Radiation Balance
In atmosphere, radiation can
be…
 transmitted
 absorbed
 reflected
transmission
 No change
absorption
 Energy is transferred to absorber; absorber
emits energy
 Energy emitted in photons (energy bundles)
 Hydrogen atom as absorber
 Absorption: atom receives energy; electron
moves to higher energy level
 Emission: electron moves to lower energy
level; gives off energy as a photon
 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.
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
Earth’s average albedo, March
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 ”.
Albedos of various surfaces:

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
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Earth’s surface
Cumulonimbus clouds
Stratocumulus clouds
Cirrus clouds
Fresh snow
Melting snow
Sand
Grain crops
Deciduous forest
Coniferous forest
Tropical rainforest
Water bodies
0.31 (31%)
0.9 (90%)
0.6 (60%)
0.4 -0.5 (40 – 50%)
0.8 – 0.9 (80 – 90%)
0.4 – 0.6 (40 – 60%)
0.3 – 0.35 (30 – 35%)
0.18 – 0.25 (18 – 25%)
0.15 – 0.18 (15 – 18%)
0.09 – 0.15 (9 – 15%)
0.07 – 0.15 (7 – 15%)
0.06 – 0.10 (6 – 10%) increases at low sun
angles
Scattering / diffuse radiation
 A form of reflection
Types of scattering:
 Rayleigh
 Mie
 Nonselective
1. Rayleigh
 Happens when diameter of
gas is
< 1/10th diameter of
wavelength
of incoming radiation
 favors smaller wavelengths
 Scatters forward and back
Longer path through atmosphere at decreasing angle of sun; other
wavelengths have been scattered away; leaving long wavelengths (red)
Optical path at point of tangency is 20 x as long as at SSP.
2. Mie
 Caused by aerosols:
 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
 Pollution: high aerosol content
 Grey sky : aerosols scatter entire visible range
towards surface
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
Radiation Balance
 Balance maintained by earth and
atmosphere between incoming and
outgoing radiation
 Imagine shortwave solar radiation
entering the top of the atmosphere
as total we start with.
“100%”
incoming
70% is absorbed by earth/atmosphere
30% is reflected by earth/atmosphere
(albedo = 30%)
70% absorbed by:
 Ground (51%)
 Gases, dust in atmosphere (17%)
 Clouds (2%)
70%
Shortwave!
30% reflected by
 Ground (5%)
 Clouds (20%)
 Scattered by atmosphere (5%)
30%
Albedo of earth/atmosphere = 30%
shortwave
Shortwave
absorption
 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.
117 units (%) of longwave emitted from
earth surface to atmosphere
111 absorbed by atmosphere
6 transmitted to space
111
6
117
Earth’s surface:
ABSORBS SHORTWAVE, EMITS LONGWAVE
Agents in atmosphere that absorb longwave :
(clouds, water vapor, carbon dioxide, ozone, other greenhouse gases)
Their energy level is raised; emit longwave
6
111
117
amount re-emitted (160) exceeds
amount absorbed (111)
40 + 24 + 96 = 160
40
24
H2O,CO2, O3 clouds
111
96
6
117
surface
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 ?
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
Net all wave radiation
includes long and shortwave
 Atmosphere:
Shortwave:
Absorbs : how much?
Net all wave radiation
includes long and shortwave
 Atmosphere:
Shortwave:
Absorbs : 17 + 2 = 19
Net all wave radiation
includes long and shortwave
 Atmosphere:
Shortwave:
Absorbs : 17 + 2 = 19
Longwave
Loses : net of - 49
Net all-wave deficit for atmosphere is
19 – 49 = - 30
Net all-wave
 Surface:
Shortwave
Absorbs (gains): 51
Longwave
Emits (losses) : - 21
Net all-wave surplus for surface: 51 – 21 = 30
The balancing act…..
 The net deficit of the atmosphere equals
the net surplus of the earth’s surface.
But, there’s more…
 If radiation were only means of
transferring energy, our feet would
scorch and our heads would freeze.
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
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)
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.
 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
“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.
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
Groundhog Day yesterday!
Quiz 4
1. This represents
a. Incoming shortwave
b. Incoming longwave
c. albedo
Quiz 4
2. This represents
a. Outgoing shortwave
from surface
b. Albedo
c. Outgoing longwave
from surface
Quiz 4
2. These represent
a. Shortwave reflection
by the atmosphere and
clouds.
b. Radiation of longwave
by clouds and greenhouse gases to earth
and space.
Quiz 4
3. This represents
transfer of surplus
heat from surface to
atmosphere by the
combined processes
of :
and
Quiz 4
4. This represents
transfer of surplus
heat from surface to
atmosphere by:
Enhanced greenhouse effect
 enhancement of normal greenhouse
heating caused by increased
concentration of greenhouse gases
 (carbon dioxide, methane, nitrous oxides,
etc)
Differences in insolation* / radiation
caused by:
1.Latitude
2.Season
3.Atmospheric obstruction
*Insolation = intercepted solar radiation
1.Latitude:
 Low latitudes: net surplus of radiation
 High latitudes: net deficit
 Balanced by circulation of ocean and
atmosphere
2.Season:
energy per unit area diminishes with
sun angle
3.Atmospheric obstruction:
 clouds and dust
 optical path length
Optical path at point of tangency is 20 x as long as at SSP.