Radiation

capricioustelephoneUrban and Civil

Nov 16, 2013 (3 years and 1 month ago)

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MET 60: Chapter: 4

(W&H) and 2 (Stull)

Radiative Transfer


Dr. Craig Clements

San Jos
é
State University


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.

Energy transmission

Conduction

Convection

Electromagnetic radiation


Radiation is the transfer of energy by rapid oscillations
of electromagnetic fields.



The most important general characteristic is its
wavelength (

), ______________________________.



Radiation travels through space at the speed of light


(3 x 10
8

m s
-
1
) or
670,616,630 MPH
.



Defined as the crest
-
to
-
crest distance

The Spectrum of Radiation



/
1

Electromatic radiation may be viewed as an ensemble of
waves propagating at the speed of light (c*=2.998 x 10
8

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


~



/
*
*
~
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
]









d
I
d
I
I




2
1
2
1
The “Spectrum”

Wavelength,
λ

I
λ


λ
2

λ
1

Spectrum of Monochromatic Intensity

(theoretical)

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:


E

=
σ
T
4

λ

=
w

/
T

λ

=
maximum wavelength (
μ
m)

w = constant = 0.2897 (
μ
m K)

T= temperature of the object (K)

Stefan
-
Boltzmann Law:

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?



If you were examining the radiation emitted by both
the Sun and Earth, which would have a longer
wavelength?



What wavelength radiation are you emitting right
now?

Sun

Earth

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
wavelengths _____


than those
contained in solar
radiation

Solar Radiation
(“Shortwave”)

Terrestrial
Radiation
(“Longwave”)

longer

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:


Clouds


Atmosphere


Surface


Or absorbed; either by atmosphere (e.g.
clouds or ozone) or Earth’s surface.



Albedo

Recap


______________ radiation comes from the sun
and is composed of both ultraviolet and visible
radiation



__________________ 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.

Shortwave or solar

Longwave, terrestrial or infrared


Review
-

sensors that measure radiation


A _________________ measures solar
radiation.



A__________________ measures infrared
radiation (terrestrial) that comes from the Earth.


Pyranometer

Pyrgeometer