NATS 101 Lecture 5 TR Greenhouse Effect Seasons

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

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

Absorption Visible (0.4-0.7 μm) is absorbed very little UV Visible IR 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 4

Total Atmospheric Absorption Ahrens, Fig. 2.9 Visible radiation (0.4-0.7 μm) is not absorbed Infrared radiation (5-20 μm) is selectively absorbed, but there is an emission window at 10 μm

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

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

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

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

Atmospheric Heating Net Effect: Atmosphere is Heated From Below Ahrens, Fig. 2.11 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

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

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

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

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?

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

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

Quantifying Beam Spreading Schematic Ignores Earth’s Curvature

Atmospheric Path Length Schematic Ignores Earth’s Curvature Cloud

Reflectivity of Smooth Water Schematic Ignores Earth’s Curvature 20

Length of Day Lutgens & Tarbuck, p33

Daylight at Solstices – US Cities Summer-Winter Tucson (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 Arctic Circle Gedzelman, p67 Sunrise-Sunset and Twilight Calendar

Path of Sun Hours 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

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

Noon Zenith at Solstices Summer-Winter Tucson 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? Consider Average Daily Temperature for Chicago IL: warming warming equilibruim cooling USA Today WWW Site

Astronomical (Insolation) vs. Meteorological Seasons W W C http://web.geog.arizona.edu/~comrie/nats101/wa/wa1insol.jpg

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

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 Lecture Temperature Variations Reading - Ahrens 3rd - Pg. 53-68 4th - Pg. 55-69 5th - 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