Transcript Lecture 2: Properties of Radiation

Lecture 2: Properties of
Radiation
Chapter 2 & 3 Petty
Properties of Radiation
• What is radiation?
• How it behaves at the most fundamental
physical level?
• What conventions are used to classify it
according to wavelength and other properties?
• How do we define the characteristics (e.g.
intensity) that appear in quantitative descriptions
of radiation and its interaction with the
atmosphere
Properties of Radiation
• The Nature of Electromagnetic Radiation
- Electric and Magnetic fields (detectable at some distance
from their source)
F1=F2=Kc*q1q2
r2
Properties of Radiation
• The Nature of Electromagnetic Radiation
- A changing magnetic field produces an electric field (this
is the phenomenon of electromagnetic induction, the
basis of operation for electrical generators, induction
motors, and transformers).
- Similarly, a changing electric field generates a magnetic
field.
- Because of this interdependence of the electric and
magnetic fields, it makes sense to consider them as a
single coherent entity—the electromagnetic field
Properties of Radiation
• The electromagnetic spectrum is a continuum of
all electromagnetic waves arranged according to
frequency and wavelength. The sun, earth, and
other bodies radiate electromagnetic energy of
varying wavelengths.
• Electromagnetic energy passes through space
at the speed of light in the form of sinusoidal
waves. The wavelength is the distance from
wavecrest to wavecrest (see the figure below)
Properties of Radiation
Electromagnetic Waves
-EM wave propagate as rays will spread the wave’s energy over a larger area
and weaken as it gets further away.
- EM waves follow principle of superposition.
- EM waves are “transverse” waves.
http://paws.kettering.edu/~drussell/Demos/superposition/superposition.html
WAVE NATURE OF LIGHT
l
Wavelength
Blue: l = 400 nm
Light is an electromagnetic wave.
Different wavelengths in the
visible spectrum are seen by the
eye as different colors.
Red: l = 700 nm
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Properties of Radiation
• Electromagnetic Waves
Basic concepts and definitions
Electromagnetic radiation:
* Energy propagated in the form of an advancing electric and
magnetic field disturbance.
* Travels in wave form at the speed of light (c).
* Wavelength ( l ) is the physical distance between adjacent
maxima or minima in the electric or magnetic field. unit:  m
* Wave number ( k ) is the number of wavelengths in a unit
distance, i.e., k  1 / l unit: 1/cm
* Frequency ( ) is the number of successive maxima or
minima passing a fixed point in a unit of time,   c / l
unit: cycle-per-second (cps) or 1/s
Frequency and wavelength
Speed of light
Frequency (Hz)
v= c
l
Wavelength
1 hertz (Hz) = one cycle per second
c = 3.0 x 108 ms-1
Weather Radar,
3GHz
wavelength??
Frequency
Frequency Decomposition
What if electromagnetic disturbance is not a
steady oscillating signal?
-
Frequency Decomposition
• Eq. (2.2): any EM fluctuation can be
thought of as a composite of a number of
different “pure” periodic fluctuation
Broadband vs. Monochromatic
Broadband vs. Monochromatic
ELECTROMAGNETIC SPECTRUM
Blue
Green
Yellow
Red
Visible
Gamma Ray
X-ray
Ultraviolet
Short Wavelength
Infrared
Radio
Microwaves
Long Wavelength
Lasers operate in the ultraviolet, visible, and infrared.
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Radio
Major Spectral Bands --Visible
Band
Relevant to remote sensing
• As a proportion of total solar irradiance:
• Total energy from 0 – 0.75μm
54% – all energy up to
infra-red
• Total energy from 0.39μm – 0.75μm
43% – visible light only
• Total energy from 0 – 4μm
99% – all “shortwave”
• Total energy from 4-infinity
1% – all “longwave”
• Total energy from 13μm-infinity
0.03% – major 15μm CO2
band and above
• Terminology:
• >0.75μm is infra-red (slightly different conventions exist about the
maximum value for visible light, but nothing substantial)
• 0-4μm is “shortwave” – a climate science convention referring to
solar radiation
• 4μm-infinity is “longwave“- a climate science convention referring to
terrestrial radiation
Answer:
Radiation properties
• Quantum description
• Wave description
Quantum Properties of Radiation
STIMULATED EMISSION
Incident Photon
Excited Atom
Incident Photon
Stimulated Photon
same wavelength
same direction
in phase
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•
When energy is absorbed by an atom, some of the electrons in that atom move into
larger, higher energy orbits. When energy is released by the atom, the electrons
move to smaller orbits. The lowest energy state is called the ground state. This is
when all the electrons are as close to the nucleus as possible. Higher energy states
are called excited states. Excited atomic states are not stable. Excited atoms tend to
release energy in the form of photons and drop to lower energy states.
•
Ordinary light is produced by spontaneous emission as excited atoms drop to lower
energy levels and release photons spontaneously. The result is light that is a mixture
of many different wavelengths, is emitted in all directions, and has random phase
relationships.
