NATS 101 Lecture 2 Atmospheric Composition and Vertical Structure.

NATS 101

Lecture 2Atmospheric Composition

and Vertical Structure

Lecture 2-Nats 101 2

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.


Atmospheric CompositionImportant Trace Gases

Ahrens, Table 1.1, 3rd ed.


CO2 Trend

Ahrens, Fig. 1.3, 3th Ed.

Keeler Curve from Hawaii Obs Some gases can vary by season and can vary over many years

CO2

increases in spring decreases in fall


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 molecular weight (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


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 aboveLower pressure

Lower elevation More air above Higher pressureBottom

Top


Density and Pressure VariationKey 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



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



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.



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


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


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


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• Temperature

Complex 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

NATS 101 Lecture 2 Atmospheric Composition and Vertical Structure.

Documents

density pressure slide

mass slide

hg slide

b slide

moleculesmole slide

fall slide

daily slide

pressure decreases