back to the main page

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


back to the main page