Guido Cervone EOS 753 Lecture III

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Chapter 2

• Guido CervoneEOS 753 Lecture III

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Solar Radiation and the Seasons

• Energy
• Kinetic Vs. Potential
• Transfer of Energy
• EM Spectrum
• Temperature Scale
• Solar Cycle
• Earths Seasons

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Energy and the Seasons

• What is energy
• How much energy does the Earth receive from the
  Sun? And other sources?
• How can we measure it?
• What is the Solar Cycle?
• What are the Sun Spots?
• Why do we have seasons?
• How does energy change from season to season?

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Energy

• Ability to do work
• In physics, mechanical work is the amount of
  energy transferred by a force acting through a
  distance
• According to the work-energy theorem if an
  external force acts upon an object, causing its
  kinetic energy to change from Ek1 to Ek2, then
  the mechanical work (W) is given by
• where m is the mass of the object and v is the
  object's speed.

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Energy Unit

• The SI unit of work is the joule (J), which is
  defined as the work done by a force of one newton
  acting over a distance of one meter
• The dimensionally equivalent newton-meter (Nm)
  is sometimes used instead.
• A Joule corresponds to 0.239 calories

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Power

• Power is measured in Watts (W), and is the rate
  at which energy is released, transferred, or
  received.
• 1 W 1 Joule/sec

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Sources of Earths Energy

• Almost all of the energy comes from the Sun
• Very small amount of energy come from Earths
  interior
• Infinitesimally small and negligible amounts come
  from other sources, like distant stars

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Forms of Energy

• Kinetic
• Extra energy which it possesses due to its motion
• It is defined as the work needed to accelerate a
  body of a given mass from rest to its current
  velocity
• Negative work of the same magnitude would be
  required to return the body to a state of rest
  from that velocity
• Potential
• Energy stored within a physical system
  (mechanical, chemical, etc)
• It is called potential energy because it has the
  potential to be converted into other forms of
  energy, particularly kinetic energy, and to do
  work in the process

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Energy Transfer Mechanisms

• Energy can be transferred in many ways
• For example powering an electrical car using
  batteries involves at least three forms of
  energy
• Chemical
• Electromagnetic
• Kinetic
• The potential energy stored in the batteries is
  converted into electricity, and then into kinetic
  energy through the use of a motor

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Different Temperature Scales

• Temperature of absolute zero, the ice point of
  water, and the stream point of water using
  various temperature measurement scales.

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Thermal Energy Transfer

• Conduction
• Heat conduction or thermal conduction is the
  spontaneous transfer of thermal energy through
  matter, from a region of higher temperature to a
  region of lower temperature, and acts to equalize
  temperature differences
• It is also described as heat energy transferred
  from one material to another by direct contact

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Thermal Energy Transfer

• Convection
• Convection in the most general terms refers to
  the movement of molecules within fluids or gases
• Convective heat and mass transfer take place
  through both
• Diffusion the random Brownian motion of
  individual particles
• Advection, in which matter or heat is transported
  by the larger-scale motion of currents

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Thermal Energy Transfer

• Radiation
• Radiation is the transfer of heat energy through
  empty space.
• All objects with a temperature above absolute
  zero radiate energy
• No medium is necessary for radiation to occur
• The energy from the Sun travels through the
  vacuum of space before warming the earth.
• Also, the only way that energy can leave earth is
  by being radiated to space

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EM Spectrum
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Black body

• The amount of radiation an object emits depends
  on its temperature
• Perfect emitters, called blackbodies, emits EM
  energy according to the Stefan-Bolzmann law
• I sT4

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Emitted Wavelength Vs. Temperature
Questions Why is it important that the Sun
(6000K) peaks in the visible? What can we tell
about a distant star observed through a telescope
using this graph?
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Examples of Thermal IR
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How to Convert Thermal IR Radiation to Temperature

• The Planck Radiation Law gives the rate at which
  Blackbody objects radiate thermal energy

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Relation Between Wavelength and Temperature

• The relation between peak wavelength and radiant
  body temperature is the Wien Displacement Law
• ?m T 2898
• where ?m is the wavelength at maximum radiant
  emittance and T is the absolute temperature in
  degrees Kelvin (C 273). The constant, 2898, is
  in units of µm K.
• What is the peak wavelength for the Sun (Peak
  temperature 6000k) Which part of the EM
  spectrum is this wavelength in? What color should
  it be?
• What is the peak wavelength for a lamp that glows
  at 1800 C? Which part of the EM spectrum is this
  wavelength in?

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Emitted Radiation and Temperature

• We have seen that Planks formula can be used to
  convert radiated energy into temperature
• Is this the actual temperature of the body?

