Lecture 5: Solar radiation and the seasons (Ch 2)

1. Lecture 5: Solar radiation and the seasons (Ch 2) • energy transfer mechanisms • thermal radiation emission spectra of sun and earth • earth’s seasons

2. Energy Transfer Mechanisms • Radiation. Emission-flight-absorption of “photons” (energy packets). Ultimately, radiative energy transfer “drives” the atmosphere. • Conduction. Local exchange of energy by molecule-to-molecule interaction. Not very important in atmosphere except at boundaries, where mass motion of the air is impeded. • Convection. Transport due to “bulk motion” ie. “mass movement” of the air. Rate of transport is proportional to air velocity... and transport flux is parallel to velocity vector. Convective fluxes of “sensible” and “latent” heat are enormously important in the atmosphere.

3.  Electromagnetic radiation … generated by the acceleration of charges in matter at any temperature above absolute zero (thermal motion guarantees such accelerations) Fig. 2-5 Black body Object or medium that perfectly absorbs all radiation striking it (need not be visually black) and emits radiation at the maximum possible rate for a given temperature

4. Black body emission spectrum Fig. 2-7 Wien’s Law gives the wavelength at which peak emission occurs: Wien (Germany, 1864- 1928; experimentalist) Sun, T  6000 K Earth, T  300 K

5. Black body emission spectrum Fig. 2-7 Boltzmann (Austria, 1844-1906; theoretician) Stefan (Austria, 1835- 1893; experimentalist) Stefan-Boltzmann Law gives the area I under the curve, the total energy emitted per second per square metre of emitter surface: Emissivity  equals 1 for a “black body”… for most terrestrial surfaces  > 0.9

6. Interaction of radiation and matter • individual atoms and molecules of a gas are “selective” emitters and absorbers, whereas solids and liquids emit and absorb over a wide and continuous range of wavelengths (see Sec. 2-1, p38) • emission and absorption of photons by atmospheric gases is confined to just those wavelengths that cause the participating molecule (or atom) to move from and to an allowable energy state. Thus the atmosphere is not a black (or gray) body • later we’ll come back to interaction of radiation with the atmosphere A.H. Compton, Nobel prize winner (1927), interaction of light and matter (USA) emitabsorb.ppt JD Wilson, EAS U. Alberta Sep, 2005

7. Problem: Sun’s radius is about R=700,000 km and surface temperature is about T=6000 K. How much radiant energy Esun does the sun produce in one second, assuming it may be treated as a black body? Having computed Esun, and given earth is at distance r =1.5 x 108 km from the sun, can you compute the intensity S0 [W m-2] of the solar bean at the top of our atmosphere?

8. Two-band decomposition of environmental radiation commonly used in earth science… The “NIR” about half sun’s emission Solar (= shortwave) radiation band Longwave (= terrestrial = thermal infrared) radiation band

9. Solar constant (S0 ) Strength (ie. intensity) of the solar (shortwave) beam measured outside the atmosphere. Seen from planets, sun is a distant point source: so On earth, S0 =1367 W m-2 r Fig. 2-9

10. The seasons are regulated by the amount of solar radiation received per unit of ground area per day at earth’s surface. This is affected by sun-earth geometry which controls: • noon solar elevation & daylength • beam spreading (controlled by solar elevation) • beam depletion (solar pathlength through atmosphere) Fig. 2-10

11. If earth’s spin axis lay in the ecliptic plane… (it doesn’t) spin axis // to sun-earth radius: 24-hour day in S.H. spin axis  to sun-earth radius: 12-hour day everywhere Fig. 2-11

12. Fig. 2-12 spin axis 23.5o from being  to sun-earth radius spin axis  to sun-earth radius: 12-hour day everywhere

13. Configuration during summer solstice… “Subsolar point” – position on earth’s surface where solar beam meets surface at  incidence Fig. 2-13 Tropic of Cancer Tropic of Capricorn “Solar declination” – latitudinal position of the sub-solar point (negative during N.H. winter). It depends only on time of year

14. Edmonton, Jun 21: =90–53.5+23.5=60o annual migration of the sub-solar point Noontime solar elevation angle (p48):  = 90o – latitude + solar declination (taken as +ve in both hemispheres) Fig. 2-14 Edmonton, Dec 21: =90–53.5–23.5= 13o Sydney, Australia (34oS), Jun 21: =90 – 34 – ( 23.5) = 32.5o Dec 21: =90 – 34 – (-23.5) = 79.5o

15. Intensity of the incident beam proportional to sin(), i.e. to the “sine” of the solar elevation angle Solar elevation =90o , sin(90)=1, so ignoring absorption/scattering of the solar beam, intensity at surface is 1367 W m-2 Fig. 2-16 Solar elevation =30o, sin (30)=1/2, so ignoring absorption/scattering of the solar beam, intensity at surface is 684 W m-2… i.e. oblique incidence spreads the incident energy over a greater area, reducing intensity