Fundamentals of Atmospheric Radiation: An Introduction with 400 Problems 1st Edition by Craig F. Bohren (PDF)

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Meeting the need for teaching material suitable for students of atmospheric science and courses on atmospheric radiation, this textbook covers the fundamentals of emission, absorption, and scattering of electromagnetic radiation from ultraviolet to infrared and beyond. Much of the contents applies to planetary atmosphere, with graded discussions providing a thorough treatment of subjects, including single scattering by particles at different levels of complexity. The discussion of the simple multiple scattering theory introduces concepts in more advanced theories, such that the more complicated two-stream theory allows readers to progress beyond the pile-of-plates theory. The authors are physicists teaching at the largest meteorology department in the US at Penn State. The problems given in the text come from students, colleagues, and correspondents, and the figures designed especially for this book facilitate comprehension. Ideal for advanced undergraduate and graduate students of atmospheric science. * Free solutions manual available for lecturers at www.wiley-vch.de/supplements/

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Editorial Reviews: Review “… a highly enthusiastic and useful book … highly recommended.” CHOICE From the Inside Flap Meeting the need for teaching material suitable for students of atmospheric science and courses on atmospheric radiation, this textbook covers the fundamentals of emission, absorption, and scattering of electromagnetic radiation from ultraviolet to infrared and beyond. Much of the contents applies to planetary atmosphere, with graded discussions providing a thorough treatment of subjects, including single scattering by particles at different levels of complexity. The discussion of the simple multiple scattering theory introduces concepts in more advanced theories, such that the more complicated two-stream theory allows readers to progress beyond the pile-of-plates theory. ? Free solutions manual available for lecturers at www.wiley-vch.de/supplements/ From the Back Cover Meeting the need for teaching material suitable for students of atmospheric science and courses on atmospheric radiation, this textbook covers the fundamentals of emission, absorption, and scattering of electromagnetic radiation from ultraviolet to infrared and beyond. Much of the contents applies to planetary atmosphere, with graded discussions providing a thorough treatment of subjects, including single scattering by particles at different levels of complexity. The discussion of the simple multiple scattering theory introduces concepts in more advanced theories, such that the more complicated two-stream theory allows readers to progress beyond the pile-of-plates theory. ? Free solutions manual available for lecturers at www.wiley-vch.de/supplements/ About the Author Craig F. Bohren is Distinguished Professor Emeritus of Meteorology at the Pennsylvania State University. In 1988 he was elected a Fellow of the Optical Society of America. He is author of several books, his most recent book being ‘Atmospheric Thermodynamics’ (with Bruce A. Albrecht). Professor Bohren is the first recipient of the American Meteorological Society’s Louis J. Battan Award for Authors. Eugene E. Clothiaux is an Associate Professor of Meteorology at the Pennsylvania State University. He remained at the Pennsylvania State University as a Research Associate from 1994-1999 before becoming an Assistant Professor in 1999. Prof. Clothiaux has written several contributions on millimeter-wave cloud radar and atmospheric radiation. Excerpt. © Reprinted by permission. All rights reserved. Fundamentals of Atmospheric RadiationAn Introduction with 400 ProblemsBy Craig F. Bohren Eugene E. ClothiauxJohn Wiley & SonsCopyright © 2006 Wiley-VCH GmbH & Co. KGaA, WeinheimAll right reserved.ISBN: 978-3-527-40503-9Chapter OneEmission: The Birth of Photons This is the first of three foundation chapters supporting those that follow. The themes of these initial chapters are somewhat fancifully taken as the birth, death, and life of photons, or, more prosaically, emission, absorption, and scattering. In this chapter and succeeding ones you will encounter the phrase “as if”, which can be remarkably useful as a tranquilizer and peacemaker. For example, instead of taking the stance that light is a wave (particle), then fiercely defending it, we can be less strident and simply say that it is as if light is a wave (particle). This phrase is even the basis of an entire philosophy propounded by Hans Vaihinger. In discussing its origins he notes that “The Philosophy of ‘As If’ … proves that consciously false conceptions and judgements are applied in all sciences; and … these scientific Fictions are to be distinguished from Hypotheses. The latter are assumptions which are probable, assumptions the truth of which can be proved by further experience. They are therefore verifiable. Fictions are never verifiable, for they are hypotheses which are known to be false, but which are employed because of their utility.” 