•
Laser light is produced by stimulated emission when excited atoms are struck by
photons in the laser beam. This stimulates the excited atoms to emit their photons
before they are emitted randomly by spontaneous emission. The result is that each
stimulated photon is identical to the stimulating photon. This means that all photons
produced by stimulated emission have the same wavelength, travel in the same
direction, and are in phase. Thus the stimulated emission process leads to the
unique properties of laser light.
Flux and Intensity
Flux and Intensity
Flux
Intensity
• - Spherical Polar Coordinate
Fig. 2.3
Solid angle
• The ratio of the area of
the sphere intercepted by
the cone to the square of
the radius
–    / r2
– Units: Steradians (sr)
• What is the area cut out
of a sphere by one
steradian?
• What is the solid angle
representing all directions
at a point?
HW2: A meteorological satellite circles the earth at a height h above
the earth’s surface. Let the radius of the
aeearth be
and show that the solid angle
under which the earth is seen by the satellite sensor is
  2 [1  (2ae h  h2 )1/ 2 /(ae  h)]
Solid angle and definition of
steradian
Solid angle and definition of
steradian
Chapter 3 Electromagnetic
Spectrum
Blackbody radiation
• Examine relationships between
temperature, wavelength and energy
emitted
• Blackbody: A “perfect” emitter and
absorber of radiation... does not exist
Measuring energy
• Radiant energy: Total energy emitted in all
directions (J)
• Radiant flux: Total energy radiated in all
directions per unit time (W = J/s)
• Irradiance (radiant flux density): Total energy
radiated onto (or from) a unit area in a unit
time (W m-2)
• Radiance: Irradiance within a given angle of
observation (W m-2 sr-1)
• Spectral radiance: Radiance for range in l
Radiance
Normal
to surface
Toward satellite
Solid angle, measured in steradians
(1 sphere = 4 sr = 12.57 sr)
Stefan-Boltzmann Law
M BB =  T 4
Total irradiance
Stefan-Boltzmann constant
emitted by a blackbody
(sometimes indicated as E*)
The amount of radiation emitted by a blackbody is
proportional to the fourth power of its temperature
Sun is 16 times hotter than Earth but gives off 160,000 times
as much radiation
Planck’s Function
• Blackbody doesn't emit equal amounts
of radiation at all wavelengths
• Most of the energy is radiated within a
relatively narrow band of wavelengths.
• The exact amount of energy emitted at
a particular wavelength lambda is given
by the Planck function:
Planck’s function
First radiation constant
Wavelength of radiation
c1l-5
B l (T) =
exp (c2 / lT ) -1
Absolute temperature
Second radiation constant
Irridance:
Blackbody radiative flux
for a single wavelength at temperature T (W m-2 m-1)
Total amount of radiation emitted by a blackbody is a function of
its temperature
c1 = 1.19x10-16 W m-2 sr-1
c2 = 1.44x10-2 m K
Planck curve
Wein’s Displacement Law
lmT = 2897.9 m K
Gives the wavelength of the maximum emission of a
blackbody, which is inversely proportional to its temperature
Earth @ 300K: ~10 m
Sun @ 6000K: ~0.5 m
Intensity and Wavelength of Emitted
Radiation : Earth and Sun
Solar Spectrum
window
Atmosphere
Window
Rayleigh-Jeans Approximation
Bl (T) = (c1 / c2) l -4 T
When is this valid:
1. For temperatures encountered on Earth
2. For millimeter and centimeter wavelengths
At microwave wavelengths, the amount of radiation emitted
is directly proportional to T... not T4
Bl (T)
TB =
(c1 / c2) l -4
Brightness temperature (TB) is often used for microwave and
infrared satellite data, where it is called equivalent blackbody
temperature. The brightness temperature is equal to
the actual temperature times the emissivity.
Emissivity and Kirchoff’s Law
 l l l
Actual irradiance by
a non-blackbody
at wavelength l
Emittance: Often referred to as emissivity
Emissivity is a function of the wavelength of radiation
and the viewing angle and) is the ratio of energy radiated
by the material to energy radiated by a black body at the
same temperature
l l
absorbed/
l
incident
Absorptivity (rl , reflectivity; tl , transmissivity)
Solar Constant
• The intensity of radiation from the Sun
received at the top of the atmosphere
• Changes in solar constant may result in
climatic variations
• http://www.space.com/scienceastronomy/0
71217-solar-cycle-24.html
CLOUD RADIATIVE FORCING
Clouds can either warm or cool the climate
depending on the cloud type
Cooling
•
By reflecting solar radiation back to
space
•
Particularly low clouds
•
Global average short wave cooling is - 48
W m-2
Warming
•
By acting as a greenhouse absorber and
emitter of
•
long wave radiation
•
Particularly thin cirrus clouds
•
Global average long wave warming is +
28 W m-2
Net Effect
•
Global cooling of about - 20 W m-2
•
But what will the cloud feedback be with
global