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16 Νοε 2013 (πριν από 4 χρόνια και 7 μήνες)

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MET 60: Chapter: 4

(W&H) and 2 (Stull)

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.

the propagation of electromagnetic waves
through space.

Energy transmission

Conduction

Convection

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

/
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:

(or
spectral intensity

or
)

λ

The integral of the monochromatic intensity over some
finite range of electromagnetic spectrum is called the
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)

All objects with a temperature greater than
0
°
K emit some type of radiation (energy)

Examples:

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 =
-
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

Sunlight is primarily made up of the
following:

Visible Light (44%)

Unit: 1

m =

0.000001 m

Planets mainly emit

infrared

planets occurs
mainly at
wavelengths _____

than those
contained in solar

(“Shortwave”)

Terrestrial
(“Longwave”)

longer

The sun is much
hotter than planets;
therefore, sunlight
consists of shorter
wavelengths than
;

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 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

Greenhouse gas

molecules absorb certain

Absorption of
Earth’s
Atmosphere

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

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
-

A _________________ measures solar