NATS 101 Lecture 2 Atmospheric Composition and Vertical Structure

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NATS 101Lecture 2Atmospheric Compositionand
Vertical Structure

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Atmospheric CompositionPermanent 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

Ahrens, Table 1.1, 3rd Ed.
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Atmospheric CompositionImportant Trace Gases
Ahrens, Table 1.1, 3rd ed.
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CO2 Trend Keeling Curve
Some gases can vary by season and can vary over
many years
Increases in fall Decreases in spring
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H2O Vapor VariabilityPrecipitable Water (mm)
Some gases can vary spatially and daily
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Two Important Concepts

• Lets introduce two new concepts...
• Density
• Pressure

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What is Density?

• Density (?) Mass (M) per unit Volume (V)
• ? M/V
• ? Greek letter rho
• Typical Units kg/m3, gm/cm3
• Mass
• molecules ? molecular weight (gm/mole)
• Avogadro number (6.023x1023 molecules/mole)

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Density Change

• Density (?) changes by altering either
• a) molecules in a constant volume
• b) volume occupied by the same molecules

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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

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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
Top
Bottom
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Density and Pressure Variation

• Key Points
• Both decrease rapidly with height
• Air is compressible, i.e. its density varies

Ahrens, Fig. 1.5
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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.

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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
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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.

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

1 mb
48 km
10 mb
32 km
100 mb
16 km
Ahrens, Fig. 1.5
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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

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Equation for Pressure Variation

• We can Quantify Pressure Change with Height

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What is Pressure at 2.8 km?(Summit of Mt. Lemmon)

• Use Equation for Pressure Change

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What is Pressure at Tucson?

• Use Equation for Pressure Change
• Lets get cocky
• How about Denver? Z1,600 m
• How about Mt. Everest? Z8,700 m
• You try these examples at home for practice

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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.

Ahrens, Fig. 1.7
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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
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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

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Summary

• Many gases make up air
• N2 and O2 account for 99
• Trace gases CO2, H2O, O3, etc.
• Some are very importantmore later
• Pressure and Density
• Decrease rapidly with height
• Temperature
• Complex vertical structure

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Reading Assignment

• Ahrens
• Pages 13-22 427-428 (Appendix C)
• Problems 1.17, 1.18, 1.20
• (1.17 ? Chapter 1, Question 17)
• Dont Forget the 4x6 Index Cards