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NATS 101 - 34Lecture 2

Hurricane Dean & 2006 climate anomalies

Atmospheric Composition

Density, Pressure & Temperature

http://www.ncdc.noaa.gov/oa/climate/research/2006/ann/ann06.html

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 CompositionImportant 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 VariabilityPrecipitable 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 eithera) # molecules in a constant volumeb) volume occupied by the same # molecules

ab

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

PressureCan 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 aboveLower pressure

Lower elevation More air above Higher pressureBottom

Top

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 compressiblecompressible.

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.

massmass

massmass

massmass

massmass

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.

pp

pp

pp

pp

A Thinning Atmosphere

Bottom

Top Lower density, Gradual drop

Higher densityRapid decrease

NASA photo gallery

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

100 mb

10 mb

1 mb

16 km

32 km

48 km

Ahrens, Fig. 1.5

Water versus Air

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

Air:Lower density, Gradual drop

Higher densityRapid decrease Bottom

Top

Bottom

Top Water:Constant drop

Constant drop

Equation for Pressure Variation

We can Quantify Pressure Change with Height /(16km)

MSL

MSL

where

is elevation in kilometers (km)

is pressure in millibars (mb)

at elevation z in meters (km)

is pre

(at elevation zin km)

ssure (mb

1

) at mean sea l

0

leve

Z

z

p

p

p

p −= ×

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

Use Equation for Pressure Change/(16 km)

MSL

(2.8km) /(16 km)

0.175

MSL

(at elevation Zin km) 10

(2.8 km) 1013mb 10

(2.8 km) 1013mb

set = 2.8 km, 10

10

(2.8 km) 1013mb 0.668 677mb

13 mb

Zp p

p

p

p

Z

p

−

−

−

= ×

= ×

= ×= × =

=

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

/(16km)M

MS

S

L

L(at e

set =

levation Zin

800 m

km) 10

, 1013 mb

Z

Z p

p p −= ×=

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

Higher AtmosphereMolecular 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

• TemperatureComplex vertical structure

Reading Assignment

• Ahrens

Pages 13-22; 427-428 (Appendix C)

Problems 1.17, 1.18, 1.20

(1.17 Chapter 1, Question 17)

Don’t Forget the 4”x6” Index Cards