NATS 101

Lecture 5 TRGreenhouse Effect

Seasons

2

Review Key Concepts• All objects above 0K emit radiation• Hotter the object, shorter the wavelength

of maximum emission: Wien’s Law

• Hotter objects radiate more energy than colder objects: Stefan-Boltzman Law

• Objects that are good absorbers of radiation are also good emitters…today’s lecture!

λmax μm⎛

⎝⎜

⎞

⎠⎟≈2900μmK⎛

⎝⎜⎞⎠⎟T−1=

2900μmKT

3

Review Key Concepts

• Radiative equilibrium and temperature Energy In = Energy Out (Eq. Temp.)

• Each molecule has a unique distribution of permitted, quantum energy states

Unique spectrum of absorption and emission frequencies of radiation

• Air is transparent to incoming solar opaque to outgoing infrared

Absorption Visible (0.4-0.7 μm) is

absorbed very little

O2 and O3 absorb UV (shorter than 0.3 μm)

Infrared (5-20 μm) is selectively absorbed

H2O & CO2 are strong absorbers of IR

Little absorption of IR around 10 μm – atmospheric window

Ahrens, Fig. 2.9

IR

VisibleUV

The Importance of the Greenhouse Effect

The presence of the gases in our atmosphere that absorb and emit infrared radiation helps maintain the Earth’s average temperature at about 59 °F.

Slide courtesy C. Castro

The Greenhouse EffectDOES NOT EQUAL

Global Warming or Climate Change!

Global warming: The increase in Earth’s mean temperature that would result because of the increase in greenhouse gases due to human activities. This would enhance the greenhouse effect.

Climate change: Long-term change in global, regional, or local climate resulting from both enhanced greenhouse gases and/or other human activities.

Slide courtesy C. Castro

Greenhouse Effect: Venus, Earth, and Mars

VENUS(Same size as Earth) EARTH

MARS(Half size of Earth)

Pressure = 93,000 mbAtmosphere composed of 96% CO2

Temperature = 482 °C

Pressure = 1,013 mbAtmosphere composed of less than 1% CO2

Temperature = 15 °C

Pressure = 8 mbAtmosphere composed of 95% CO2

Temperature = -63 °C

GREENHOUSE EFFECTON STERIODS!

GREENHOUSE EFFECTJUST RIGHT

VIRTUALLY NO ATMOSPHERE TO HAVE A GREENHOUSE EFFECT

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

Atmospheric Heating

Ahrens, Fig. 2.11 old ed.

Solar radiation heats the ground

Air next to ground heats by conduction

Air above ground heats by convection and absorption of 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

Ground heats by absorption of SR

Global Atmo Energy BalanceAhrens, Fig. 2.14

Solar

Ground

AtmosphereAtmosphere

Take Home Points

• Greenhouse Effect…a MisnomerSFC Warmer than Rad. Equil. Temp

Reason: selective absorption of atmosphere

H2O and CO2 most absorbent GHG’s of IR

• Energy Balance Complex system with a delicate balance

All modes of Heat Transfer are important

Reasons for Seasons

• Eccentricity of Earth’s Orbit

Elongation of Orbital Axis

• Tilt of Earth’s Axis - Obliquity

Angle between the Equatorial Plane and the Orbital Plane

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

Eccentricity of Orbit

AphelionPerihelion

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

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

Why is July warmer than January in Northern Hemisphere?

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

Quantifying 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

Reflectivity of Smooth Water

Zenith Angle Reflectivity 0o 2% 10o 2% 30o 2% 50o 4% 70o 13% 80o 35%

Horizon 100%

Schematic Ignores Earth’s Curvature

Length of Day

Lutgens & Tarbuck, p33

Daylight at Solstices – US Cities

Summer-WinterTucson (32o N) 14:15 -

10:03

Seattle (48o N) 16:00 - 8:25

Anchorage (61o N) 19:22 - 5:28

Fairbanks (65o N) 21:47 - 3:42

Hilo (20o N) 13:19 - 10:46

Gedzelman, p67

Arctic Circle

Sunrise-Sunset and Twilight Calendar

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 ZenithsDanielson et al., p75

Alaska: Land of the Midnight Sun

MIDNIGHTMIDNIGHTSUN LOWEST IN SKYSUN LOWEST IN SKY

DUE NORTHDUE NORTH

Noon Zenith at Solstices

Summer-WinterTucson AZ (32o N)

09o - 56o (always south) Seattle WA (48o N)

24o - 71o (always south) Anchorage AK (61o N)

38o - 85o (always south) Fairbanks AK (65o N)

41o - 88o (always south) Hilo HI (20o N)

4o (north) - 43o (south) Aguado & Burt, p46

Incoming Solar

Radiation (Insolation) at the Top

of the Atmosphere

http://web.geog.arizona.edu/~comrie/nats101/wa/wa1insol.jpg

Is Longest Day the Hottest Day?

USA Today WWW Site

Consider Average Daily Temperature for Chicago IL:

equilibruimwarmingwarming

cooling

Astronomical (Insolation)

vs.Meteorologica

l Seasons

http://web.geog.arizona.edu/~comrie/nats101/wa/wa1insol.jpg

C

C

W

W

Annual Energy Balance

Heat transfer done by winds and ocean currents

NH SH

Radiative WarmingRadiative

CoolingRadiative Cooling

Ahrens, Fig. 2.21

Differential heating drives winds and currentsWe will examine later in course

Take Home Points

• Tilt (23.5o) is primary reason for seasons

Tilt changes two important factors Angle at which solar rays strike the earth Number of hours of daylight each day

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

• Poleward Heat Transport Requirement Done by Atmosphere-Ocean System

Assignment for Next LectureTemperature Variations

• Reading - Ahrens

3rd - Pg. 53-684th - Pg. 55-695th - Pg. 55-72

• Homework02 – D2L (Due Mon. Feb 1st)

3rd - Pg. 72: 3.1, 2, 5, 6, 14

4th - Pg. 74: 3.1, 2, 5, 6, 14

5th - Pg. 75: 3.1, 2, 5, 6, 14