Chapter 2
The Earth’s Global
Energy Balance
Energy Balance
What is energy balance?
 Energy Balance- equilibrium between flow
of energy or radiation reaching Earth and
flow of energy or radiation leaving Earth
 69% absorbed, 31% reflected
 Importance: controls daily and seasonal
variations in temperature, currents, and life
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Global Radiation Balance
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Absorption of shortwave (solar) radiation must
equal emission of longwave (terrestrial) radiation
Globally, some areas have surplus of solar
radiation while others have deficit  circulation
in atmosphere and oceans
Areas receive different amounts of solar radiation
at different times of year  circulation patterns
vary through year
Annually, lower latitudes (~35°N to ~35°S) tend
to have surplus, while higher latitudes (~35°N to
90°N and ~35°S to 90°S) tend to have deficit
Why does the Earth not receive all of
the Sun’s energy?
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Absorption-when molecules and particles in the
atmosphere intercept and absorb radiation at
particular wavelengths (Solar radiation)
 Gasses?
Scattering-when incoming solar radiation is turned
in a different direction, often deflected back into
space
 Received vs. Reflected
Electromagnetic Spectrum
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Electromagnetic Radiation-Wave-like form of energy radiated
by any object either as light (visible) or heat (invisible)
Emitted at different wavelengths
 Crest and Troughs
 Measured in micrometers (one-millionth of a meter)
Short wavelengths=Higher heat
Longer wavelengths=Lower heat
Wien’s Law
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Inverse relationship between object’s temp and
maximum wavelength of emitted radiation
Hotter objects  shorter maximum wavelengths
Example: Sun (~5800K) vs. Earth (~288K)
Example: The surface
temperature of Venus is
773K. At what wavelength
does Venus emit radiation?
*3.75μm (Infrared)
Stephan-Boltzmann Law
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Describes amount of energy emitted by a black body
 A black body is an object that absorbs 100% of the
radiation striking it and radiates heat at a max. rate
(blacktop)
Earth not perfect black body, but can approximate
Amount energy emitted directly related to temp
What does this mean?
 Hotter objects emit more energy
 T4= E/8.17 x 10-11
 Example: What is the average temperature of a body with
an intensity of incoming radiation of 7.29L?
 T=547K
CAVEAT-If you are calculating the Temperature of a Planet,
you must divide the solar constant by 4 before using it in the
Equation
Example
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You have crash landed on Mercury and your
instruments tell you that the solar constant
is 13.64L. What is the average temperature
of Mercury?
 1. 13.64/4=3.41
2. T4=3.41/8.17 x 10-11
 3. T=452K
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Net Radiation
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Net Radiation=Incoming – Outgoing Radiation
Rn=I (1-a) + R↓ - σT4e
Example: What would the net radiation be if
insolation were 1200 W/m2; the albedo of the
surface was 0.33; emissivity was 0.90,
temperature was 325K, and downward longwave
radiation was 313 W/m2?
n
-8
4
 R =1200(1-.33)+313-(5.67 x 10 ) x 325 x .90
 Rn=547.68 W/m2
Global Radiation Balance
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Scattering of shortwave vs. longwave radiation
Insolation-interception of solar energy
(shortwave radiation) by an exposed surface
 W/m²
Insolation
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Insolation (incoming solar radiation) is dependent
on 4 things:
 1. Angle of Incidence- the angle between the
sun’s rays and a line perpendicular to the
Earth’s surface (also called the Zenith angle)
 2. Length of daylight
 3. Transparency of atmosphere (dust, smog)
 4. Variation in Solar Irradiance- insolation
received at the outer edge of the atmosophere
Insolation
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Angle of the Sun (SA)-the angle between the Earth’s
surface and the sun’s rays
 SA + ZA = 90°
Solar Constant- the maximum potential intensity of
insolation available to the Earth at the outer edge of the
atmosphere
 Solar Constant = 1.94 Langleys
Sine Law
 Summarizes the relationship between the SA and
radiation intensity
 Ib=(Ia) x (Sin SA)
 Example: What is the maximum potential intensity of
radiation if the SA = 60°?
 1.68L
Net Radiation, Latitude, and
Energy Balance
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Net radiation: difference
between incoming and
outgoing radiation
Varies strongly by latitude,
but balance on global
scale due to heat transfer
Transfer occurs by
methods discussed in both
atmospheric and oceanic
circulation
Terrestrial Radiation
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Energy emitted by earth
Earth absorbs certain
wavelengths of solar
radiation, then is
converted to another form
and emitted at different
wavelength
Wavelength much longer
than that of solar radiation
(Wien’s Law)
World Latitude Zones
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Equatorial (10°N to 10°S): intense annual insolation
Tropical (10°N to 25°N; 10°S to 25°S): some seasonal
variation, but still high annual insolation
Subtropical (25°N to 35°N; 25°S to 35°S): more seasonal
variation; fairly high annual insolation
Mid-latitude (35°N to 55°N; 35°S to 55°S): strong
seasonal contrasts in insolation
Sub-Arctic (55°N to 60°N), Sub-Antarctic (55°S to 60°S):
very strong seasonal contrasts in insolation
Arctic (60°N to 75°N), Antarctic (60°S to 75°S): intense
seasonal contrasts in insolation
Polar (75°N to 90°N; 75°S to 90°S): greatest seasonal
contrasts in insolation... Range is ~500W/m2
Other Types of Heat Transfer
2 main ways in addition to longwave and
shortwave radiation...
 1. Sensible heat
 Sensible heat transfer
 Conduction OR convection
 2. Latent heat
 Latent heat transfer
 Occurs during change of state
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Composition of the Atmosphere
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Important to energy flow and energy balance
Different gases impact heat absorption in air
 Nitrogen, oxygen, and argon – three most
abundant; low importance to climate
 Carbon dioxide, water vapor, ozone, and
methane – low content in
atmosphere, but very
important to climate
- “Greenhouse Gasses”
Diffuse Radiation
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Diffuse Radiation-when incoming radiation is
scattered by dust and other atmospheric particles
causing radiation to bounce in various directions
 Clouds can reflect a lot of radiation
Direct Absorption
 15% incoming radiation absorbed by CO² and
water vapor
 17% absorbed by diffuse radiation
 80% reaches ground
Miscellaneous Topics
Albedo-the proportion of shortwave
radiation that is reflected and scattered by
an object’s surface
 Different surfaces and colors
 Counterradiation-LW radiation of the
atmosphere directed downward toward the
Earth’s surface
 CO² and Water vapor
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Greenhouse Effect
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Greenhouse Effect- the ability of the Earth’s atmosphere to
admit most of the insolation and prevent its re-radiation
from the Earth
 Although the natural “greenhouse effect” enables the
Earth to remain livable…increased pollutants have
caused an adverse reaction creating an “enhanced
effect”
 The “enhanced effect” is when human activities
increase the ability for the Earth’s atmosphere to absorb
radiation causing a net warming of the atmosphere over
time
Greenhouse Gasses?
 Water Vapor, Ozone, Carbon Dioxide, and Methane