1. Chapter 3 Solar and Terrestrial Radiation

2. Electromagnetic Spectrum Earth bombarded by electromagnetic radiation from the sun Various forms of electromagnetic radiation classified by their wavelengths – which determine how they interact with the earth and its atmosphere…make up the electromagnetic spectrum

3. Electromagnetic radiation travels in the form of waves, and are classified in three ways: –Wavelength, distance from wave crest(trough) to crest(trough) –Frequency, number of waves passing a point in a given amount of time –Energy – higher frequency=higher energy Electromagnetic Spectrum

4. Get familiar with this!

5. Radiation Laws Q: All objects emit all forms of electromagnetic radiation, but how do we know how much radiation a given object will emit? A: Blackbody Radiation Laws Blackbody, an object that absorbs all radiation that is incident upon it, and emits all radiant energy it absorbs No perfect blackbodies, but the sun and the earth can be approximated by such a description

6. The wavelength of the most intensely emitted radiation by a blackbody is inversely proportional to the absolute temperature: Wein’s Displacement Law max = C/T C=2897, and T is in Kelvin (273.15 K = 0º C) Radiation Laws Hot object’s maximum emission wavelengths are shorter, cold object’s max emission wavelengths are longer.

7. max = C/T Sun Earth 10 μm (Infrared) 0.5 μm (Green light)

8. Law relating the temperature of a blackbody to the amount of energy emitted: Stefan- Boltzman Law E~T 4 Radiation Laws E=total energy flux across all wavelengths emitted by the blackbody. T=absolute temperature of the object

9. Angle of incoming radiation: Wherever the sun is at its maximum solar angle (90 degrees) is the spot where solar rays are most concentrated. Earth’s Motion in Space and the Seasons

10. Angle of incoming radiation, continued…

11. Earth’s Motion in Space and the Seasons Distance from source of incoming radiation: Inverse square law…

12. Earth’s Motion in Space and the Seasons Earth rotates once a day – 24 hours – day and night SIDE NOTE: There are 365. 2422 days in a solar year. That is only 11 minutes and 14 seconds less than 365. 25 days, but in 125 years the accumulated excess in the leap years amounts to more than one day. Therefore, in centesimal years not exactly divisible by 400 -- such as 1900, 1800 and 1700 -- there is no leap year. Earths orbital eccentricity – departure from circular orbit – Earth-to-sun distance varies 3.3% through the year Earth closest to the sun on January 3 (perihelion) and farthest from the sun (aphelion) on July 4 – earth receives 6.7% more radiation at perihelion than at aphelion. – thanks to the inverse square law.

13. So…the earth is closest to the sun in OUR winter…then why is it colder here during the winter? Earth’s Motion in Space and the Seasons The tilt of the Earth’s spin axis – 23 degrees 27 minutes. Because of the tilt, the spot on earth where maximum intensity of solar radiation is (sun is directly overhead), shifts continuously.

14. Sun’s direct rays at Equator Sun’s direct rays at Tropic of Cancer. Sun’s direct rays at Tropic of Capricorn Sun’s direct rays at Equator

15. Equator KY North Pole

17. Solar Radiation and the Atmosphere Solar radiation interacts with gasses and aerosols as it travels through the atmosphere… –Reflection –Scattering –Absorption

18. Reflection Occurs at the interface between two different media – like the air and a cloud Law of reflection: angle of incident radiation equals angle of reflected radiation

19. Scattering Particles disperse solar radiation in all directions Both molecules and aerosols scatter solar radiation Scattering by molecules is wavelength-dependant –In fact, the preferential scattering of blue-violet light by oxygen and nitrogen molecules is the reason the sky is blue – Q: why are clouds white? A: Water droplets in clouds scatter without a wavelength dependence…so clouds appear white. Q: What color is the moon’s daytime sky? Hint – there’s no atmosphere on the moon. A: black

20. Absorption Reflection and scattering only alter the direction in which solar radiation is traveling Absorption is actually an energy conversion process – radiation striking the surface of a particle is converted to heat energy Absorption by atmospheric gasses varies greatly by wavelength – will strongly absorb some wavelengths and others little or not at all Absorption eliminates radiation in an associated wavelength band Get into this more later…

21. Solar radiation is reflected without being converted to heat The fraction of reflected radiation in known as the albedo [albedo = (reflected radiation)/incident radiation)] Clouds are the most important reflectors of solar radiation Reflection

22. Solar Radiation and the Atmosphere

23. Solar radiation and the earth’s surface The fraction of solar radiation that does make it to the earth’s surface is either reflected or absorbed (increasing the surface’s temperature) –Common Albedos Urban area: 14-18 – daytime highs warmer during sunny days Cirrus clouds: 40-50 – keeps nighttime temperatures warmer – daytime cooler Fresh snow: 75-95 – daytime temps cooler

24. Infrared Response and the 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. Remember that “Absorption by atmospheric gasses varies greatly by wavelength – will strongly absorb some wavelengths and others little or not at all” Greenhouse gasses are very transparent to solar radiation, but absorb infrared radiation (emitted by the earth) very well. – These gasses in turn heat, and emit infrared radiation back toward the surface = Greenhouse effect

25. Principle greenhouse gasses: –Water vapor – evaporated water – example=DESERT DAILY TEMPERATURE VARIATION –Carbon dioxide – respiration, burning fossil fuels –Methane – decomposition –Nitrous oxide –Ozone… Infrared Response and the Greenhouse Effect

26. The Stratospheric Ozone Shield Ozone (O 3 ) – 3 oxygen atoms – relatively unstable Near the surface – ozone is a air pollutant, and a major contibutor photochemical smog (Chapter 2) Ozone in the stratosphere shields us from lethal intensities of Ultraviolet (UV) radiation.

27. Ozone is both created and destroyed by UV light Creation – UV strikes O 2 atoms, causing them to split, and then the two free O attoms collide with existing )two molecules to form O 3 (ozone) Destruction – ozone absorbs UV radiation, slitting the O 3 into an O 2 molecule and O atom. The free O atom then collides with an O 3 atom, forming two separate O 2 atoms. The Stratospheric Ozone Shield

28. CFCs – chlorofluorocarbons UV radiation breaks CFCs up, yielding Chlorine (Cl) gas…which reacts with and destroys ozone The Stratospheric Ozone Shield