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2: Chapter Solar Energy to Earth and the Seasons Introduction Universe has bils galaxies; incoming solar energy tht goesEarth's atmosphere=>pattern of energy input that drives Earth's phys sys's; this + Earth's tilt/rotation causes daily, annual, seasonal patterns of changing daylength/Sun angle; Sun= ultimate energy source for most life in our biosphere The Solar System, Sun, and Earth Our solar sys located on remote edge of Milky Way Galaxy- flattened, disk-shaped mass w/~400 bil stars; is way+ out fr galactic center in Milky Way's spiral Orion Arm; huge black holeSagittarius A*- sits in center Solar System Formation and Structure Our solar sys condensed fr lg, slowly rotating/collapsing nebula; gravity- mutual attracting force exerted by mass of object on othr objects- key force; as nebular cloud org'd into disk shape, early protosun grew in mass at center, drawing more matter to it; also Protoplanets- sml accumulations Starts of Sun, Solar Sys (SS) occurred 4.6+ bil yrs ago; early protoplants- planetesimals- orbited ~same distances fr Sun as modern planets; planetesimal hypothesis (dust-cloud hyp)- says how suns condense fr nebular clouds w/planetesimals forming in orbits about central mass Dimensions and Distances Speed of light= 300K km/s; light-year- great distance, 9.5 tril km, light travels in yr; measurement unit for Universe; known Universe seen fr Earth ~12 bil light-yrs in all directions; MW 100K L-yrs long; our Solar Sys ~11 hrs in diameter, as measured by L-yr speed Earth's Orbit Earth's orbit around Sun elliptical; its avg distance fr Sun 150 mil km; Earth is at perihelionclosest position to Sun- in N Hemisphere winter (Jan 3); is at aphelion (farthest fr Sun) during N Hem summer; this seasonal diff causes slight variation ini solar energy coming to earth Plane of the elliptic- plane touching all pts of Earth's orbit; earth's tilted axis stays fixed relative to this plane as earth revolves round Sun; this plane impt to Earth seasons; structure of Earth's orbit changes over long pers; diff'g distance fr sun 1 of svrl factors tht create Earth's cyclical pattern of glaciations (colder)/interglacial (warmer) pers Solar Energy: From Sun to Earth Sun captured ~99.9% of the matter fr rig solar nebula; rest 0.1% formed the planets, thr satellites, asteroids, comets, debris; thus Sun dominant object in our reg of space; only thing in Solar Sys having enough mass/interior density/pressure to sustain nuclear rxns/produce radiant energy: Sun's H atoms fuel together in fusion reactions- H nuclei form He, releasing huge qtys energy Solar Activity and Solar Wind Sun constantly emits clouds of electrically charged particles (H nuclei, free electrons)- travels slower than light- this= solar wind; extends fr Sunpast Pluto's orbit; sunspots due to magnetic storms on Sun- diameter ranges 10K50K km, bigger than Earth's; they produce flares, prominences Also coronal mass ejections, outbursts of charged material; regular cyc for sunspots- avg 11 yrs Solar Wind Effects As approaches Earth, Solar wind's charged particles 1st interact w/its mag field; magnetospheremag field surrounding Earth, created by its dynamo-like motions; deflects solar wind toward Earth's poles so only sml part enters atmosphere; interaction of solar wind/upper atmosph creates auroras: Aurora borealis (northern lights)/-australis (southern); solar wind disrupts some radio broadcasts, satellite transmissions, causes overloads on electrical sys's, may affect weather patterns; our understanding of it incr'g w/data fr satellites: SOHO, FAST, WIND, Ulysses, etc. Weather Effects Solar wind might influence weather/climate cycs; wetter pers in sme midlatitude areas occur w/evry other solar max; sunspot cycs for 260+ yrs fr 1740-1998 coincided w/pers of wetness, drought; failure in current planning worldwide is lack of attention given to these cycs Electromagnetic Spectrum of Radiant Energy Diff wavelengths of elctromag energy key solar inpt 2 life; solar radiation prt of electromagnetic spectrum; wavelength- distance btwn corresponding pts on any 2 successive waves; # of waves passing fixed pt in 1 sec= frequency; Sun's radiant energy- 8% UV, X-ray, gamma-ray w-lengths Impt phys law states all objects radiate energy in w-lengths related to thr indv surface temps: hotter object= shorter w-lengths; Sun's surface temp ~6000 C; its emission curve similar to that predicted for idealized 6000 C surface, or blackbody radiator; ideal blackbody