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| NATS 101 - 34 Lecture 2 Hurricane Dean & 2006 climate anomalies Atmospheric Composition Density, Pressure & Temperature |
| "http://www.ncdc.noaa.gov/oa/climate/research/2006/ann/ann06..." |
| http://www.ncdc.noaa.gov/oa/climate/research/2006/ann/ann06.html |
| Slide 4 |
| Atmospheric Composition Permanent Gases |
| N2 and O2 are most abundant gases | |
| Percentages hold constant up to 80 km | |
| Ar, Ne, He, and Xe are chemically inert | |
| N2 and O2 are chemically active, removed & returned |
| Atmospheric Composition Important Trace Gases |
| CO2 Trend |
| H2O Vapor
Variability Precipitable Water (mm) |
| Two Important Concepts |
| LetÕs introduce two new concepts... | |
| Density | |
| Pressure |
| What is Density? |
| Density (r) = Mass (M) per unit Volume (V) | |
| r = M/V | |
| r = Greek letter ÒrhoÓ | |
| Typical Units: kg/m3, gm/cm3 | |
| Mass = | |
| # molecules (mole) « molecular mass (gm/mole) | |
| Avogadro number (6.023x1023 molecules/mole) |
| Density Change |
| Density (r) changes by altering either | |
| a) # molecules in a constant volume | |
| b) volume occupied by the same # molecules |
| 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 |
| Pressure |
| Can be thought of as weight of air above you. | |
| (Note that pressure acts in all directions!) | |
| So as elevation increases, pressure decreases. |
| Density and Pressure Variation |
| Key Points | |
| Both decrease rapidly with height | |
| Air is compressible, i.e. its density varies |
| Why rapid change with height? |
| Consider a spring with 10 kg bricks on top of it | |
| The spring compresses a little more with each addition of a brick. The spring is compressible. |
| 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. |
| 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. |
| 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 Dp. | |
| Thus, pressure must decrease most rapidly near the bottom. |
| A Thinning Atmosphere |
| 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 |
| Water versus Air |
| Pressure variation in water acts more like bricks, close to incompressible, instead of like springs. |
| Equation for Pressure Variation |
| We can Quantify Pressure Change with Height | |
| What is Pressure at 2.8 km? (Summit of Mt. Lemmon) |
| Use Equation for Pressure Change | |
| What is Pressure at Tucson? |
| Use Equation for Pressure Change | |
| LetÕs get cockyÉ | |
| How about Denver? Z=1,600 m | |
| How about Mt. Everest? Z=8,700 m | |
| You try these examples at home for practice |
| 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. |
| 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. |
| Summary |
| Many gases make up air | |
| N2 and O2 account for ~99% | |
| Trace gases: CO2, H2O, O3, etc. | |
| Some are very importantÉmore later | |
| Pressure and Density | |
| Decrease rapidly with height | |
| Temperature | |
| Complex vertical structure |
| Reading Assignment |
| Ahrens | |
| Pages 13-22; 427-428 (Appendix C) | |
| Problems 1.17, 1.18, 1.20 | |
| (1.17 Þ Chapter 1, Question 17) | |
| DonÕt Forget the 4Óx6Ó Index Cards |