1.1 Wave and Particle Languages We may discuss electromagnetic radiation using two languages: wave or particle (photon) language. As with all languages, we sometimes can express ideas more succinctly or clearly in the one language than in the other. We use both, separately and sometimes together in the same breath. We need fluency in both. Much ado has been made over this supposedly lamentable duality of electromagnetic radiation. But no law requires physical reality to be described by a single language. We may hope for such a language, but Nature often is indifferent to our hopes. Moreover, we accept without protest or hand-wringing the duality of sound. We describe sound waves in air as continuous while at the same time recognizing that air, and hence sound, is composed of discrete particles (molecules) in motion. How do we choose which language to use? Simplicity. Life is short. To understand nature we take the simplest approach consistent with accuracy. Although propagation of sound in air could be described as the motions of molecules, had this approach been taken acoustics would have floundered in a mathematical morass. In the photon language a beam of radiation is looked upon as a stream of particles called photons with the peculiar property that they carry energy, linear momentum, and angular momentum but not mass. The mass of the photon often is said to be identically zero. But given the near impossibility of measuring zero in the face of inevitable errors and uncertainties, it would be more correct to say that the upper limit of the photon mass keeps decreasing, its present value being about [10.sup.-24] times the mass of the electron. If it bothers you that a particle without mass can carry momentum this is because you are stuck on the notion that momentum is mass times velocity. Sometimes this is true (approximately), sometimes not. Momentum is momentum, a property complete in itself and not always the product of mass and velocity. Photons are of one kind, differing only in their energy and momenta, whereas waves are of unlimited variety and often exceedingly complex, the simplest kind a plane harmonic wave characterized by a single (circular) frequency w and direction of propagation (see Secs. 3.3 and 3.4). The dimensions of circular frequency are radians per unit time. You may be more familiar with just plain frequency, often denoted by v (sometimes f), which has the dimensions of cycles per unit time. The unit of frequency is the hertz, abbreviated Hz, one cycle per second. Because one cycle corresponds to 2[pi] radians, the relation between frequency and circular frequency is simple: w = 2[pi]v. (1.1) All electromagnetic waves propagate in free space (which does not strictly exist) with the same speed c, about 3 x [10.sup.8] [ms.sup.-1]. A plane harmonic wave in free space can just as well be characterized by its wavelength [lambda], related to its frequency by [lambda]v = c. (1.2) You sometimes hear it said that frequency is more fundamental than wavelength. In a sense, this is correct, but wavelength is often more useful. When we consider the interaction of electromagnetic waves with chunks of matter, the first question we must ask ourselves is how large the waves are. Big and small have no meaning until we specify a measuring stick. For electromagnetic radiation the measuring stick is the wavelength. The mathematical expressions describing the interaction of such radiation with matter can be quite different depending on the size of the matter relative to the measuring stick. How do we translate from wave to photon language? A plane harmonic wave with circular frequency w corresponds to a stream of photons, each with energy [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (1.3) where h is Planck’s constant (6.625 x [10.sup.-34] Js) and [??] = h/2[pi]. The frequency of visible electromagnetic radiation (light) is about [10.sup.14] Hz, and hence the photons that excite the sensation of vision have energies around [10.sup.-20] J. This isn’t much energy; the kinetic energy of a golf ball as it slices through air is about [10.sup.13] times greater. Understanding what happens when an electromagnetic wave is incident from air on the smooth surface of glass, say, is not especially difficult if one uses the wave language. The incident wave excites molecules in the glass to radiate secondary waves that combine to form (approximately) a net reflected wave given by the law of reflection and a net transmitted wave given by the law of refraction. There is no such thing as an absolutely smooth surface, so what is meant is smooth on the scale of the wavelength. All this makes intuitive sense and causes no perplexity. But now consider what happens when we switch to photon language. If we look upon reflection as the rebound of photons at a surface and transmission as their penetration through it, then why, if all photons are identical, are some reflected and some transmitted? This is indeed puzzling; even more so is why photons should be specularly (by which is meant mirror-like) reflected, because for photons imagined as particles of vanishingly small dimensions, all surfaces are rough. This is not to say that one couldn’t describe reflection and transmission at smooth interfaces in photon language, only that to do so would be exceedingly