nats 101 lecture 2 tr atmospheric composition vertical structure weather & climate

NATS 101

Lecture 2 TRAtmospheric Composition

Vertical Structure Weather & Climate

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.


N2

Boiling point: 77 °K or -196°C or –320 °F

O2

Boiling point: 90 °K or -183 °C or -297 °F

N2 and O2

Balance between input (production) and output (destruction):

Input: plant/animal decaying

Sink: soil bacteria;

oceanic plankton-->nutrients

Input: plant photosynthesis

Sink: organic matter decay

chemical combination (oxidation)

breathing


Atmospheric CompositionImportant Trace Gases

Ahrens, Table 1.1, 3rd ed.

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


Sourcesvegetative decayvolcanic eruptionsanimal exhalationcombustion of fossil fuels(CH4 + 2 O2 > 2 H2O + CO2)

Sinksphotosynthesis (oxygen production)dissolves in waterphytoplankton absorption (limestone formation)

Carbon Dioxide COCarbon Dioxide CO22


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

http://earthguide.ucsd.edu/globalchange/keeling_curve/01.html

http://earthguide.ucsd.edu/globalchange/keeling_curve/01.html


H2O Vapor VariabilityPrecipitable Water (mm)

Some gases can vary spatially and daily


Aerosols

1 cm3 of air can contain as many as 200,000non-gaseous particles.

– dust– dirt (soil)– salt from ocean spray– volcanic ash– water– pollen– pollutants


Aerosols - Volcanic Ash

Fig. 1-4, p.6


Aerosols - Dust Particles

Dust Storm on Interstate 10, between Phoenix and Tucson, AZ.


Aerosols

• Provide surfaces upon which water vapor can condense.

• Provide a surface area or catalyst needed for much atmospheric chemistry.

• Aerosols can deplete stratospheric ozone. They can also cool the planet by reflecting sunlight back to space.


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

w/elevation

Higher density,Rapid decrease

w/elevation 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/(16km)

MSL

(2.8km) /(16km)

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


Atmospheric Layers Essentials

• Thermosphere-above 85 kmTemps 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


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


Climate and Weather

“Climate is what you expect.

Weather is what you get.”-Robert A. Heinlein


WeatherWeather – The state of

the atmosphere:

for a specific place

at a particular time

Weather Elements

1) Temperature

2) Pressure

3) Humidity

4) Wind

5) Visibility

6) Clouds

7) Significant Weather


Surface Station Model

Temperatures

Plotted F in U.S.

Sea Level Pressure

Leading 10 or 9 is not plotted

Examples:

1013.8 plotted as 138


1036.0 plotted as 360Ahrens, p 431

Responsible for boxed parameters


Sky Cover and Weather Symbols

Ahrens, p 431

Ahrens, p 431


Wind Barbs

Direction

Wind is going towards

WesterlyWesterly from the West

Speed (accumulated)

Each flag is 50 knots

Each full barb is 10 knots

Each half barb is 5 knotsAhrens, p 432

65 kts from west


temperature dew point

SLP pressure

wind

cloud cover

Ohio State website

http://twister.sbs.ohio-state.edu/


Practice Surface Station

Temperate (oF)Pressure (mb) Last

Three Digits (tens, ones, tenths)

Dew Point (later) Moisture

Wind Barb Direction and Speed

Cloud Cover Tenths total coverage

Ahrens, p 431

72

58

111

Decimal point

What are Temp, Dew Point, SLP, Cloud Cover, Wind Speed and Direction?


Practice Surface Station

Sea Level Pressure

Leading 10 or 9 is not plotted

Examples:



1036.0 plotted as 360Ahrens, p 431

42

18

998

Decimal point

What are Temp, Dew Point, SLP, Cloud Cover, Wind Speed and Direction?


Surface Map Symbols

• Fronts

Mark the boundary between different air masses…later

Significant weather occurs near fronts

Current US MapAhrens, p 432

http://www.atmo.arizona.edu/gifs/USPLOT.GIF


Radiosonde

Weather balloons, or radiosondes, sample atmospheric to 10 mb.

They measuretemperaturemoisturepressure

They are tracked to get windsAhrens, Fig. 1


Radiosonde Distribution

Radiosondes released at 0000 and at 1200 GMT for a global network of stations.

Large gaps in network over oceans and in less affluent nations.

Stations ~400 km apart over North America


Reading Assignment

• Ahrens

Pages 13-22

Problems 1.17, 1.18, 1.20

(1.17 Chapter 1, Question 17)

Pages 25-30

Problems 2.1-2.4

(2.1 Chapter 2, Problem 1)

Don’t Forget the 4”x6” Index Cards

nats 101 lecture 2 tr atmospheric composition vertical structure weather & climate

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