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Emitted Radiation and Temperature

• A body's temperature can represent one thermal
  state but be expressed by two temperatures
• The internal temperature from the kinetic motion
  of its atoms, measured using a thermometer
• The external temperature measured by its emitted
  radiation
• The radiant flux FB (rate of flow of EM energy,
  commonly measured as Watts W - a unit of power
  1 Watt 1 Joule per second per square
  centimeter) emanating from a blackbody is related
  to its internal (kinetic) temperature Tk
  (temperature in Kelvin units) by the
  Stefan-Boltzmann Law
• FB sTk4, where s is a constant given as 5.67 x
  10-12 W x cm-2 x K-4

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Emitted Radiation and Temperature

• Strictly, the previous equation holds only for
  perfect blackbodies
• For other bodies (so-called real or
  "graybodies"), the radiant flux will always be
  less than the blackbody flux, as calculated by
• FR e sTk4
• e Emissivity and is a dimensionless number that
  expresses the ratio of the radiant flux of a real
  material FR to the radiant flux of a perfect
  blackbody FB
• FR / FB e

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Emitted Radiation and Temperature

• Values of e vary from 0 to 1 and are spectrally
  dependent

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Is the Radiant Temperature Equal to the Internal
Temperature?

• The radiant (sensed) temperature TR differs from
  a body's kinetic (internal) temperature TK
  according to the relation TR e 1/4 TK
• What does this mean? Can you give me a real
  example?

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To Remember

• For real bodies (graybodies) radiant temperatures
  are always less than kinetic temperatures
• The radiant temperature is significantly higher
  for a blackened surface (high e ) than for a
  shiny surface (lower e ), even if the two
  materials are at the same kinetic temperature

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Sunlight

• Is the total spectrum of the electromagnetic
  radiation given off by the Sun
• On Earth, sunlight is filtered through the
  atmosphere, and the solar radiation is obvious as
  daylight when the Sun is above the horizon
• When the direct radiation is not blocked by
  clouds, it is experienced as sunshine

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Solar Energy

• Direct sunlight has a luminous efficiency of
  about 93 lumens per watt of radiant flux, which
  includes infrared, visible, and ultra-violet
  light
• Bright sunlight provides luminance of
  approximately 100,000 candela per square meter at
  the Earth's surface
• Lumens measure light output at the source, while
  candelas measure the light that falls on a
  surface
• Sunlight is a key factor in the process of
  photosynthesis

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Solar Constant

• The solar constant is the amount of incoming
  solar electromagnetic radiation per unit area,
  measured on the outer surface of Earth's
  atmosphere in a plane perpendicular to the rays
• The solar constant includes all types of solar
  radiation
• It is measured by satellite to be roughly 1,366
  watts per square meter (W/m²), though this
  fluctuates by about 6.9 during a year (from
  1,412 W/m² in early January to 1,321 W/m² in
  early July) due to the earth's varying distance
  from the Sun, and typically by much less than one
  part per thousand from day to day.

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Solar Cycle

• The solar cycle, or the solar magnetic activity
  cycle, is the main source of periodic solar
  variation driving variations in space weather
• The cycle is observed by counting the frequency
  and placement of sunspots visible on the Sun

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Exact amount of Energy

• To calculate the amount of sunlight reaching the
  ground, both the elliptical orbit of the Earth
  and the attenuation by the Earth's atmosphere
  have to be taken into accoun
• The extraterrestrial solar illuminance (Eext),
  corrected for the elliptical orbit by using the
  day number of the year (dn), is
• where dn1 on January 1 dn2 on January 2 dn32
  on February 1, etc. In this formula dn-3 is used,
  because in the modern times Earth's perihelion,
  the closest approach to the Sun and therefore the
  maximum Eext, occurs around January 3 each year.
• The solar illuminance constant (Esc), is equal to
  128103 lx. The direct normal illuminance (Edn),
  corrected for the attenuating effects of the
  atmosphere is given by
• where c is the atmospheric extinction coefficient
  and m is the relative optical airmass.

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Atmospheric Effects
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Insulation at Earths Surface (July)
Average absorbed solar insolation at the Earth's
surface July 1983-1990 Color range blue - red -
white, Values 0 - 350W/m2. Global mean
158/m2, Minimum 0W/m2, Maximum 323/m2.
(Source NASA Surface Radiation Budget Project)
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Seasons

• A season is one of the major divisions of the
  year, generally based on yearly periodic changes
  in weather
• Seasons result from the yearly revolution of the
  Earth around the Sun and the tilt of the Earth's
  axis relative to the plane of revolution
• In temperate and polar regions, the seasons are
  marked by changes in the intensity of sunlight
  that reaches the Earth's surface, variations of
  which may cause animals to go into hibernation or
  to migrate, and plants to be dormant.

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http//jonesview.files.wordpress.com/2008/03/seaso
ns.jpg
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Astronomical Illustration
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Earths Tilt
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Earths Seasons From Space
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Energy and the Seasons

• The amount of solar radiation emitted is assumed
  constant (Solar Constant)
• Earths surface insulation changes as a function
  of
• Solar Angle
• Period of Daylight
• Beam Depletion

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North Star
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No South Star
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Animation

• http//www.cs.sbcc.cc.ca.us/physics/flash/Lengtho
  fDay.swf