NATS 101 Lecture 5 Greenhouse Effect and Earth-Atmo Energy Balance and the Seasons.

NATS 101

Lecture 5

Greenhouse Effect and Earth-Atmo Energy Balance

and the Seasons

Review Items

• Heat Transfer• Latent Heat • Wien’s Displacement Law Ramifications

• Stefan-Boltzman Law Ramifications

max2900 mKm

Tμμλ ⎛ ⎞

⎜ ⎟⎝ ⎠

≈

8-2 -2 -4 4(W m ) (5.67 10 Wm K )E T−= ×

New Business

• Selective Absorption and Emission

• Earth-Atmo Energy Balance

Modes of Heat Transfer

Conduction Convection Radiation

Williams, p. 19

Latent Heat

Remember this thought experiment and

the incandescent light bulb thru the prism

Latent Heat Take 2

Williams, p 63

Takes energy from environment

Emits energy to environment

General Laws of Radiation

• All objects above 0 K emit radiant energy• Hotter objects radiate more energy per unit area

than colder objects, result of Stefan-Boltzman Law

• The hotter the radiating body, the shorter the wavelength of maximum radiation, result of

Wien’s Displacement Law• Objects that are good absorbers of radiation are

also good emitters…today’s lecture!

Sun’s Radiation Spectrum

Ahrens, Fig. 2.7Planck’s Law

Key concept: Radiation is spread unevenly across all wavelengths

Sun - Earth Radiation SpectraAhrens, Fig. 2.8

Planck’s Law

Key concepts: Wien’s Law and Stefan-Boltzman Law

What is Radiative Temperature of Sun if Max Emission Occurs at 0.5

m?• Apply Wien’s Displacement Law

max

max

2900

2900

29000.5

5800

mKT

mK

mKm

T K

T

T

μ

μλ

μμ

λ

≈

≈

≈

≈

How Much More Energy is Emitted by the Sun than the Earth?

• Apply Stefan-Boltzman Law

2

2

-2 -2 -4 48

8 4 4 54448

25 43

(W m ) W m K(5.67 10 )

(5.67 10 ) 5800 5800 1.6 10202905.67( 10 ) 290

4 7.0 10 1.2 10 (12,000 )4 6.4 10

2.0 1

Sun

Earth

Sun Sun

Earth Earth

Sun Sun

EarthEarth

E T

EE

A r times largerA r

A EA E

ππ

⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠

−

−

−

= ×

× = ×≈ ==×

×= ×≈ ≈×

×≈ 90 (2 )billion times more

Radiative Equilibrium

• Radiation absorbed by an object increases the energy of the object.– Increased energy causes temperature to

increase (warming).

• Radiation emitted by an object decreases the energy of the object.– Decreased energy causes temperature to

decrease (cooling).

Radiative Equilibrium (cont.)

• When the energy absorbed equals energy emitted, this is called Radiative Equilibrium.

• The corresponding temperature is the Radiative Equilibrium Temperature.

Why Selective, Discrete Absorption/Emission?

Life as we perceive it: A continuous world!

Atomic perspective: A quantum world!Gedzelman 1980, p 103

Energy States for Atoms

Electrons can orbit in only permitted states

A state corresponds to specific energy level

Only quantum jumps between states

Intervals correspond to specific wavelengths

Gedzelman 1980, p 104

Hydrogen Atom

Energy States for Molecules

Molecules can

rotate, vibrate

But only at specific energy levels or frequencies

Quantum intervals between modes correspond to specific wavelengths

Gedzelman 1980, p 105

H2O molecule H2O Bands

Selective Absorption

The Bottom LineEach molecule has a

unique distribution of quantum states!

Each molecule has a unique spectrum of absorption and emission frequencies of radiation!

H2O molecule

Williams, p 63

Absorption Visible (0.4-0.7 m) is

absorbed very little

O2 an O3 absorb UV (shorter than 0.3 m)

Infrared (5-20 m) is selectively absorbed

H2O & CO2 are strong absorbers of IR

Little absorption of IR around 10 m – atmospheric window

Visible

IR

Ahrens, Fig. 2.9

Total Atmospheric Absorption

Visible radiation (0.4-0.7 m) is not absorbed

Infrared radiation (5-20 m) is selectively absorbed, but there is an emission window at 10 m

Ahrens, Fig. 2.9

Simple Example of the Greenhouse Effect

(0% Solar absorbed, 100% IR absorbed)

1 Unit Incoming Solar

1

1/2 1/4 1/8 1/16

1/2 1/4 1/8 1/16

1 Unit Outgoing IR to Space

2 Units IR Emitted by Ground

½ emitted to space

½ emitted to ground

Take Home Point: Surface is warmer with selectively absorbing atmosphere than it would be without it.

Radiative Equilibrium

Global Solar Radiation Balance (Not all Solar Radiation SR reaches the surface)

Ahrens, Fig. 2.13

70% SR absorbed by earth-atmosphere

~50% SR absorbed by surface~50% SR absorbed by surface

30% SR reflects back to space30% SR reflects back to space

Albedo: percent of total SR reflected

~20% absorbed by atmosphere

Atmosphere Heated from Below

Ahrens, Fig. 2.11 old ed.

