| NATS 101 Intro to Weather and Climate Section 05: 2:00PM TTh ILC 150 |
| Dr. E. Robert Kursinski | |
| TAs: Mike Stovern & | |
| April Chiriboga |
| NATS 101 - 05 |
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| NATS 101 -
05 Lecture 2 Density, Pressure & Temperature Climate and Weather |
| 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 inaltitude, pressure drops by factor of 10. | |
| 48 km - 1 mb 32 km - 10 mb 16 km - 100 mb 0 km - 1000 mb |
| 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 |
| 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. |
| 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 ÒallÓ H2O vapor, weather of public interest |
| 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 |
| Climate and Weather |
| ÒClimate is what you expect. | |
| Weather is what you get.Ó | |
| -Robert A. Heinlein |
| Weather |
| Weather – The state of the atmosphere: | |
| for a specific place | |
| at a particular time | |
| Weather Elements | |
| 1) Temperature | |
| 2) Pressure | |
| 3) Humidity | |
| 4) Wind | |
| 5) Visibility | |
| 6) Clouds | |
| 7) Significant Weather |
| Surface Station Model |
| Temperatures | |
| Plotted ¡F in U.S. | |
| Sea Level Pressure | |
| Leading 10 or 9 is not plotted | |
| Examples: | |
| 1013.8 plotted as 138 | |
| 998.7 plotted as 987 | |
| 1036.0 plotted as 360 |
| Sky Cover and Weather Symbols |
| Wind Barbs |
| Direction | |
| Wind is going towards | |
| Westerly Þ from the West | |
| Speed (accumulated) | |
| Each flag is 50 knots | |
| Each full barb is 10 knots | |
| Each half barb is 5 knots |
| Slide 31 |
| Practice Surface Station |
| Temperate (oF) | |
| Pressure (mb) Last Three Digits (tens, ones, tenths) | |
| Dew Point (later) Moisture | |
| Wind Barb Direction and Speed | |
| Cloud Cover Tenths total coverage |
| Practice Surface Station |
| Sea Level Pressure | |
| Leading 10 or 9 is not plotted | |
| Examples: | |
| 1013.8 plotted as 138 | |
| 998.7 plotted as 987 | |
| 1036.0 plotted as 360 | |
| Surface Map Symbols |
| Fronts | |
| Mark the boundary between different air massesÉlater | |
| Significant weather occurs near fronts | |
| Current US Map |
| Slide 35 |
| Radiosonde |
| Weather balloons, or radiosondes, sample atmospheric to 10 mb. | |
| They measure temperature moisture pressure | |
| They are tracked to get winds |
| Radiosonde Distribution |
| Radiosondes released at 0000 and at 1200 GMT for a global network of stations. | |
| Large gaps in network over oceans and in less affluent nations. | |
| Stations ~400 km apart over North America |
| Radiosonde for Tucson |
| Example of data taken by weather balloon released over Tucson | |
| Temperature (red) | |
| Moisture (green) | |
| Winds (white) | |
| Note variations of all fields with height | |
| UA Tucson 1200 RAOB |
| Climate |
| Climate - Average weather and range of weather, computed over many years. | |
| Whole year (mean annual precipitation for Tucson, 1970-present) | |
| Season (Winter: Dec-Jan-Feb) | |
| Month (January rainfall in Tucson) | |
| Date (Average, record high and low temperatures for Jan 1 in Tucson) |
| Slide 40 |
| Slide 41 |
| Climate of Tucson Monthly Averages |
| Slide 43 |
| Climate of Tucson Probability of Last Freeze |
| Climate of Tucson Probability of Rain |
| Climate of Tucson Extreme Rainfall |
| Climate of Tucson Snow! |
| Summary |
| Weather - atmospheric conditions at specific time and place | |
| Weather Maps Þ Instantaneous Values | |
| Climate - average weather and the range of extremes compiled over many years | |
| Statistical Quantities Þ Expected Values | |
| Reading Assignment |
| Ahrens | |
| Pages 25-42 | |
| Problems 2.1-2.4, 2.7, 2.9-2.12 | |
| (2.1 Þ Chapter 2, Problem 1) | |
| DonÕt forget the clickers É |