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Wolfgang’s
Workshop of Atmospheric Physics
Radiation
in the Atmosphere
Everything
emits radiation. Any solid or liquid body not at absolute zero transmits energy
to its surroundings by radiation. The warmer an object is, the more radiation
energy it emits, and the shorter is the wavelength that characterizes that
radiation. The farther a receiver is from the radiation source, the less
radiation is received. If we increase the separation from a distance r1
to a distance r2, the
radiation intensity reaching this body will be reduced by the factor (r1/r2)2.
The dependence is known as the ‘inverse square law’ for radiation.
The
radiation intensity depends also on the angle at which the radiation strikes the
surface. If the radiation arrives at right angles to the surface, the radiation
energy received by a given area is a maximum. The more oblique the incident
radiation, the smaller the amount of energy received. For this reason, the
ground is warmed very little by sunlight near sunrise and sunset and is heated
most strongly at local noon. Also, winter is colder than summer because the sun
is generally low in winter sky on the northern hemisphere.
Radiation
can generally be defined as a transfer of energy by the rapid oscillations of
electromagnetic fields in space. These oscillations can be considered as
travelling waves, with a characteristic wavelength, frequency and speed. All
electromagnetic waves are basicelly the same phenomenon, they are travelling
with the speed of light and require no intervening medium, that means, they are
capable of transmitting energy through a vacuum. The basic difference between
all forms of radiation is the wavelength. The oscillatory force field is capable
of causing charged electrons to move, thereby transferring energy. When the
electrical charges in the molecules of the earth, the ground, or our skin are
subjected to this oscillating force, they begin to vibrate faster, increasing
their kinetic energy or energy of motion. We feel this increase in kinetic
energy as a rise in temperature.
A
rise in temperature causes an increase in molecular exitation within the
material, which favours the acceleration of particular electrical charge
carriers and, hence, the generation of radiation. The wavelength of emitted
radiation varies inversely with the transition energy. It is important to note
that, in the infrared region, the wavelengths are long and the radiation energy
is low. If the material allows all possible transitions, like in liquid or solid
bodys, the wavelength distribution will therefore be uniform and the radiation
is then said to have a continuous spectrum. Certain substances like all gases
allow only a few well-defined transitions. The emission can occur only at
certain frequencies and takes the form of discrete wavelengths because there are
quantum selection rules. Each atom or molecul has a special and characteristic
line spektrum and can therefore identified by its absorption- or emission lines.
All
objects are sources of radiation, but the amount and kind of radiation each
object emits depend on the temperature and emissivity of the object, which are
measures of how efficiently the object emits radiation. According to
‘Kirchhoff’s law’, the absorptivity and emissivity of a body are equal; a
good admitter is a good absorber. In the visible wavelengths, black coal dust is
a good absorber. White snow, on the other hand, reflects most of the incident
radiation in the visible range. But an object that is a good absorber of certain
wavelengths is not necessarily a good absorber of all wavelengths. Thus, fresh
snow is very nearly a white body in visible wavelengths, whereas it is almost a
black body in infrared wavelengths. Because nearly 50 percent of the energy of
the sun is infrared radiation, fresh white snow is melting like butter in the
sun.
A
perfect absorber/emitter is called a ‘black body’ and has a emissivity of
100 percent. A perfect reflector or
‘white body’ has an emissivity of 0 percent. A ‘blackbody’ is an
idealised body and is one that absorbs all of the electromagnetic radiation
striking it. A blackbody is also a perfect emitter of radiation. In any event
the definition does not imply that the object must be black in color. As above
mentioned, snow is an excellent blackbody, but only in the infrared part of the
spectrum. Consequently snow brings strong coldness during clear nights!
For
a perfect all-wave blackbody the intensity of the
emitted radiation and the wavelength distribution depend only on the absolute
temperature of the body. The ‘Stefan’s law’ applies: S = σ T4
W/m2. This ‘fourth-power law’ was formulated before ‘Planck’s
law’. ‘Wien’s distribution law’ lm
= 2898/T mm
shows that the wavelength of maximum energy λm is inversely
proportional to absolute temperature. ‘Wien’s law’ describes the
relationship between the wavelength at which the maximum amount of radiation is
emitted and the temperature of the body. If we assume the earth a blackbody with
a temperature of 288 K ( +15° C) then the maximum of the emitted energy lies by
10 μm. The heated and cooled earth with ground temperatures between -50°
and +50° C emits the maximum amount of radiation in wavelengths between 8 and
14 mm.
The
atmosphere surrounding the Earth is a gaseous envelope, held by gravity, having
its maximum density and pressure just above the solid/liquid surface, and
becoming gradually thinner with distance from the ground, until it finally
becomes indistinguishable from the interplanetary gas. The atmosphere is
transparent to long wave radiation emitted by the earth’s surface in certain
wavelength intervals, particular within a spectral range of approximately 8 to
14 μm, which is called the open atmospheric radiation window. Therefore
under clear skies an object can be cooled below ambient air temperature by
radiation heat loss.
An
important property of the earth’s radiation is that its absorption by gases in
the atmosphere is not continuous over the spectrum but occurs in a series of
discrete lines. In some parts of the spectrum, infrared radiation can move
upward relatively freely and be lost to space when skies are clear; the most
notable example is the radiation window between 8 to 14 μm. This window is
by nature open and can’t be closed by the absorption bands of the atmospheric
gases, especially not by the strong absorption bands of CO2 at 15 mm.