costly in mental effort. And the reverse sometimes is true. Many years ago one of the authors attended a colloquium entitled “The photoelectric effect without photons.” By the photoelectric effect is usually meant the emission of electrons by a surface (often metallic) because of illumination by radiation (often ultraviolet). In photon language the photoelectric effect is simple to describe. When a photon of energy hv is absorbed by the surface, the maximum kinetic energy E of the electrons thereby set free is E = hv – p, (1.4) where p is the minimum energy an electron loses in breaking free of the surface. A single photon interacting with a single electron gives up its entire energy to that electron, which if sufficient enables the electron to break free of the forces binding it to the metal. According to this equation the energies of the emitted electrons are independent of the incident power whereas the photocurrent (rate and number of emitted electrons) is proportional to it, which accords with experiment. This simple equation, first written down by Einstein in 1905, is one of the keystones of the modern theory of radiation and matter. Yet the speaker at that colloquium years ago, in an effort to describe and explicate the photoelectric effect without photons, assailed the audience with dozens of complicated equations. And even at that, part way through his mathematical tour de force his mind and tongue betrayed him and he blurted out the forbidden word “photon”. At that point, your author who was there leapt up from his seat and shouted, “Photons! Photons! You promised no photons.” A mirror illuminated by an incident beam gives rise to a reflected beam. Is this reflected beam redirected incident photons? Alas, we cannot do an experiment to answer this question. To determine if reflected photons are the same as incident photons would require us to be able to identify them. But photons are indistinguishable. We cannot tell one from another. We cannot tag a photon and follow its progress. Thus if you want to believe that reflected photons are the same as incident photons, you may do so. No one can prove you wrong. But you cannot prove you are right. When faced with an undecidable proposition, you may believe whatever you wish. Note that in the wave language we would not likely even ask if the reflected wave is the same as the incident wave. It is not often acknowledged that there is a third language for talking about light, what might be called the who-gives-a-hoot-what-light-is? language. This is geometrical or ray optics, in which the nature of light isn’t addressed at all. Fictitious rays are imagined to be paths along which the energy carried by light is transported, and these paths meander and bifurcate according to simple geometrical laws. But which language is the more useful? In a letter to American Journal of Physics, M. Psimopoulos and T. Theocharis ask the rhetorical questions: “What new discoveries have (i) the particle or photon aspect of light, and (ii) the wave aspect of light, given rise to? Answer: (i) we are not aware of any; (ii) holography, laser, intensity interferometry, phase conjugation.” To this list we add radar, all of interferometry, on which much of the science of measurement is based, and interference filters, which have many applications. The view of these authors is extreme, but they also quote the more measured words of Charles Townes, a pioneer in masers and lasers: “Physicists were somewhat diverted by an emphasis in the world of physics on the photon properties of light rather than its coherent aspects.” That is, the photon language has been the more fashionable language among physicists, just as French was the fashionable language in the Imperial Russian court. When prestigious and munificent prizes began to be awarded for flushing “ons” (electron, positron, neutron, meson, and so on) from the jungle, shooting them, and mounting their stuffed heads on laboratory walls, the hunt was on, and slowed down only with the demise of the Superconducting Supercollider. Although the wave language undoubtedly has been and continues to be more fruitful of inventions, the photon language is perhaps more soothing because photons can be incarnated, imagined to be objects we can kick or be kicked by. Waves extending through all space are not so easily incarnated. We can readily conceive of the photon as a thing. And yet an electromagnetic wave is just as much a thing as a photon: both possess energy and momentum (linear and angular) but not, it seems, mass. 1.2 Radiation in Equilibrium with Matter We often are told that when bodies are heated they radiate or that “hot” bodies radiate. True enough, but it is just as true that when bodies are cooled they radiate and that “cold” bodies radiate. All matter – gaseous, liquid, or solid – at all temperatures emits radiation of all frequencies at all times, although in varying amounts, possibly so small at some frequencies, for some materials, and at some temperatures as to be undetectable with today’s instruments (tomorrow’s, who knows?). Note that there is no hedging here: all means all. No exceptions. Never. Even at absolute zero? Setting aside that absolute zero is unattainable (and much lower than temperatures in the