Solar radiation heats the ground

Air contacting ground heats by conductionAir contacting ground heats by conduction

Air above ground heats by convection and absorption of some IR from ground

Ground heats further through absorption of IR from atmosphere

Net Effect: Net Effect: Atmosphere is Heated Atmosphere is Heated

From BelowFrom Below

Global Atmo Energy BalanceAhrens, Fig. 2.14

SolarSolar

GroundGround

AtmosphereAtmosphere

Summary

• Greenhouse Effect (A Misnomer)Surface Warmer than Rad. Equil. Temp

Reason: selective absorption of air

H2O and CO2 most absorbent of IR

• Energy Balance Complex system has a delicate balance

All modes of Heat Transfer are important

NATS 101Intro to Weather and Climate

Next subject:The Seasons

Supplemental References Supplemental References for Today’s Lecturefor Today’s Lecture

Aguado, E. and J. E. Burt, 2001: Understanding Weather & Climate, 2nd

Ed. 505 pp. Prentice Hall. (ISBN 0-13-027394-5)

Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp. McGraw-Hill. (ISBN 0-697-21711-6)

Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN 0-471-02972-6)

Lutgens, F. K. and E. J. Tarbuck, 2001: The Atmosphere, An Intro-duction to the Atmosphere, 8th Ed. 484 pp. Prentice Hall. (ISBN 0-13-087957-6)

Wallace, J. M. and P. V. Hobbs, 1977: Atmospheric Science, An Introductory Survey. 467 pp. Academic Press. (ISBN 0-12-732950-1)

Reasons for Seasons

• Tilt of Earth’s Axis - Obliquity

Angle between the Equatorial Plane and the Orbital Plane

• Eccentricity of Earth’s Orbit

Elongation of Orbital Axis

Earth is 5 million km closer to sun in January than in July.

Solar radiation is 7% more intense in January than in July.

Why is July warmer than January in Northern Hemisphere?

Eccentricity of Orbit

AphelionPerihelion

Ahrens (2nd Ed.), akin to Fig. 2.15

147 million km 152 million km

Ahrens, Fig. 2.17

Solar Zenith Angle

Depends on latitude, time of day & season

Has two effects on an incoming solar beam

Surface area covered or Spreading of beam

Path length through atmosphere or Attenuation of beam

Ahrens, Fig. 2.19L

arge

Large

Area

Area

Small Small AreaArea

Short Path

Long Path

Equal Energy 23.523.5

oo

Beam Spreading

Low Zenith - Large Area, Much Spreading

High Zenith - Small Area, Little Spreading

Ahrens, Fig. 2.16

Large Zenith Angle Zero

Zenith Angle Large

Zenith Angle

Small Zenith Angle

Beam Spreading

Zenith Angle Equivalent Area 0o 1.00

10o 1.02 30o 1.15 50o 1.56 70o 2.92 80o 5.76

Horizon Infinite

Schematic Ignores Earth’s Curvature

Atmospheric Path Length

Zenith Angle Equivalent Atmospheres 0o 1.00

10o 1.02 30o 1.15 50o 1.56 70o 2.92 80o 5.70

Horizon 45.0

Schematic Ignores Earth’s Curvature

Cloud

Length of Day

Lutgens & Tarbuck, p33

Day Hours at Solstices - US Sites

Summer-WinterTucson (32o 13’ N)

14:15 - 10:03

Seattle (47o 38’ N) 16:00 - 8:25

Anchorage (61o 13’ N) 19:22 - 5:28

Fairbanks (64o 49’ N) 21:47 - 3:42

Hilo (19o 43’ N)

13:19 - 10:46 Gedzelman, p67

Arctic Circle

Path of SunHours of daylight

increase from winter to summer pole

Equator always has 12 hours of daylight

Summer pole has 24 hours of daylight

Winter pole has 24 hours of darkness

Note different Zeniths

Danielson et al., p75

Noon Zenith Angle at Solstices

Summer-WinterTucson AZ (32o 13’ N)

08o 43’ - 55o 43’Seattle WA (47o 38’ N)

24o 08’ - 71o 08’ Anchorage AK (61o 13’

N) 37o 43’ - 84o 43’ Fairbanks AK (64o 49’

N) 41o 19’ - 88o 19’ Hilo HI (19o 43’ N)

3o 47’ (north) - 43o 13’Aguado & Burt, p46

Is Longest Day the Hottest Day?

USA Today WWW Site

Consider Average Daily Temperature for Chicago IL:

Annual Energy Balance

Heat transfer done by winds and ocean currentsDifferential heating drives winds and currents

We will examine later in course

NH SH

Radiative WarmingRadiative

CoolingRadiative Cooling

Ahrens, Fig. 2.21

Summary

• Tilt (23.5o) is primary reason for seasons

Tilt changes two important factors 1. Angle at which solar rays strike the earth

2. Number of hours of daylight each day

• Warmest and Coldest Days of Year Occur after solstices, typically around a month

• Requirement for Heat Transport Done by Atmosphere-Ocean System

Assignments for Next Lectures

• Ahrens (next lecture)

Pages 42-52, 55-64

Problems:

2.15, 2.16, 2.18

3.1, 3.2, 3.5, 3.6, 3.14