1. NATS 101 Lecture 5 TR Greenhouse 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! 2

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 3 3

4. Absorption Visible (0.4-0.7 μm) is absorbed very littleUV
Visible IR
Visible ( 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 4

5. Total Atmospheric Absorption
Ahrens, Fig. 2.9
Visible radiation ( μm) is not absorbed
Infrared radiation (5-20 μm) is selectively absorbed, but there is an emission window at 10 μm

6. Slide courtesy C. Castro
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 6

7. Global Warming or Climate Change!
The Greenhouse Effect
DOES 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 7

8. VIRTUALLY NO ATMOSPHERE TO HAVE A GREENHOUSE EFFECT
Greenhouse Effect: Venus, Earth, and Mars
VENUS
(Same size as Earth)
EARTH
MARS
(Half size of Earth)
Pressure = 93,000 mb
Atmosphere composed
of 96% CO2
Temperature = 482 °C
Pressure = 1,013 mb
of less than 1% CO2
Temperature = 15 °C
Pressure = 8 mb
of 95% CO2
Temperature = -63 °C
GREENHOUSE EFFECT
ON STERIODS!
JUST RIGHT
VIRTUALLY NO ATMOSPHERE TO HAVE A GREENHOUSE EFFECT 8

9. Global Solar Radiation Balance (Not all Solar Radiation SR reaches the surface)
30% SR reflects back to space
Albedo: percent of total SR reflected
~20% absorbed by atmosphere
70% SR absorbed by earth-atmosphere
Ahrens, Fig. 2.13
~50% SR absorbed by surface

10. Atmospheric Heating Net Effect: Atmosphere is Heated From Below
Ahrens, Fig old ed.
Air above ground heats by convection and absorption of IR from ground
Net Effect: Atmosphere is Heated From Below
Air next to ground heats by conduction
Ground heats by absorption of SR
Ground heats further through absorption of IR from atmosphere
Solar radiation heats the ground

11. Global Atmo Energy Balance
Ahrens, Fig. 2.14
Solar
Atmosphere
Ground

12. Take Home Points Greenhouse Effect…a Misnomer Energy Balance
SFC 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

13. 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

14. Eccentricity of Orbit
Perihelion
Aphelion
Ahrens (2nd Ed.), akin to Fig. 2.15
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?

15. 147 million km
152 million km
Ahrens, Fig. 2.17

16. 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
Long Path
Large Area
Equal Energy
23.5o
Short Path
Small Area
Ahrens, Fig. 2.19

17. Beam Spreading High Sun – Power Spread over Smaller Area
Ahrens, Fig. 2.16
Large Zenith Angle
Zero Zenith Angle
Small Zenith Angle
High Sun – Power Spread over Smaller Area
Low Sun – Power Spread over Larger Area

18. Quantifying Beam Spreading
Schematic Ignores Earth’s Curvature

19. Atmospheric Path Length
Schematic Ignores Earth’s Curvature
Cloud

20. Reflectivity of Smooth Water
Schematic Ignores Earth’s Curvature 20

21. Length of Day
Lutgens & Tarbuck, p33

22. Daylight at Solstices – US Cities
Summer-Winter
Tucson (32o N) : :03
Seattle (48o N) :00 - 8:25
Anchorage (61o N) :22 - 5:28
Fairbanks (65o N) :47 - 3:42
Hilo (20o N) : :46
Arctic Circle
Gedzelman, p67
Sunrise-Sunset and Twilight Calendar

23. Path of Sun Hours of daylight increase from winter to summer pole
Equator always has hours of daylight
Summer pole has hours of daylight
Winter pole has hours of darkness
Note different Zeniths
Danielson et al., p75

24. Alaska: Land of the Midnight Sun
SUN LOWEST IN SKY
DUE NORTH 24

25. Noon Zenith at Solstices
Summer-Winter
Tucson AZ (32o N) o - 56o (always south)
Seattle WA (48o N) o - 71o (always south)
Anchorage AK (61o N) 38o - 85o (always south)
Fairbanks AK (65o N) 41o - 88o (always south)
Hilo HI (20o N) o (north) - 43o (south)
Aguado & Burt, p46

26. Incoming Solar Radiation (Insolation) at the Top of the Atmosphere

27. Is Longest Day the Hottest Day?
Consider Average Daily Temperature for Chicago IL:
warming
warming
equilibruim
cooling
USA Today WWW Site

28. Astronomical (Insolation) vs. Meteorological SeasonsW W C

29. Annual Energy Balance Heat transfer done by winds and ocean currents
Radiative Warming
Radiative Cooling
Radiative Cooling NH SH
Ahrens, Fig. 2.21
Heat transfer done by winds and ocean currents
Differential heating drives winds and currents
We will examine later in course

30. 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

31. Assignment for Next Lecture Temperature Variations
Reading - Ahrens
3rd - Pg th - Pg th - Pg
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