1. Radiation Balance

2. In atmosphere, radiation can be…
transmitted
absorbed
reflected

3. transmission
No change

4. absorption Energy is transferred to absorber; absorber emits energy
Energy emitted in photons (energy bundles)

5. Hydrogen atom as absorber

6. Absorption: atom receives energy; electron moves to higher energy level

7. Emission: electron moves to lower energy level; gives off energy as a photon

8. Remember... Energy levels emit photons of different wavelength
Atmospheric gases selectively absorb and emit only at certain wavelengths.
Atmosphere does not absorb ALL of the incoming solar radiation.
Remember...

9. reflection Energy re-directed; not absorbed
Our eyes detect reflected visible wavelengths.
Albedo is the reflective quality of a surface
Percent of incoming radiation reflected

10. Earth’s average albedo, March

11. Albedo is an important variable in global climate change
“A drop of as little as 0.01 in Earth’s albedo would have a major warming influence on climate—roughly equal to the effect of doubling the amount of carbon dioxide in the atmosphere, which would cause Earth to retain an additional 3.4 Wm-2 ”.

12. Albedos of various surfaces:
Earth’s surface (31%)
Cumulonimbus clouds 0.9 (90%)
Stratocumulus clouds 0.6 (60%)
Cirrus clouds (40 – 50%)
Fresh snow – 0.9 (80 – 90%)
Melting snow – 0.6 (40 – 60%)
Sand – 0.35 (30 – 35%)
Grain crops – 0.25 (18 – 25%)
Deciduous forest – 0.18 (15 – 18%)
Coniferous forest – 0.15 (9 – 15%)
Tropical rainforest – 0.15 (7 – 15%)
Water bodies – (6 – 10%) increases at low sun angles

13. Scattering / diffuse radiation
A form of reflection

14. Types of scattering:
Rayleigh
Mie
Nonselective

15. 1. Rayleigh Happens when diameter of gas is
< 1/10th diameter of wavelength
of incoming radiation
favors smaller wavelengths
Scatters forward and back

18. Longer path through atmosphere at decreasing angle of sun; other wavelengths have been scattered away; leaving long wavelengths (red)

19. Optical path at point of tangency is 20 x as long as at SSP.

20. 2. Mie Caused by aerosols: Scatter forward
particles in atmosphere
microscopic but larger than gas molecules (pollen, dust, smoke, small water droplets )
Scatter forward
Do not favor short wavelengths; scatter all visible wavelengths

21. Pollution: high aerosol content
Grey sky : aerosols scatter entire visible range towards surface

24. 3. nonselective
No wavelength preference; particles much larger than wavelength
Big water droplets; large dust particles
E.g., fog and clouds reflect all wavelengths of light, appear white or grey

27. Radiation Balance
Balance maintained by earth and atmosphere between incoming and outgoing radiation
Mention that this is averaged over entire globe over entire year.

28. Imagine shortwave solar radiation entering the top of the atmosphere
as total we start with.
70% is absorbed by earth/atmosphere
30% is reflected by earth/atmosphere
(albedo = 30%)
“100%”
incoming

29. 70% absorbed by: Ground (51%) Gases, dust in atmosphere (17%)
Clouds (2%)
70%
Shortwave!

30. 30% reflected by Ground (5%) Clouds (20%) Scattered by atmosphere (5%)
Albedo of earth/atmosphere = 30%
shortwave

31. Shortwave
absorption
Point out 51 shortwave absorbed by earth; 19 by atmosphere

32. Because energy is transferred between atmosphere and earth.
Consider absorption:
Earth absorbs far more solar radiation than atmosphere.
Why aren’t we boiling up?
Because energy is transferred between atmosphere and earth.
Shortwave solar radiation is absorbed and longwave radiation is emitted.

34. 117 units (%) of longwave emitted from earth surface to atmosphere
111 absorbed by atmosphere
6 transmitted to space
117
Earth’s surface:
ABSORBS SHORTWAVE, EMITS LONGWAVE

35. Point out 51 shortwave absorbed by earth; 19 by atmosphere

36. Agents in atmosphere that absorb longwave :
(clouds, water vapor, carbon dioxide, ozone, other greenhouse gases)
Their energy level is raised; emit longwave
111
117 6

38. amount re-emitted (160) exceeds amount absorbed (111)
Notice!
amount re-emitted (160) exceeds amount absorbed (111)
H2O,CO2, O3 clouds 96
117
= 160
atmosphere
surface

40. Difference between amount emitted and amount absorbed
Net radiation
Difference between amount emitted and amount absorbed
For atmosphere:
net longwave radiation = 111 – 160 = - 49
For earth’s surface
What is net longwave radiation ?

