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2. NATS 101 - 34 Lecture 2 Hurricane Dean & 2006 climate anomalies Atmospheric Composition Density, Pressure & Temperature Hurricane Dean

3. http://www.ncdc.noaa.gov/oa/climate/research/2006/ann/ann06.html

5. 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, 4 th Ed.

6. N 2 Boiling point: 77 °K or -196°C or –320 °F O 2 Boiling point: 90 °K or -183 °C or -297 °F N 2 and O 2 Balance between input (production) and output (destruction): Input:plant/animal decaying Output: soil bacteria; oceanic plankton-->nutrients Input:plant photosynthesis Output: organic matter decay chemical combination (oxidation) breathing

7. Atmospheric Composition Important Trace Gases Ahrens, Table 1.1, 3 rd ed. Which of these is now wrong even in the 4th edition of Ahrens?

8. Sources vegetative decay volcanic eruptions animal exhalation combustion of fossil fuels (CH 4 + 2 O 2 > 2 H 2 O + CO 2 ) Sinks photosynthesis (oxygen production) dissolves in water phytoplankton absorption (limestone formation) Carbon Dioxide CO 2

9. CO 2 Trend “Keeling Curve” Some gases vary by season and over many years. The CO2 trend is the cause for concern about global warming. CO 2 increases in northern spring, decreases in northern fall See http://earthguide.ucsd.edu/globalchange/keeling_curve/01.html

10. H 2 O Vapor Variability Precipitable Water (mm) Some gases can vary spatially and daily

11. Aerosols 1 cm 3 of air can contain as many as 200,000 non-gaseous particles. –dust –dirt (soil) –ocean spray –volcanic ash –water –pollen –pollutants

12. Aerosols - Volcanic Ash Fig. 1-4, p.6

13. Aerosols - Dust Particles Dust Storm on Interstate 10, between Phoenix and Tucson, AZ.

14. Aerosols Provide condensation nuclei for water vapor. Provide a surface area or catalyst needed for much atmospheric chemistry. Aerosols can deplete stratospheric ozone. They can also cool the planet by reflecting sunlight back to space.

15. Two Important Concepts Let’s introduce two new concepts... Density Pressure

16. 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 (mole)  molecular mass (gm/mole) Avogadro number (6.023x10 23 molecules/mole)

17. Density Change Density (  ) changes by altering either a) # molecules in a constant volume b) volume occupied by the same # molecules a b

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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.

24. 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

25. A Thinning Atmosphere Bottom Top Lower density, Gradual drop Higher density Rapid decrease NASA photo gallery

26. 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

27. 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

28. 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

29. Equation for Pressure Variation We can Quantify Pressure Change with Height

30. What is Pressure at 2.8 km? (Summit of Mt. Lemmon) Use Equation for Pressure Change

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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