CHAPTER 2 Solar Radiation & Earth ’ s Seasons

1. CHAPTER 2 Solar Radiation & Earth’s Seasons

2. From last time: temperature • Temperature measures the average speed of air molecules • This also means it is a measure of kinetic energy

3. Heat Transfer • Three ways for heat to be transferred: • Conduction: Heat transfer within a substance: touching a metal pan

4. Conduction Energy travels from hot to cold Metal is a good conductor, air is a poor conductor

5. Heat Transfer • Three ways for heat to be transferred: • Conduction: Heat transfer within a substance: touching a metal pan • Convection: Heat transfer by a fluid (such as water or air): Warm, less-dense air rising • In meteorology, we only call vertical motions “convection”, and we use “advection” for horizontal motions such as the wind

6. Convection Energy has been transported upward Remember: at the same pressure, warm air is less dense than cold air

7. Airflow over a mountain • As a parcel of air (think of a large balloon) is lifted up a mountain, the pressure surrounding it decreases – it must expand • The energy that goes into the expansion is lost, and the parcel cools • As it sinks, the pressure outside the parcel increases – it is compressed • As it compresses, the molecules inside move faster, leading to a higher temperature • Rising air expands and cools, sinking air compresses and warms

8. Heat Transfer • Three ways for heat to be transferred: • Conduction: Heat transfer within a substance: touching a metal pan • Convection: Heat transfer by a fluid (such as water or air): Warm, less-dense air rising • In meteorology, we only call vertical motions “convection”, and we use “advection” for horizontal motions such as the wind • Radiation: Heat transfer that does not require the substances touching or a fluid between them: energy from the sun

9. What do these have to do with the weather? • Conduction: Only important very near the ground (air is a poor conductor) • Convection: Many clouds form as a result of convection, as warm, moist air rises • Radiation: Energy from the sun warms the planet; causes daily changes in temperature, and much more

10. Radiation from the sun and earth (The scale on the left is 100,000 times greater than the scale on the right)

11. Radiation from the sun and earth • Solar radiation is often called “shortwave” radiation • Much of the solar radiation is in the visible part of the spectrum – we can see the sun, and the reflection and absorption of solar radiation allows us to see other things • Earth’s radiation is “infrared” or “longwave” radiation • Not visible to our eyes • Transfers much less energy

12. Balancing act • If the Earth is radiating energy all the time, why is it not extremely cold and always getting colder?

13. Emission and absorption • Objects with a temperature don’t just emit, they also absorb! • If something emits more than it absorbs, it will cool, if it absorbs more than it emits, it will warm

14. Kirchoff’s Law Day • Objects that are good absorbers are also generally good emitters Consider an asphalt road: • During the day the asphalt absorbs solar radiation and warms • At night the asphalt emits infrared radiation and cools relative to its surroundings Warm Asphalt Road (warms due to solar radiation) Night Cool Asphalt Road (cools by IR radiation)

15. Radiative equilibrium • Averaged over a long period of time, the amount of shortwave energy received from the sun is equal to the amount of longwave energy emitted by the earth’s surface – the planet is in radiative equilibrium –on average, the planet does not heat or cool • But this calculation gives an average temperature of 255 K (0° F) – a frozen earth! • What we actually observe, however, is an average surface temperature of 288 K (59° F) – much more livable. Why? Radiative equilibrium: incoming = outgoing

16. Radiation by latitude • Radiation surplus in the Tropics; deficit near the poles • Do the poles get colder and colder, and the tropics hotter and hotter every year? • No! Circulations in the atmosphere and ocean transfer heat from the Tropics to the poles.

17. Radiation • Radiation travels in the form of waves, which move at the speed of light in a vacuum (186,000 miles per second) • The shorter the wave, the more energy it carries! Our eyes can only see radiation between 0.4-0.7 μm 1 um = 0.001 mm

18. Putting them all together…

19. “Blackbody radiation” • Objects that absorb all radiation hitting them and emit all possible radiation • They don’t need to be black • The sun and the earth’s surface behave as blackbodies, but the atmosphere does not • The intensity of energy radiated by a blackbody increases to the fourthpower of its absolute temperature.

20. Stefan-Boltzmann Law expressed as I = σT4 where I is the intensity of radiation in watts per square meter, σis a constant (5.67 x 10-8watts per square meter) and T is the temperature of the body in kelvins.

