chapter 3 solar and terrestrial radiation. driving question how does energy flow into and out of the...
TRANSCRIPT
Chapter 3
Solar and Terrestrial Radiation
Driving Question
How does energy flow into and out of the Earth-Atmosphere system?
Law of Energy Conservation – Energy cannot be created nor destroyed (First
Law of Thermodynamics)
Electromagnetic Spectrum
Earth is continuously bombarded by electromagnetic radiation from the sun All objects emit electromagnetic radiation (except
at absolute zero) Types:
Radio waves IR Visible UV X and Gamma Rays
Together these form the electromagnetic spectrum
Waves
Wavelength – the distance between successive waves crests or troughs
Frequency – the number of crests or troughs that pass a given point in a given amount of time (1 second) 1 cycle/1 second = 1 Hertz
Radiation Laws
Blackbody – an object at a constant temperature that absorbs all radiation incident on it and emits all radiation at every wavelength Perfect absorber and perfect emitter Earth and Sun are NOT blackbodies, but
they are close enough that blackbody laws can be applied
Radiation Laws
Wien’s Displacement Law The wavelength of most intense radiation
is inverse to the temperature of the object λmax = C/T
– λ: wavelength (μm)– C: constant = 2897μm K– T: Temperature (K)
The sun emits short wave radiation The earth emits long wave radiation
Radiation Laws
Stefan-Boltzman Law - Law relating the temperature of a blackbody to the amount of energy emittedE = σT4
E: Energy Emission (W/m2)σ: Stefan Boltzman Constant = 5.67e-8
W/m2K4
T: Temperature (K)Average T earth = 288KAverage T sun = 6000K
Earth’s Orbit
Earth’s orbit is slightly elliptical Closest to the sun in early January (91
million miles (perihelion) Farthest from the sun in early July (94
million miles (aphelion) Earth’s axis is tilted 23 degrees 27
minutes
Solar Altitude
The angle of the sun above the horizon Greatest = 90 Lowest = 0
Solar intensity is greatest when the solar altitude is at 90 degrees
Important Latitude Lines
Equator Tropic of Cancer: line where solar altitude is
90 degrees at summer solstice (June 21) Tropic of Capricorn: line where solar altitude
is 90 degrees at winter solstice (December 21)
Arctic Circle: At winter solstice, 24 hours of darkness northward
Antarctic Circle: At summer solstice, 24 hours of darkness southward
Equinox – “Equal Night”
First day of spring/fall
Solar altitude is 90 degrees at equator
Night and day are generally equal at 12 hours
Summer Solstice
First day of summer in NH
Solar altitude is 90 degrees at the Tropic of Cancer
24 hours of darkness south of Antarctic Circle
Winter Solstice
First day of winter in NH
Solar altitude is 90 degrees at Tropic of Capricorn
24 hours of darkness north of Arctic Circle
Why is it colder in the winter if the earth is closer to the sun? Tilt and Solar Altitude Less Daylight – decrease in amount of solar energy
Why is it colder in the winter if the earth is closer to the sun? Decreased Solar Intensity in Atmosphere
Why is it colder in the winter if the earth is closer to the sun? Decreased Solar Intensity at Surface
Solar Radiation Reflection
Occurs when radiation hitting a surface is reflected
Law of reflection: angle of incidence equals the angle of reflection
Solar Radiation
Scattering A particle (gas molecule, aerosol)
disperses solar radiation in all directions Scattering is wavelength dependent
Oxygen and Nitrogen tend to scatter blue/violet light – reason why the sky is blue
Water and ice crystals scatter light equally at all wavelengths – reason why clouds are white
Solar Radiation
Absorption Process where some of the radiation on an
object is converted to heat Different from reflection and scattering:
energy conversion and not energy redirection
Absorption by atmospheric gases varies greatly by wavelength
O3 < 0.3μm H2O > 0.8μm
Solar Radiation
Albedo
The fraction of incident radiation that is reflected by a surface
Albedo = (reflected radiation/incident radiation) Recall that 30% of solar radiation was “lost to
space” Earth’s Albedo is 30% or 0.30
Dark objects have low albedos and bright objects have high albedos
Moon’s albedo is about 7% - no atmosphere to reflect the radiation
Greenhouse Effect
Global radiative equilibrium keeps the planet’s temperature in check – emission of heat to space in the form of infrared radiation balances the solar radiation’s heating.
Solar radiation and terrestrial radiation emit at different wavelengths – allows “trapping” of radiation Recall that different gases absorb radiation at
different wavelengths
Greenhouse Effect
Without the greenhouse effect the earth’s surface temperature would be about 0oF – too cold Average temperature of earth’s surface is about
59oF Most IR radiation escapes through
atmospheric windows Gases that prevent IR radiation from entering
space are greenhouse gases Water vapor, carbon dioxide, ozone, nitrous oxide,
methane
Greenhouse Warming Examples The Gulf Coast and desert Southwest
Similar solar radiation and daytime highs, but different morning lows – why?
Reason: amount of water vapor More water vapor exists near gulf coast and it
traps IR radiation leaving the surface. Drier air in the southwest does not trap radiation allowing temperatures to drop
Clouds – generally composed of water droplets Cloudy nights are warmer – trap IR radiation Cloudy days are cooler – block solar radiation
Ozone
Unstable molecule of 3 oxygen atoms that has positive and negative effects Positive: blocks harmful UV rays in the
stratosphere from reaching the surface Negative: smog at the surface
Chemical reactions (UV) in the stratosphere account for the destruction and creation of ozone
Destruction by CFC’s (banned in US in 1979)
Global Warming
Increasing CO2 concentrations have been observed
Enhances the natural greenhouse effect
Methane and Nitrous Oxide concentrations also increasing
Global Warming Possible Effects
Shifting climate zones Melting of ice sheets and glaciers leading to an
increase in sea level Positives
Longer growing season Less energy used (warmer in winter months)
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