depths of the Antarctic winter or in the coldest regions of the atmosphere), even at absolute zero radiation still would be associated with matter because of temperature fluctuations. Temperature is, after all, an average, and whenever there are averages there are fluctuations about them. Radiation emitted spontaneously, as distinguished from scattered radiation (see Ch. 3), is not stimulated by an external source of radiation. Scattered radiation from the walls of the room in which you read these words may be stimulated by emitted radiation from an incandescent lamp. Turn off the lamp and the visible scattered radiation vanishes, but the walls continue to emit invisible radiation as well as visible radiation too feeble to be perceptible. We are interested in the spectral distribution of radiation – how much in each wavelength interval – emitted by matter. Consider first the simpler example of an ideal gas in a sealed container held at absolute temperature T (Fig. 1.1). When the gas is in equilibrium its molecules are moving in all directions with equal probability, but all kinetic energies E are not equally probable. Even if all the molecules had the same energy when put into the container, they would in time have different energies because they exchange energy in collisions with each other and the container walls. A given molecule may experience a sequence of collisions in which it always gains kinetic energy, which would give it a much greater energy than average. But such a sequence is not likely, and so at any instant the fraction of molecules with kinetic energy much greater than the average is small. And similarly for the fraction of molecules with kinetic energy much less than the average. The distribution of kinetic energies is specified by a probability distribution function f(E) which, like all distribution functions, is defined by its integral properties, that is, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.5) is the fraction of molecules having kinetic energies between any two energies [E.sub.1] and [E.sub.2]. Note that f does not specify which molecules have energies in a given interval, only the fraction, or probability, of molecular energies lying in this interval. If f is continuous and bounded then from the mean value theorem of integral calculus [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.6) where [bar.E] lies in the interval ([E.sub.1],[E.sub.2]). If we denote [E.sub.1] by E and [E.sub.2] by E + [DELTA]E we have [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (1.7) Because of Eq. (1.7) f(E) is sometimes called a probability density. When the limits of the integral in Eq. (1.5) are the same (interval of zero width) the probability is zero. The probability that a continuous variable has exactly a particular value at any point over the interval on which it is defined is zero, as it must be, for if it were not the total probability would be infinite. A distribution function such as f(E) is sometimes defined by saying that f(E) dE is the fraction (of whatever) lying in the range between E and E + dE. This is sloppy mathematics because although E represents a definite number dE does not. Moreover, this way of defining a distribution function obscures the fact that f is defined by its integral properties. As we shall see, failure to understand the nature of distribution functions can lead to confusion and error. It would be better to say that f(E) [DELTA]E is approximately the fraction of molecules lying between E and E + [DELTA]E, where the approximation gets better the smaller the value of [DELTA]E. You also often encounter statements that f(E) is the fraction of molecules having energy E per unit energy interval. This can be confusing unless you recognize it as shorthand for saying that f(E) must be multiplied by [DELTA]E (or, better yet, integrated over this interval) to obtain the fraction of molecules in this interval. This kind of jargon is used for all kinds of distribution functions. We speak of quantities per unit area, per unit time, per unit frequency, etc., which is shorthand and not to be interpreted as meaning that the interval is one unit wide. (Continues…) Excerpted from Fundamentals of Atmospheric Radiationby Craig F. Bohren Eugene E. Clothiaux Copyright © 2006 by Wiley-VCH GmbH & Co. KGaA, Weinheim. Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site. Read more

Reviews from Amazon users which were colected at the time this book was published on the website:

⭐I doubt I would have enjoyed this book as a student (too dense and long winded), but as an instructor many years later I absolutely love it! All the other books on this same difficult and treacherous subject are dry and lack the perspective that this one offers. The authors take no prisoners and actually make you laugh. I’m very thankful for the frank and refreshing writing of Drs. Bohren and Clothiaux, and also of Drs. Bohren and Albrecht for a very similar style book on Atmospheric Thermodynamics.

⭐この本の前書きに書いてあるように、どんな些細なことも仮定せずに説明し尽くすように配慮されて書かれている。ユーモアも入っている。厳密であるのに非常に読みやすい。おそらく論理に飛躍が無いからである。二週間かけて読破したが、何回でも読みたくなる。Fundamentalというタイトルは、初歩的なという意味よりも、基本的な、重要なという意味に捉えるべきである。大気放射の本は初心者にとっつきやすいものがないと思い悩んでいる方にぜひお勧めしたい。

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