1. NATS 101 Intro to Weather and Climate Section 05: 2:00PM TTh ILC 150
Dr. E. Robert Kursinski
TAs: Mike Stovern &
April Chiriboga
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4. NATS 101 - 05 Lecture 2 Density, Pressure & Temperature Climate and Weather

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

6. What is Density? Density () = Mass (M) per unit Volume (V)  = M/V
 = Greek letter “rho”
Typical Units: kg/m3, gm/cm3
Mass =
# molecules (mole)  molecular mass (gm/mole)
Avogadro number (6.023x1023 molecules/mole)

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

8. 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

9. (Note that pressure acts in all directions!)
Can be thought of as weight of air above you.
(Note that pressure acts in all directions!)
So as elevation increases, pressure decreases.
Top
Higher elevation
Less air above
Lower pressure
Lower elevation
More air above
Higher pressure
Bottom

10. Density and Pressure Variation
Key Points
Both decrease rapidly with height
Air is compressible, i.e. its density varies
Ahrens, Fig. 1.5

11. 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.
10 kg

12. 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
 mass
 mass
 mass

13. 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.

14. 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

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

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

17. Exponential Variation
Logarithmic Decrease
For each 5.5 km height increase, pressure drops by factor of 2.
16.5 km mb 11 km mb km mb km mb

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

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

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

21. 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

22. 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
6.5oC/km
Ahrens, Fig. 1.7

23. 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

24. 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” H2O vapor, weather of public interest

25. Summary Many gases make up air N2 and O2 account for ~99%
Trace gases: CO2, H2O, O3, etc.
Some are very important…more later
Pressure and Density
Decrease rapidly with height
Temperature
Complex vertical structure

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

27. 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

28. Responsible for boxed parameters
Surface Station Model
Responsible for boxed parameters
Temperatures
Plotted F in U.S.
Sea Level Pressure
Leading 10 or 9 is not plotted
Examples:
plotted as 138
998.7 plotted as 987
plotted as 360
Ahrens, p 431

29. Sky Cover and Weather Symbols
Ahrens, p 431
Ahrens, p 431

30. Wind Barbs Direction Wind is going towards Westerly  from the West
Speed (accumulated)
Each flag is 50 knots
Each full barb is 10 knots
Each half barb is 5 knots
65 kts from west
Ahrens, p 432

31. SLP pressure
temperature dew point
cloud cover
Ohio State website
wind

32. 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
Ahrens, p 431 72 58
111
Decimal point
What are Temp, Dew Point, SLP, Cloud Cover, Wind Speed and Direction?

33. Practice Surface Station
Sea Level Pressure
Leading 10 or 9 is not plotted
Examples:
plotted as 138
998.7 plotted as 987
plotted as 360
Ahrens, p 431 42 18
998
Decimal point
What are Temp, Dew Point, SLP, Cloud Cover, Wind Speed and Direction?

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

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

37. 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

38. 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
stratosphere
tropopause
troposphere
temperature profile
moisture profile
wind profile

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)

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 Center

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

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

47. Cool Site: Western Region Climate Center
Climate of Tucson Snow!
Cool Site: Western 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 25-42 Problems 2.1-2.4, 2.7, 2.9-2.12
(2.1  Chapter 2, Problem 1)
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