absorbs all radiant energy.. It receives/emits all it receives; hotter object emits more energy/unit area of its surface than does similar area of cooler object; tho cooler than Sun, Earth ~acts like blackbody, but emits longer wlengths as radiation, mostly in infrared prt of spectrum; Suns radiated energy=shortwave radiation: Peaks in short visible w-lengths; Earth's= longwave radiation concentrated in infrared w-lengths Reg at top of atmosph, 480 km above Earth's surface,= thermopause- outer boundary of Earth's energy sys/useful pt at which to assess arriving solar radiation before scattered/absorbed in atmosph; Earth's distance fr Sun=>interception of only 2 billionth of Sun's total energy output- tho this still lg Solar radiation tht reaches horizontal plane at Earth= insolation (incoming solar radiation), specifically for radiation arriving at Earth's atmosph/surface Solar constant- avg insolation received at thermopause when Earth at its avg distance fr Sun; value is 1372 Watts/m2; constancy of solar constant over time impt- sml variations drastic for earth; its value reduced by thru reflection, scattering, absorption of shortwave radiation Earth's curved surface a cont'sly varying angle to incoming parallel rays of insolation; thus= uneven distribution of it, heating; max insolation at pt where insolation rays perpendicular to surface, called subsolar point; other places get < 90 angle; solar beam angles more pronounced @higher lats Thermopause above equator reg gets 2.5X more insolation than above poles; plus lower-angle solar rays near poles pass thru thicker atmosph; decrease in insolation fr equatorial regs N-ward, S-ward toward poles; tho in June, N Pole gets 500 W/m2 per day- more than equator- due to: Long daylengths at poles in summer; pattern reverses in Dec, w/S Pole getting most insolation; due to Earth's closer location to Sun @perihelion; on equator, max pers of 430 W/m2 at spring/fall equinoxes, when subsolar pt is at equator Incoming Energy at the Top of the Atmosphere Solar Constant Uneven Distribution of Insolation Global Net Radiation Net radiation- balance btwn incoming shortwave/outgoing longwave radiation- energy inputs minus outputs; latitudinal energy imbalance on globe- positive values in lower negative lats, toward poles; in middle/high lats, poleward of 36 N & S lats, net radiation neg b/c earth's climate sys loses: More energy to space than it gains fr Sun, as measured at top of atmosph; in lower atmos, these deficits offset by flows of energy fr tropical energy surpluses; lgst net radiation values, 80 w/m2, above tropical oceans near equator; mins over Antarctica; -20 W/m2 area over Sahara reg N Africa: Clear skies (good for longwave radiation losses fr earth surface), light-colored reflective surfaces reduce net radiation values at thermopause; clouds, lower atmos'ic pollution do ths too; atmos/ocean form giant heat engine, driven by diffs in energy, causing major circulations w/in lower atmos/ocean ..Incl global winds, ocean currents, weather sys's; The Seasons Seasonality Seasonality- seasonal variation of Sun's position above horizon/changing daylengths during yr; variations due to changes in Sun's altitude- angle btwn horizon/Sun; Sunrise/set- Sun at horizon, altitude 0o; directly overhead- 90 alt (zenith)- only at subsolar point, where insolation at max Sun's declination- latitude of subsolar pt; annually migrates thru 47o of lat, btwn Tropic of Cancer/of Capricorn; daylength- duration of exposure to insolation; varies during yr, depending on lat; equator always gets equal hrs of day/night; at N/S poles, range of daylength extreme... Extends fr 6-mth per of no isolation to 6-mth per of cont's 24-hr insolation Reasons for Seasons Seasons due to variations in Sun's alt above horizon, Sun's declination, daylength during yr; these in turn created by svrl phys factors tht operate in concert: Earth's revolution in orbit around Sun, daily rotation on its axis, its titled axis, unchanging orientation of its axis, its sphericity Essential ingredient- having single source of radiant energy- Sun Revolution Earth's orbital revolution about Sun- speed ~107,280 km/h; this speed+ Earth's distance fr Sun determines time req'd for 1 revolution around Sun & thus length of yr/duration of seasons; earth completes annual revolution in 365.2422 days- based on tropical year, measured equinox-equinox Rotation Earth's rotation- turning on its axis- complex motion, avgs ~24 hrs; determines daylength, creates deflection of winds/ocean