Transcript Introduction

Radiative Transfer
Dr. Craig Clements
San José State University
MET 60: Chapter: 4
(W&H) and 2 (Stull)
Energy transmission
There are three modes of energy transmission
in the atmosphere.
 Conduction: the transfer of energy in a substance by
means of molecular excitation without any net external
motion.
 Convection: the transfer of energy by mass motions
within a fluid or gas, resulting in actual transport of
energy.
 Radiation: the propagation of electromagnetic waves
through space.
Conduction
Convection
Electromagnetic radiation

 Radiation is the transfer of energy by rapid oscillations
of electromagnetic fields.
 The most important general characteristic is its
Defined as the crest-to-crest distance
wavelength (), ______________________________.
 Radiation travels through space at the speed of light
(3 x 108 m s-1) or 670,616,630 MPH.
The Spectrum of Radiation
Electromatic radiation may be viewed as an ensemble of
waves propagating at the speed of light (c*=2.998 x 108 m/s
through vacuum).
As for any wave with a known speed of propagation,
frequency ~, wavelength λ, and wave number, ν (i.e., the
number of waves per unit length in the direction of
propagation) are interdependent. Wave number is the
reciprocal of wavelength
  1/ 
~  c *  c * / 
The electromagnetic radiation in a specific direction
passing through a unit area (normal to direction
considered) is called:
monochromatic radiance (or spectral intensity or
monochromatic radiance) λ
The integral of the monochromatic intensity over some
finite range of electromagnetic spectrum is called the
intensity or radiance, I [W m-2]
2
2
1
1
I   I  d   I d
Spectrum of Monochromatic Intensity
(theoretical)
Iλ
The “Spectrum”
λ1 λ2
Wavelength, λ
Radiation
 What emits radiation?
– All objects with a temperature greater than
0°K emit some type of radiation (energy)
 Examples:
 Radiation laws:
– Warmer objects emit more intensely than cold
objects. (Stefan-Boltzmann Law)
– Warmer objects emit a higher proportion of
their energy at short wavelengths than cold
objects. (Wien’s Law)
Wien’s Law:
λ=w/T
λ = maximum wavelength (μm)
w = constant = 0.2897 (μm K)
T= temperature of the object (K)
Stefan-Boltzmann Law:
E = σT4
E = radiation emitted (W m-2)
σ = Stefan-Boltzmann constant= 5.67 x 10-8 (W m-2 K-4)
T= temperature of the object (K)
Review questions
 Considering the previous discussion
– Which object would emit more (intensity) radiation:
Earth or Sun?
Sun
– If you were examining the radiation emitted by both
the Sun and Earth, which would have a longer
wavelength?
Earth
– What wavelength radiation are you emitting right
now?
infrared
Solar Radiation (Sunlight)
 Sunlight is primarily made up of the
following:
– Visible Light (44%)
– Infrared Radiation (48%)
– Ultraviolet Radiation (7%)
Unit: 1 m =
0.000001 m
Terrestrial or Longwave Radiation
 Planets mainly emit
infrared radiation
 Radiation emitted by
planets occurs
mainly at
longer
wavelengths _____
than those
contained in solar
radiation
Solar Radiation
(“Shortwave”)
Terrestrial
Radiation
(“Longwave”)
Solar vs. Terrestrial Radiation
 The sun is much
hotter than planets;
therefore, sunlight
consists of shorter
wavelengths than
planetary radiation;
 Thus …
Energy from the Sun
 Obviously, the Sun provides the Earth with it’s energy.
The question is, how much of the Sun’s energy does
the Earth get?
 Sun’s energy is either
– Scattered (reflected away) or
– Absorbed
 Scattering happens by bouncing off
– Particles in the atmosphere
– Earth’s surface
 Absorption happens when certain gases absorb the
energy
– The reality is the only certain gases absorb certain
wavelengths.
Absorption of radiation
 Absorption of shortwave radiation by atmospheric
gas molecules is fairly weak;
– most absorption of shortwave radiation occurs
at the Earth’s surface.
 Most gases do not interact strongly with longwave
radiation, however
– Greenhouse gas molecules absorb certain
wavelengths of longwave radiation.
Absorption of
Radiation in the
Earth’s
Atmosphere
Incoming solar radiation
 Each ‘beam’ of incoming sunlight can be either:
– Reflected back to space: Albedo
 Clouds
 Atmosphere
 Surface
– Or absorbed; either by atmosphere (e.g.
clouds or ozone) or Earth’s surface.
Recap
or solar radiation comes from the sun
Shortwave
______________
and is composed of both ultraviolet and visible
radiation
Longwave, terrestrial or infrared
 __________________ radiation comes from
the Earth and is composed of infrared radiation
 Recall that everything (above a temperature of
0K) emits some type of radiation (energy) with
a particular wavelength.
Review - sensors that measure radiation
Pyranometer
 A _________________
measures solar
radiation.
Pyrgeometer
 A__________________
measures infrared
radiation (terrestrial) that comes from the Earth.