42. Difference between amount emitted and amount absorbed
Net radiation
Difference between amount emitted and amount absorbed
For atmosphere:
net longwave radiation = 111 – 160 = - 49
For earth’s surface:
net longwave radiation = 96 – 117 = - 21

43. Net all wave radiation includes long and shortwave
Atmosphere:
Shortwave:
Absorbs : how much?

44. Point out 51 shortwave absorbed by earth; 19 by atmosphere

45. Net all wave radiation includes long and shortwave
Atmosphere:
Shortwave:
Absorbs : = 19

46. Net all wave radiation includes long and shortwave
Atmosphere:
Shortwave:
Absorbs : = 19
Longwave
Loses : net of - 49
Net all-wave deficit for atmosphere is
19 – 49 = - 30

47. Surface: Net all-wave Shortwave Absorbs (gains): 51 Longwave
Emits (losses) :
Net all-wave surplus for surface: 51 – 21 = 30

48. Point out 51 shortwave absorbed by earth; 19 by atmosphere

49. The balancing act…..
The net deficit of the atmosphere equals the net surplus of the earth’s surface.

50. But, there’s more…
If radiation were only means of transferring energy, our feet would scorch and our heads would freeze.

51. Energy transfer mechanisms (other than radiation):
Conduction: transfer of heat from one molecule to another by collision
Only a few cm of air above surface are heated by conduction
Convection : transfer of heat from one area to another by physical mixing
Warm air near surface transfers heat upward by mixing

52. Looking more closely at conduction and convection…
Temperature gradient in upper few centimeters of soil
Energy conducted downward during day; upward at night
Temperature gradient in laminar boundary layer of air (few mm. thick)

53. Sensible Heat: can be sensed with thermometer.
Types of heat energy:
Sensible Heat: can be sensed with thermometer.
Latent Heat : heat released or absorbed during phase changes (solid-liquid-gas)
Energy used to change phase is not lost
energy evaporating water is “held” in water vapor to be released in reverse process
Some of energy received at surface is used to evaporate water rather than to raise surface temp. even more.

54. Surplus of 30 units of net all wave at surface
7 are transferred to atmosphere by sensible heat transfer of conduction and convection
Sensible heat travels by conduction through laminar boundary layer and is dispersed upward by convection
23 are transferred to the atmosphere as latent heat (evaporation)
Evaporation of water makes energy available to atmosphere that otherwise would have warmed surface

55. Point out 51 shortwave absorbed by earth; 19 by atmosphere

56. “Natural” Greenhouse effect
Maintains earth’s mean surface temperature at 59°F (15°C)
Otherwise -4°F (-20°C)
Caused by counterradiation:
greenhouse gases in atmosphere absorb longwave emitted from earth’s surface, some of which is radiated back to earth.

57. Greenhouse analogy
Glass of greenhouse allows shortwave radiation IN, but does not allow escape of longwave
BUT…UNLIKE the atmosphere, a greenhouse prevents loss of heat by convection

58. Groundhog Day yesterday!

59. Quiz 4
1. This represents
Incoming shortwave
Incoming longwave
albedo

60. Quiz 4 2. This represents Outgoing shortwave from surface Albedo
c. Outgoing longwave

61. Quiz 4 2. These represent Shortwave reflection by the atmosphere and
clouds.
b. Radiation of longwave
by clouds and green-
house gases to earth
and space.

62. Quiz 4 3. This represents transfer of surplus heat from surface to
atmosphere by the
combined processes
of :
and

63. Quiz 4 4. This represents transfer of surplus heat from surface to
atmosphere by:

64. Review: How did we get a net all wave surplus of 30 for surface and deficit of 30 for atmosphere?
Look at other balances

65. Enhanced greenhouse effect
enhancement of normal greenhouse heating caused by increased concentration of greenhouse gases
(carbon dioxide, methane, nitrous oxides, etc)

66. Differences in insolation* / radiation caused by:
1.Latitude
2.Season
3.Atmospheric obstruction
*Insolation = intercepted solar radiation

67. 1.Latitude: Low latitudes: net surplus of radiation
High latitudes: net deficit
Balanced by circulation of ocean and atmosphere

68. 2.Season:
energy per unit area diminishes with sun angle

71. 3.Atmospheric obstruction:
clouds and dust
optical path length

72. Optical path at point of tangency is 20 x as long as at SSP.