21. Celsius Temperature = (oF- 32) / 1.8 Fahrenheit Temperature = 1.8 x oC + 32 Kelvin Temperature = oC + 273

23. Wien’s law • Determines the wavelength of peak emission • for any radiating body • (in micrometers): • max = constant (2900)/T • where maxrefers to the wavelength of energy • radiated with greatest intensity. • Wien’s law tells us that hotter objects radiate energy • at shorter wavelengthsthan do cooler bodies.

24. Incoming radiation from the sun (shortwave) Can be: • Absorbed by the atmosphere (19% of incoming radiation: atmosphere is relatively transparent to solar radiation) • Reflected back to space by clouds, aerosols, and the atmosphere (26%) • Transmitted down to the surface • This can be reflected (4%) • Or absorbed by the surface (51%)

25. Albedo • Reflectivity of a surface (such as Earth’s ground). • Average Earth albedo: 30% • Each surface has a different albedo – snow and clouds are very reflective, water and dark ground are not • More reflective = more albedo

27. Atmospheric greenhouse effect • This downward longwave radiation warms the surface • When this is accounted for, we can calculate the average temperature of 288 K • Without the greenhouse effect, Earth’s temperature would not be suitable for life!

28. Atmospheric greenhouse effect

29. Fig. 2.13, p. 50

30. Latent heat • Heat energy is required to change the phase of water – this heat is “hidden” or “latent” – we can’t measure it with a thermometer • Instead of being used to change the temperature of the substance, the heat is used to change the phase

31. Latent heat • The evaporation of water from oceans and lakes transfers heat from the surface to the atmosphere • When warm, moist air rises and clouds form, latent heat is released (condensation) – This is “moist convection”, and is another way that things are brought back into balance

32. Why do we have seasons? “If you graduated from Harvard, do you think you would know why it is warmer in summer than in winter? Educators who surveyed Harvard students on their graduation day in 1986 discovered that most of them could not correctly answer this question.” -- Harvard Gazette, 1997

33. Is it the shape of Earth’s orbit around the sun?

34. Or the sun angle? • When the sun is directly overhead, the radiation is concentrated over a smaller area • When at an angle, that same energy is spread out over a much larger area

35. Earth is tilted on its axis • …at an angle of about 23.5°

36. Tilt of Earth on its axis

37. Seasons • The tilt of Earth on its axis is the primary reason there are seasons • In December, the Southern Hemisphere is strongly tilted toward the sun; they get longer days and the sun is high in the sky • The Northern Hemisphere is tilted away from the sun; we have shorter days and winter • In June, the opposite is true • March and September are the “equinoxes”, when the solar energy is maximized at the equator

38. Northern Hemisphere Summer

39. If the sun is out for all 24 hours in Alaska, why isn’t it hotter there than in College Station where it’s only light for 14 hours?

40. Fig. 2.19, p. 56

41. Stepped Art Fig. 3-8, p. 63

42. Net radiation by month (incoming minus outgoing) http://profhorn.meteor.wisc.edu/wxwise/AckermanKnox/chap2/ERBE%20Net.html

43. Terms • Equinox, “equal night” • Day and night are the same length; sun is directly over the equator (March 20 and September 22) • Solstice, “sun stands still” • Summer solstice: June 21 – longest day of year in northern hemisphere • Winter solstice: December 21 – shortest day of year in northern hemisphere

44. Meteorological winter vs. astronomical winter • In meteorology, seasons are DJF (winter), MAM (spring), JJA (summer), SON (autumn) • The “first official day of winter” on December 21 is the astronomical definition

45. Why isn’t June the hottest month? Record high Average high Average low Record low http://www.srh.noaa.gov/hgx/?n=climate_cll

46. Seasonal variations at 40°N 15 hrs daylight 12 hrs daylight 12 hrs daylight 9 hrs daylight March June Sept. Dec. South

47. “The sun is a mass of incandescent gas…” • The core: estimated to be ~15 million degrees Celsius • The photosphere (what we see) is about 6000°C • Sunspots: cooler, dark regions • Corona: much hotter (2 million °C) • Chromosphere: cooler region between the photosphere and the corona • Solar flares and prominences: jets of gas that shoot up into the corona • Solar flares can disrupt Earth’s magnetic field, causing problems with radio and satellite communications

48. Magnetic field • Much like a bar magnet, Earth has a magnetic field

49. Solar wind • Charged particles from the sun, called the “solar wind”, distorts its shape

50. Aurora • Charged particle from the solar wind “excites” atoms or molecules in the upper atmosphere (thermosphere) • This causes the electron to jump to a higher energy level • When it returns to normal, it emits light (Solar wind) (Air molecule or other atom)