Chapter 2 Lecture
Understanding
Weather and
Climate
Seventh Edition
Solar Radiation
and the Seasons
Frode Stordal, University of Oslo
Redina L. Herman
Western Illinois University
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Energy
• Energy is traditionally described as “the ability to
do work.”
• About one two-billionth of the energy emitted by
the Sun is transferred to Earth as
electromagnetic radiation.
• Some electromagnetic radiation is absorbed by the
atmosphere and some by the Earth’s surface.
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Energy
• Kinds of Energy
– Energy can be classified as either kinetic or potential.
• Kinetic energy is energy in use or motion.
• Potential energy is energy in reserve or stored.
– Power is the rate at which energy is used, released,
transferred, or received.
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Energy
• Kinetic
Gas molecules have no bonds to other
molecules and move in random motion.
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Energy
• Heat Transfer Mechanisms
– Energy can be transferred from one place to another by
three processes:
• Conduction
• Convection
• Radiation
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Energy
• Conduction
– Conduction is the movement of heat through a substance
without the movement of molecules in the direction of the
heat transfer (from molecule to molecule).
• Heat moves to the handle of a warmed pot and this is
conduction.
• Heat moves into the ground by conduction.
• Conduction is most effective in solid materials.
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Energy
• Convection
– Convection is the transfer of heat by mixing of a fluid.
– Both liquids and gases can move energy by convection.
• A pot of boiling water is an example of convection.
• Convection in the atmosphere occurs when the heating of
the Earth’s surface warms the 1 mm layer of air in contact
with the surface.
• Winds are natural convection currents (forced convection).
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Energy
• Radiation
– Radiation is the transfer of energy that requires no
physical medium (can occur through empty space).
• Continually emitted by all substances
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Characteristics of Radiation
• Radiation Quantity and Quality
– Radiation quantity
• Refers to the amount of energy transferred
• Associated with wave height, or amplitude
– Radiation quality
• Relates to radiation wavelength, or the distance between the
wave crests
• Identifies the type of radiant energy
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Characteristics of Radiation
• Radiation Quantity and Quality
Electromagnetic radiation
E = electric wave
M = magnetic wave
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Characteristics of Radiation
• Intensity and Wavelengths of Emitted Radiation
– Categorized into a few individual “bands” along the
electromagnetic spectrum, visible light is a narrow band
bounded by infrared and ultraviolet.
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Characteristics of Radiation
• Intensity and Wavelengths of Emitted Radiation
– All matter radiates energy over a wide range of
electromagnetic wavelengths.
– Physical laws defining amount and wavelength of emitted
energy apply to hypothetical perfect emitters of radiation
known as blackbodies.
– The Earth and Sun are similar to blackbodies.
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Characteristics of Radiation
• Energy radiated by substances occurs over a wide
range of wavelengths.
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Characteristics of Radiation
• Intensity and Wavelengths of Emitted Radiation
– The intensity of radiation depends on the temperature
raised to the fourth power (Stefan-Boltzmann law):
I = σ T4
I
T
σ
intensity
temperature
Stefan Bolzmann’s constant
– The surface of the Sun is about 5800 K (5500°C or 9900°F)
and emits about 64 million watts per square meter.
– Most liquids and solids are graybodies, meaning they emit
some percentage of the maximum amount of radiation
possible at a given temperature.
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Characteristics of Radiation
• Intensity and Wavelengths of Emitted Radiation
– Emissivity refers to the percentage of energy radiated by a
substance relative to that of a blackbody.
– Radiation intensity is a function of both emissivity and
temperature
I = ε σ T4
ε
emissivity
– Most natural surfaces have emissivities above 0.9 (that is,
above 90 percent of blackbody).
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Characteristics of Radiation
• Intensity and Wavelengths of Emitted Radiation
– For any radiating body, the wavelength of peak emission
(in micrometers) is given by Wien’s law.
– Warmer objects radiate energy at shorter wavelengths
than do cooler bodies.
– Wavelengths less than 4 µm are considered shortwave
radiation.
– Wavelengths longer than 4 µm are considered longwave
radiation.
– Warmer bodies radiate more energy than do cooler bodies
at all wavelengths.
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Characteristics of Radiation
Wiens law
λmax = c/T
λ
C
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wavelength
constant
Characteristics of Radiation
Wiens law
λmax = c/T
λ
C
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wavelength
constant
The Solar Constant
• Intensity of electromagnetic radiation is not depleted or
reduced as it moves toward Earth.
• The intensity is reduced as a result of radiation being
distributed over a large area, not because of the
distance from the Sun.
– Radiation intensity decreases in proportion to the distance
squared.
– Calculating this inverse square law for Earth’s average
distance from the Sun yields a solar constant of 1367 W/m 2.
– Solar emission = 3.865 x 1026 W/distance surrounding the
Sun = 4 (1.5 x 1011m)2 = 1367 W/m 2.
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The Solar Constant
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The Causes of Earth’s Seasons
• Earth’s Revolution
– Earth revolves around the Sun once every 365.25 days.
– Earth revolves the Sun in an ecliptic plane annually,
known as the revolution.
– Distance from the Sun varies.
• Perihelion (Jan 3; 147 million km
• Aphelion (July 3; 152 million km
– Using the inverse square law, radiation intensity varies by
about 7 percent between perihelion and aphelion.
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The Causes of Earth’s Seasons
• Earth’s Revolution
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The Causes of Earth’s Seasons
• Earth’s Revolution
– The length of a day is defined by Earth’s rotation, which
occurs every 24 hours.
– Axis of rotation is offset 23.5° from the perpendicular plane.
– Northern axis aligns with the star Polaris.
– As Earth orbits the Sun, the hemispheres are impacted
seasonally.
– A particular hemisphere aligns toward or away from the Sun
or occupies a position between the extremes, creating our
solstices and equinoxes.
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The Causes of Earth’s Seasons
• Earth’s Revolution and Rotation
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The Causes of Earth’s Seasons
• Earth’s Revolution and Rotation
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The Causes of Earth’s Seasons
• Summer and Winter Solstices
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The Causes of Earth’s Seasons
• Equinoxes: March 21 and September 21
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Effects of Earth’s Changing Orientation
• Solar Angle
– Solar radiation is directly related to solar angle.
– Higher solar angles reduce beam spreading, which leads to
warming.
– Lower angles induce less intense warming.
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Effects of Earth’s Changing Orientation
• Solar Angle
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Effects of Earth’s Changing Orientation
Changes in Energy Receipt with Latitude and Season
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Effects of Earth’s Changing Orientation
Changes in Energy Receipt with Latitude and Season
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