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NATS 101 - 34Lecture 2Hurricane Dean & 2006 climate anomaliesAtmosphericCompositionDensity, Pressure & Temperature

atmospheric composition permanent gases
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, 4th Ed.

atmospheric composition important trace gases
Atmospheric CompositionImportant Trace Gases

Ahrens, Table 1.1, 3rd ed.

Which of these is now wrong even in the 4th edition of Ahrens?

co 2 trend
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


h 2 o vapor variability precipitable water mm
H2O Vapor VariabilityPrecipitable Water (mm)

Some gases can vary spatially and daily

two important concepts
Two Important Concepts

Let’s introduce two new concepts...



what is density
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 Change

Density () changes by altering either

a) # molecules in a constant volume

b) volume occupied by the same # molecules

what is pressure
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


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


density and pressure variation
Density and Pressure Variation

Key Points

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

Ahrens, Fig. 1.5

why rapid change with height

10 kg

10 kg

10 kg

10 kg

10 kg

10 kg

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 height1
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 height2
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 height3
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.





a thinning atmosphere



A Thinning Atmosphere

Lower density, Gradual drop

Higher density

Rapid decrease

NASA photo gallery

pressure decreases exponentially with height
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

water versus air
Water versus Air

Pressure variation in water acts more like bricks, close to incompressible, instead of like springs.



Lower density, Gradual drop

Higher density

Rapid decrease



Constant drop

Constant drop



equation for pressure variation
Equation for Pressure Variation

We can Quantify Pressure Change with Height

what is pressure at tucson
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

temperature t profile




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

higher atmosphere
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

  • 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
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