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 λ max = 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 emitted E = σT 4 E: Energy Emission (W/m 2 ) σ: Stefan Boltzman Constant = 5.67e-8 W/m 2 K 4 T: Temperature (K) Average T earth = 288K Average 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 O 3 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 0 o F – too cold Average temperature of earth’s surface is about 59 o F 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 CO 2 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) /ProGlobal/ /ProGlobal/ /ChallengingGlobal/