GEO 203 Notes – 9/21/10 (Lecture #4)All objects with a temperature above absolute zero emit radiation.Shortwave radiation carries more energy than longwave radiation.Blackbody: An object that absorbs all radiation striking on it and emits maximum radiation possible at itsgiven temperatureThe Sun and the Earth are blackbodiesStefan-Boltzman Law: amount of energy per square meter per second emitted by a blackbody related to the 4th power of its Kelvin temperature (E = s T4)A warmer object emits much more radiation than a cooler object. As temperature increases, the radiation energy increases by a power of four. If temperature doubles, E increases by 2^4 or 16 times!Energy radiated by sun = 6.328 x 10^7 W m^-2Energy radiated by Earth = 390 W m^-2Wien’s law: Wavelength of peak radiation emitted by an object is inversely related to temp.Solar radiation: 0.5 µm (micrometer) (shortwave)Earth radiation: 10 µm (longwave)(µm = wavelength)Sun’s peak radiation: about 0.5 µm (visible light: 0.4-0.7 µm)44% of solar radiation in visible spectrumEarth radiation balance:Energy received = energy lostAbsorbed sunshine x area = thermal loss x areaIf in perfect balance with sun, our temperature would be about -18°CThe earth is in a state of radiative equilibrium when incoming radiation is balanced by outgoing radiation.Radiative equilibrium predicts surface temp. of 255K or -18°CEarth’s observed avg. surface temperature is 15°CSelective absorbers: Objects that selectively absorb and emit radiationKirchhoff’s Law: good absorbers are good emitters at a particular wavelengthAll gases in Earth’s atmosphere are selective absorbersAtmospheric gases are transparent to visible radiationWater vapor and carbon dioxide are strong absorbers of infrared radiation (greenhouse gases)Ozone and molecular oxygen are good absorbers of ultraviolet radiationWhich gas absorbs radiation at the greatest range of wavelengths?Nitrous oxideMethaneMolecular oxygen and ozone (widest range on graph—absorbs radiation throughout entire length of electromagnetic spectrum)Water vaporCarbon dioxideSolar radiation passes rather freely through earth's atmosphere, but earth's re-emitted longwave energyeither passes through a narrow window (called atmospheric window) or is absorbed by greenhouse gases and re-radiated toward Earth.Basic radiation laws: higher temperature = greater radiation emitted, shorter wavelengthsSun and Earth are blackbodiesWater vapor and carbon dioxide are important atmospheric greenhouse gases; keep Earth’s avg. surfacetemperature warmer than it would be otherwiseAnnual avg. temperature of Earth, atmosphere remains fairly constant due to equilibrium between energy absorption and lossLecture #5: Seasonal and Daily TemperaturesEarth’s orbit is an ellipse instead of circle; actual distance from Earth to sun varies by yearCloser to Sun during January than JulyEarth’s tilt: 23.5°Revolution around the sun creates seasonal solar exposure. A solstice tilt keeps a polar region with either 24hrs. of light or darkness. An equinox tilt perfectly provides 12hrs. of night and 12hrs. of day in all non-polar regions.On which date(s) do all locations on the earth receive the same number of hours of daylight?Vernal equinox and autumnal equinoxHigh sun (summer) = more heating; low sun (winter) = less heatingSolar intensity: energy per areaGoverns Earth’s seasonal changesA sunlight beam that strikes at an angle is spread across a greater surface area, and is a less intense heatsource than a beam impinging directly.When the sun is low in the sky, the radiation is less intense and the radiation is spread out over a larger area.Seasons on the earth are defined by mean temperatures.Regulated by amount of solar energy received at top of earth’s atmosphereDetermined by angle at which sunlight strikes the surface and the length of time the sun shines each day (daylight hours)Reason for seasons:Earth is inclined on its axis as it revolves around the sunNoontime summer sun is higher in sky than in winterSummer days have more daylight hours than winter daysIf tilt of Earth was changed, more/less pronounced seasonal cycle in northern hemisphere if tilt increased/decreased Polar latitudes do not have highest temperature because of insulation between surface and top of atmosphereIn polar latitudes, thicker atmosphere reduces amount of solar energy that reaches surfaceLower sun angle has larger spread, weakens intensityLarge amount of energy is used for melting snow and icePeak seasonal temperature lags after peak solar radiation because earth continues to warm until outgoing earth’s infrared radiation exceeds incoming solar radiation.Control factors of seasonal temperature cycle:Latitude: higher latitude, lower temperatureLarge bodies of water/ocean current: closer to a large water body, smaller temperature rangeAltitude: higher altitude, lower temperatureAspect ratio/elevation: south-facing slopes receive greater solar energySurface type: desert has a large temperature rangeCloud coverSeasons are driven by variations in solar radiation; variations are a function of latitudeSan Francisco and Richmond, though at same latitude, have different temperatures—SF has moderate climate due to Pacific Ocean to west (Atlantic is east of Richmond, so it doesn’t affect its temperatures as