2. The Earth's place in the universe Location: Milky Way one of millions of galaxies…

3. THE MILKY WAY Spiral-shaped galaxy 100,000 light years across 400 billion stars Solar system on a trailing edge of the Orion Arm (Fig 2-1) Earth ~150 000 000 km from sun 8 min, 20 s from sun, 1.28 sec from moon (light) Solar system is 11 light hours across

4. Earth's Formation Planetesimal/nebula hypotheses (dust-cloud hypotheses) Basis: observations of other systems 1.Large star in Milky Way exploded 2.Nebula (cloud of dust and gas) results 3.H and He condense into Sun 4.Other elements form disk of matter around sun 5.Disk slowly accretes into clumps (planetesimals) 6.planetesimals  planetoids  planets and satellites

5. Solar Energy High pressure and temperatures in Sun's dense interior H atoms forced together; fusion of 2 H nuclei Energy is released as heat, solar wind and electromagnetic energy SUNSPOTS

6. Solar wind Stronger during sunspot activity Sunspot cycle ~11 years Solar wind does not reach Earth's surface Deflected by magnetosphere Some absorbed by atmosphere near poles (Aurora borealis and aurora australis) Image source: http://www-istp.gsfc.nasa.gov/

7. Aurora borealis

8. Radiant Energy: Electromagnetic Spectrum Wavelength - distance to same point on next wave (Fig 2-5) Frequency - number of waves in one second Solar maximum in visible wavelengths (Fig 2-6, 2-7) The hotter the object, the shorter the wavelengths Earth emits longer wavelengths (infrared)

10. Top-of-Atmosphere Energy Thermopause - outer boundary of Earth's energy system Insolation at TOA: the solar constant, 1372 W/m 2 Reduced at surface by angle of incidence, atmospheric absorption and reflection (Fig 2-8)

11. Seasonality Seasons respond to changes in solar altitude and daylength Solar declination Varies over 47  from Tropic of Capricorn to Tropic of Cancer Daylength Always 12 hours at the equator 8 hr variation at Lethbridge 24 hour variation near poles Total darkness and 24 light occur within 23.5  bands from poles (at time of solstices) For the current circle of illumination, see: http://www.jgiesen.de/sunshine/index.htm

12. Revolution determines length of year (365.24 d) How ? Axial Parallelism (Fig. 2-13) Earth axis always “points to Polaris”, but… our planet moves to opposite side of the sun Rotation determines daylength (24 hr) Lit portion of day determined by latitude and solar declination on that day December 21 and June 20 - solstices March 20 and September 22- Vernal and autumnal equinoxes (everywhere gets 12hr day)

13. Energy surpluses dominate in the tropics (incoming energy exceeds outgoing loss) In the middle-latitudes, surpluses and deficits occur seasonally, but deficits dominate (Fig. 2-9) In polar regions, there is a deficit (outgoing loss exceeds incoming energy gain) This results in a net poleward transport of the energy surplus through atmospheric and oceanic currents (more later)

14. The Atmosphere The sum of all inhalations and exhalations from the Earth over time Mixture of gases that filter sun's rays 480 km in height - exosphere beyond to 32000 km (scarce H and He atoms) Air pressure due to weight of atmosphere above (1 kg/cm 2 or 101.32 kPa) Atmosphere is most dense in the troposphere (90% of total mass)

15. Atmospheric Composition Heterosphere (above 80 km) unevenly mixed gases Homosphere (surface to 80 km) more evenly mixed (except ozone layer and near surface trace gases) Current composition reached about 500 million years ago 78% nitrogen, 21% oxygen, almost 1% argon, some CO2 (0.037%) and trace gases

16. Atmospheric Temperature Thermosphere 250 - 550 km temperature rises sharply with height due to impact of intense sun on N 2 and O 2 molecules not ‘hot’ because kinetic energy of motion not transferred to particles easily (not sensible heat) Mesosphere temperature falls with height from 0  C to -90  C at outer boundary

17. Stratosphere ranges in elevation from 18 km to 50 km ozone  temperature rises (-57  C to 0  C) Increasing temperature with altitude prevents mixing with troposphere (except for some mixing along jet streams) Troposphere altitude from surface to 8-18 km contains water vapour, clouds, weather, air pollution and life Upper limit (tropopause) is -57  C Normal lapse rate is 6.4  C per km, but environmental lapse rate varies

18. Atmospheric Function Ionosphere absorbs cosmic rays, gamma rays, X-rays and short UV wavelengths atoms are changed to positive ions auroral lights occur here Ozonosphere Contains elevated levels of ozone (O 3 ) Absorbs UV light (0.1-0.3  m) Has been stable for several hundred million years but has been destabilized