1 NATS Lecture 2 Density, Pressure & Temperature Climate and Weather

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

3 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)

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

5 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 in. Hg

6 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

7 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

8 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

9 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

10 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.

11 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

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

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

14 Exponential Variation Logarithmic Decrease For each 5.5 km height increase, pressure drops by factor of km mb 11 km mb 5.5 km mb 0 km mb

15 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

16 Equation for Pressure Variation We can Quantify Pressure Change with Height

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

18 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

19 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

20 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

21 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

22 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

23 Climate and Weather “Climate is what you expect. Weather is what you get.” -Robert A. Heinlein

24 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

25 Surface Station Model Temperatures Plotted  F in U.S. Sea Level Pressure Leading 10 or 9 is not plotted Examples: plotted as plotted as plotted as 360 Ahrens, p 431 Responsible for boxed parameters

26 Sky Cover and Weather Symbols Ahrens, p 431

27 Pressure Tendency Change in pressure over the past 3 hours is also plotted. Also called barometric tendency Ahrens, p 432

28 Wind Barbs Direction Wind is going towards Westerly Westerly  from the West Speed (accumulated) Each flag is 50 knots Each full barb is 10 knots Each half barb is 5 knots Ahrens, p kts from west

29 temperature dew point SLP pressure wind cloud cover Ohio State website

30 Practice Surface Station Temperate ( o F) Pressure (mb) Last Three Digits (tens, ones, tenths) Dew Point (later) Moisture Wind Barb Direction and Speed Cloud Cover Tenths total coverage Ahrens, p Decimal point What are Temp, Dew Point, SLP, Cloud Cover, Wind Speed and Direction?

31 Practice Surface Station Sea Level Pressure Leading 10 or 9 is not plotted Examples: plotted as plotted as plotted as 360 Ahrens, p Decimal point What are Temp, Dew Point, SLP, Cloud Cover, Wind Speed and Direction?

32 Surface Map Symbols Fronts Mark the boundary between different air masses…later Significant weather occurs near fronts Current US Map Ahrens, p 432

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34 Radiosonde Weather balloons, or radiosondes, sample atmospheric to 10 mb. They measure temperature moisture pressure They are tracked to get winds Ahrens, Fig. 1

35 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

36 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 troposphere stratosphere tropopause temperature profile moisture profile wind profile

37 Upper-Air Model Conditions at specific pressure level Wind Temperature (  C) Moisture (Later) Height above MSL UA 500mb Analysis Ahrens, p 427 Ahrens, p 431 Responsible for boxed parameters

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39 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)

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42 Climate of Tucson Monthly Averages Individual months can show significant deviations from long-term, monthly means.

43 Average and Record MAX and MIN Temperatures for Date

44 Climate of Tucson Probability of Last Freeze Cool Site: Western Region Climate CenterWestern Region Climate Center

45 Climate of Tucson Probability of Rain Cool Site: Western Region Climate CenterWestern Region Climate Center

46 Climate of Tucson Extreme Rainfall Cool Site: Western Region Climate CenterWestern Region Climate Center

47 Climate of Tucson Snow! Cool Site: Western Region Climate CenterWestern Region Climate Center

48 Summary Weather - atmospheric conditions at specific time and place Weather Maps  Instantaneous Values Climate - average weather and the range of extremes compiled over many years Statistical Quantities  Expected Values

49 Reading Assignment Ahrens Pages Problems (2.1  Chapter 2, Problem 1) Don’t Forgot the 4”x 6” Index Cards…