Transcript PowerPoint Presentation - NATS 101

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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.html
Atmospheric Composition
Permanent Gases
Ahrens, Table 1.1, 4th Ed.
• 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
Ahrens, Table 1.1, 3rd ed.
Which of these is now wrong even in the 4th edition of Ahrens?
CO2 Trend
“Keeling Curve”
Some gases vary by season and
over many years.
The CO2 trend is the cause for
concern about global warming.
CO2 increases
in northern spring,
decreases in northern fall
See http://earthguide.ucsd.edu/globalchange/keeling_curve/01.html
H2O Vapor Variability
Precipitable Water (mm)
Some gases can vary
spatially and daily
Two Important Concepts
Let’s introduce two new
concepts...
Density
Pressure
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)
Density Change
Density () changes by altering either
a) # molecules in a constant volume
b) volume occupied by the same # molecules
a
b
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.
Top
Bottom
Higher elevation
Less air above
Lower pressure
Lower elevation
More air above
Higher pressure
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
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
10 kg
10 kg
10 kg
10 kg
10 kg
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
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 p.
Thus, pressure must
decrease most rapidly
near the bottom.
p
p
p
p
A Thinning
Atmosphere
Top
Bottom
NASA photo gallery
Lower density,
Gradual drop
Higher density
Rapid decrease
Pressure Decreases
Exponentially with Height
1 mb
10 mb
48 km
32 km
100 mb 16 km
Ahrens, Fig. 1.5
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.
Top
Bottom
Air:
Lower density,
Gradual drop
Higher density
Rapid decrease
Top
Water:
Constant drop
Constant drop
Bottom
Equation for Pressure
Variation
We can Quantify Pressure Change with
Height
p (at elevation zin km)  pMSL  10 Z /(16 km)
where
z is elevation in kilometers (km)
p is pressure in millibars (mb)
at elevation z in meters (km)
pMSL is pressure (mb) at mean sea level
What is Pressure at 2.8 km?
(Summit of Mt. Lemmon)
Use Equation for Pressure Change
 Z /(16 km)
p(at elevation Zin km)  pMSL 10
set Z = 2.8 km, pMSL  1013 mb
p(2.8 km)  1013mb 10 (2.8 km) /(16 km)
p(2.8 km)  1013mb 10
0.175
p(2.8 km)  1013mb  0.668  677 mb
What is Pressure at
Tucson?
Use Equation for Pressure Change
p(at elevation Zin km)  pMSL 10 Z /(16 km)
set Z = 800 m, pMSL  1013 mb
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
Temperature (T) Profile
inversion
isothermal
6.5oC/km
Ahrens, Fig. 1.7
• 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.
Ahrens, Fig. 1.8
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
Reading Assignment
• Ahrens
Pages 13-22; Appendix A & C
Problems 1.17, 1.18, 1.20
(1.17  Chapter 1, Question 17)
Don’t Forget the 4”x6” Index Cards