currents, produces twice-daily rise/fall of ocean tides in relation to gravitational pull of Sun/Moon; counterclockwise rotation about axis- imaginary line extending thru: Planet fr geographic N Pole S Pole; whn viewed fr above equator, earth rotates E-ward- creates Sun's W-ward journey fr sunrise (E)sunset (W) as viewed on Earth, tho Sun fixed in Solar Sys; every pt on Earth takes 24 hrs for 1 rotation, but linear velocity of rotation at any pt varies w/lat This due to diffs in length of things eg equator, parallels; rotation causes cont'lly changing pattern of day/night; traveling boundary tht divides daylight/darkness=circle of illumination- b/c grt circle, tht intersects equator (grt circle)- daylength at equator always evenly divided- 12 hrs day, 12 night: Any 2 grt circles on sphere bisect each other; day defined as exactly 24 hrs/86,400 secs- mean solar time- eliminates predictable variations in rotation/revolution tht cause solar day to change slightly in length thruout yr; complexity of Earth's rotation measured exactly by GPS, etc. Earth's rotation gradually slowing, partly due to drag of lunar tidal forces Tilt of Earth's Axis Axial tilt- earth's axis tilts 23.5 fr a perpendicular to the plane of the ecliptic (plane of Earth's orbit around Sun); Earth's axial tilt changes (lessening) over complex 41K-yr cyc; it ~ranges btwn 22-24.5 fr a perpendicular to the plane of the ecliptic; present tilt 23.45 If Earth tilted on side, w/axis parallel to plane of ecliptic, we'd have max season variation wrldwide; if perpendicular, no seasonal changes- just perpetual spring/fall season w/12-hr days/nites Thruout Earth's rotation round Sun, Earth's axis keeps same alignment relative to plane of ecliptic, to Polaris/other stars; in diff moths, axis'd alwys look parallel to itself- axial parallelism Axial Parallelism Sphericity Earth's sphericity part of seasonality- produces uneven receipt of insolation fr pole-pole; 5 reasons for seasons: revolution, rotation, tilt, axial parallelism, sphericity Annual March of the Seasons During this, daylength most obvious way to sense season changes; daylength-interval btwn sunrise/ sunset; daylight extremes in Dec/June 21- solstices- specific pts in time at which Sun's declination is at its position farthest N (Tropic of Cancer)/S (Capricorn); tropicus= turn/change During yr, Earth places outside of equatorial reg experience cont's, grad'l shift in daylength & incr/ decr in Sun's alt; esp in spring, autumn; see charts, pg. 54-55; Dec 21/22, at moment of winter solstice/December solstice, circle of illumination excludes N Pole reg fr sunlight but incls S Pole reg Subsolar pt at 23.5 S lat, Tropic of Capricorn; N Hem tilted away fr these more direct sunlight raysN winter- thus lower angle for incoming solar rays/more diffuse pattern of insolation; 66.5 N lat -> 90 N (N Pole), Sun stays below horizon whole day; 66.5 N= Arctic Circle: During 24-hr per darkness, twilight, dawn give some lighting for month+ at start/end of Arctic night; following 3 mths, daylength/solar angles grad incr in N Hem; vernal equinox/March equinox occurs March 20/21- circle of illumination passes thru both poles so all Earth locations have 12-hr day/nite Summer solstice/June solstice- June 20/21; subsolar pt migrates fr equator->23.5 N lat- Tropic of Cancer; N of Arctic Circle gets 24 hrs daylight; 24-hr Antarctic darkness at this time= Midwinter's Day; Sept 22/23-autumnal equinox/Sept equinox- circle of illumination passes thru both poles: So all parts of globe have 12-hr day/night; subsolar pt returns to equator, days grow shorter to N, longer to S; N Hem, fall arrives, w/spring coming in S Hem Dawn and Twilight Dawn- per of diffused light tht occurs before sunrise; twilight- corresponding evening time after sunset; both pers, light scattered by molcs of atmos'ic gases, reflected by dust, moisture lighting up atmos; duration of both function of lat, affected by pollution, suspended particles etc At equator, where Sun's rays ~directly above horizon thruout yr, dawn/twilight only 30-45 min; times incr 1-2 hrs each at 40 lat, 2.5 hrs at 60 ; poles get 7 wks dawn/twilight Seasonal Observations In midlats of N Hem, sunrise position on horizon migrates w/day, fr SE (Dec)->NE (June); same per, pt of sunset migrates SW->NW; seasonal change noticeable across land away fr equator; recently, tho, timing of seasons changing as global climates shift in middle, high lats- spring earlier, fall later.

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