This band corresponds to a blackbody temperaature distinctly colder than that
within the atmospheric window., where outgoing radiation emamates from the
earth’s surface. The 15-mm
emission of CO2 corresponds to a blackbody temperature of about 203 K
or -70° C. With such a cold absorbed and re-emitted temperature radiation
it’s impossible to heat up the warmer earth. The width of the window,
calculated with ‘Wien’s law’, covers over a temperature range between 339
K and 213 K or between +66° C and -60° C.
The
infrared radiation window allows the possibility of remote sensing of the earth
by airplanes and satellites. The quantity most frequently measured in
present-day remote sensing systems is the electromagnetic energy emanating from
the earth surface and the objects of interest on it. Observing the earth from
outer space by weather satellites through the radiation window all objects in a
broad temperature range between +/-60° C can be detected during day and night
by its infrared radiation. The absorptivity and emissivity of gases is strongly
wavelenth dependent, varying from sero values in the window to high values in
the characteristic absorption bands. Water vapour has strong bands between 5 and
8 mm
and beyond 20 mm
and carbon dioxide around 15 mm.
While water vapour as an invisible gas is a selective absorber and emitter,
dense liquid rain- or snow-clouds act essentially as blackbodies.
Infrared
receiver systems are generally used for the remote detection of thermal
phenomena. They usually produce one- or two-dimensional images and have to
satisfy a number of more or less stringent functional criteria. Thermography
allows us not only to see the invisible, but also to detect and to evaluate it.
It extends the power of the eye beyond the limits of its sensivity. The
invisible infrared radiation permanently emitted by different objects, which
cannot directly perceived by the eye, is thus transformed into recognisable
messages that are conveyed to us by an optoelectronic detection sytem.
Thermal
imaging systems operate in the 3-3,5 mm
and 8-12 mm
bands which match the atmospheric transmission windows and produce images with
contrast proportional to variations in received radiation. There is no
difference making an infrared signature of a merchant navy ship, of a person, of
a building, of
a vertical-takeoff aircraft or of a tank in a desert.
In
1800, Sir William Herschel discovered the presence of the thermal radiation
outside the spectrum of visible light. However, it was not until 1830 that the
first detectors were developed for this type of radiation. Between 1870 and
1920, technological advances led to the development of the first quantum
detectors based on the interaction between radiation and matter. Later
photoconducting or photovoltaic detectors were found. Beyond 1930 lead sulfide
detectors, sensitive in the 1.3-3 mm,
were developed. After 1940 the spectral range was extended to middle infrared
(3-5 mm)
by the use of indium antimonide. After 1960 the far infrared between 8-14 mm
was explored by mercury-tellurium-cadmium detectors.
The
last type of detectors requires cooling. Because of their higher sensitivity and
short-response times, these quantum detectors have led to the development of
thermal imaging systems that rely on the detection of infrared radiation emitted
by matter in the range 2–15 mm.
The line-scanning infrared analyser is the answer to problems in
second-generation thermography. The concept of infrared linear scanning has been
around in the field of defense for about 30 years and has led to the development
of airborne systems for the infrared imaging of the ground with very high
spatial resolution. With a modern thermograaphic equipment it is possible to
transform an infrared image into a visible image that can be transmitted by a
video signal.
The
infrared thermography is an undoubtly proof that the IPCC-hypothesis can’t be
right. The atmosphere doesn’t act like the glasses of a greenhouse which
primarily hinder the convection! The atmosphere has an open radiation window
between 8 and 14 mm
and is therefore transparent to infrared heat from the earth’s surface. This
window can’t be closed by the distinctive absorption lines of CO2
at 4.3 and 15 mm.
Because the atmosphere is not directly heated by the Sun but indirectly from the
surface the earth looses warmth also by conduction with the air, more effective
by vertical convection of the air and to a great part by evaporation.
Summerizing
we can say: Earth’s surface gains heat from the Sun, is warmend up and looses
heat by infrared radiation. While the input of heat by solar radiation is
restricted on the daytime hours, the outgoing terrestrial radiation is a nonstop
process during day and night and depends only on the bodytemperature and the
emissivity. Therefore after sunset the earth continuous to radiate and therefore
cools. Because the air is in physical contact with the ground it also cools, so
that the vertical temperature profile changes, we get a surface inversion which
inhibits convection.
All
temperature and weather observations indicate that the earth isn’t like a
greenhouse and that there is in reality no “natural greenhouse effect” which
could warm up the earth by its own emitted energy and cause by re-emission a
“global warming effect”. With or without atmosphere every body looses heat,
gets inevitably colder. This natural fact, formulated by Sir Isaac Newton in his
“cooling law”, led Sir James Dewar to the construction of
the “Dewar flask” to minimize heat losses from a vessel. But the most
perfect thermosflask can’t avoid that the hot coffee in it gets cold after
some time.
The
hypothesis of a natural and a man-made “greenhouse effect” belongs to the
category “scientific errors”.
Oppenheim,
14th of February 2007
Dr. Wolfgang Thüne, Dipl.Met.
References:
1.
Kondratyev, K. Ya.: Radiation in the Atmosphere, New York 1969
2.
Coulson, K. L.: Solar and terrestrial radiation, New York 1975
3.
Swain, Ph. H.; Davis Sh. M.: Remote sensing: The quantitative approach, New York
1978
4.
Maldague, X. P. V.: Nondestructive evaluation of materials by infrared
thermography, London 1992
5.
Gaussorgues, G.; Chomet, S.: Infrared thermography, London 1994
6.
Anthes, R. A. et al: The atmosphere, Columbus 1981
7.
Salby, M. L.: Fundamentals of atmospheric physics, San Diego 1995
8.
Philander, S. G.: Is the temperature rising? The uncertain science of global
warming, Princeton 1998