decreases most rapidly at bottom.

15. Lecture 2-Nats 10115 Why rapid change with height? Each parcel has the same mass (i.e. same number of molecules), so the height of a parcel represents the same change in pressure  p. Thus, pressure must decrease most rapidly near the bottom. pppp pppp pppp pppp

16. Lecture 2-Nats 10116 Water versus Air Pressure variation in water acts more like bricks, close to incompressible, instead of like springs. Air: Lower density, Gradual drop Higher density Rapid decrease Bottom Top Bottom Top Water: Constant drop

17. Lecture 2-Nats 10117 A Thinning Atmosphere Bottom Top Lower density, Gradual drop Higher density Rapid decrease NASA photo gallery

18. Lecture 2-Nats 10118 Pressure Decreases Exponentially with Height Logarithmic Decrease For each 16 km increase in altitude, pressure drops by factor of 10. 48 km - 1 mb 32 km - 10 mb 16 km - 100 mb 0 km - 1000 mb 100 mb 10 mb 1 mb 16 km 32 km 48 km Ahrens, Fig. 1.5

19. Lecture 2-Nats 10119 Exponential Variation Logarithmic Decrease For each 5.5 km height increase, pressure drops by factor of 2. 16.5 km - 125 mb 11 km - 250 mb 5.5 km - 500 mb 0 km - 1000 mb

20. Lecture 2-Nats 10120 Equation for Pressure Variation We can Quantify Pressure Change with Height

21. Lecture 2-Nats 10121 What is Pressure at 2.8 km? (Summit of Mt. Lemmon) Use Equation for Pressure Change

22. Lecture 2-Nats 10122 What is Pressure at Tucson? Use Equation for Pressure Change Let’s get cocky… How about Denver? Z=1,600 m How about Mt. Everest? Z=8,700 m You try these examples at home for practice

23. Lecture 2-Nats 10123 Temperature (T) Profile More complex than pressure or density Layers based on the Environmental Lapse Rate (ELR), the rate at which temperature decreases with height. inversion isothermal 6.5 o C/km Ahrens, Fig. 1.7

24. Lecture 2-Nats 10124 Higher Atmosphere Molecular Composition Homosphere- gases are well mixed. Below 80 km. Emphasis of Course. Heterosphere- gases separate by molecular weight, with heaviest near bottom. Lighter gases (H, He) escape. Ahrens, Fig. 1.8

25. Lecture 2-Nats 10125 Atmospheric Layers Essentials Thermosphere-above 85 km Temps warm w/height Gases settle by molecular weight (Heterosphere) Mesosphere-50 to 85 km Temps cool w/height Stratosphere-10 to 50 km Temps warm w/height, very dry Troposphere-0 to 10 km (to the nearest 5 km) Temps cool with height Contains “all” H 2 O vapor, weather of public interest

26. Lecture 2-Nats 10126 Summary Many gases make up air N 2 and O 2 account for ~99% Trace gases: CO 2, H 2 O, O 3, etc. Some are very important…more later Pressure and Density Decrease rapidly with height Temperature Complex vertical structure

27. Lecture 2-Nats 10127 Reading Assignment Ahrens Pages 13-22; 427-428 (Appendix C) Problems 1.17, 1.18, 1.20 (1.17  Chapter 1, Question 17) Don’t Forget the 4”x6” Index Cards