Table of contents

Owing to the somewhat explosive development of the science of atmospheric electricity during the past decade this article covers a broad field of activity. The article begins with a description and discussion of the work that has been performed to understand the electrical properties of the basic materials involved in generating processes in the atmosphere, namely ice, water, and sand and similar particulate matter. A comprehensive treatment of the thermoelectric effect in ice is given, and the electrification of phase changes in ice is also discussed. Field measurements of the electrification of snowstorms and blizzards are described, together with simulation experiments performed in the laboratory, and the results and interpretations compared. A following section deals with the electrification of water and water drops during freezing, fragmentation, disintegration, evaporation, and splashing and bubbling, the laboratory experiments relating to a given phenomenon being critically compared. Another section considers the electrification of sand and dust, with particular reference to sandstorms and dust devils, and also measurements of the electrification of volcanic plumes and their interpretation.

The study of the electrification of convective clouds is almost certainly the most important aspect of the science of atmospheric electricity, and it is studied in detail. First, the observations of the electrification of convective clouds are described in order to lay a foundation for discussion; secondly, laboratory investigations of the electrical processes occurring within clouds are presented and compared critically. The observations and laboratory experiments so described enable theoretical models of the growth of cloud electrification to be projected; the models and their associated equations are examined thoroughly. Another aspect of cloud electrification studies is that of electrical discharges involving lightning. The electrical and optical measurements of the lightning flash are discussed and include comparisons of intra-cloud and cloud-to-ground strokes and theories of the discharge mechanisms. The phenomenon of ball lightning is also discussed. A short section is devoted to an extreme phenomenon of cloud electrification, the tornado.

Studies of the electromagnetic radiation emitted from colliding water drops are described in a section that is primarily devoted to presenting an account of recent research into electromagnetic radiation associated with the lightning discharge; spectra are shown that are thought to be associated with different components of a lightning flash. The last three sections are concerned with ions and aerosols in the atmosphere (involving ion-aerosol interactions, the electrode effect, and radio-activity), the electrification of the upper atmosphere and space, and a consideration of the global electrical circuit and its related electrical `balance sheet'.

A large number of complex and interrelated microphysical processes are involved in the formation of solid and liquid precipitation particles within clouds. These have received detailed attention in recent years and, with certain important exceptions, a moderately accurate quantitative description now exists of the growth and interactions of cloud particles, from their original formation on condensation or sublimation nuclei to their removal from the cloud as large precipitation elements. The objective of this article, which does not include a consideration of the macrophysical, dynamical approach to cloud physics, is to synthesize current knowledge of these microphysical processes and to establish which major problems have been effectively solved and which require appreciable further attention.

The basic physics of the three heterogeneous nucleation processes occurring inside clouds is now well established, although in the case of the formation of the ice phase some more specific information is required. Accurate quantitative predictions of the nucleating activity of artificially produced smokes are gradually emerging. The calculated growth rates of a population of cloud droplets growing by condensation within a rising parcel of air agree well with observation only if mixing between the parcel and its environment is considered. The efficacy of the collision and coalescence mechanism which supplants the condensation process is extremely sensitive to the values of the collision efficiencies for small cloud drop-lets. The most recent calculations suggest that the 18 μm threshold is incorrect and that droplets of smaller radii can collide. This point and the possible importance of electrical forces in triggering the coalescence process require further experimental study. The combination of the stochastic approach with recently determined collision efficiencies have produced realistic computations of the growth of populations of cloud droplets by means of coalescence. The extremely rapid growth rates observed in certain situations are probably a consequence of electrical forces. The majority of conditions under which raindrops become unstable while falling through a cloud have been established and are supported by a reasonable theory. Experimental studies of the movement of growth layers across an ice surface suggest strongly that the habit of ice crystals is controlled by variations in the surface migration distances of molecules, but further work on the prism faces of ice is required before a definitive conclusion can be drawn. Collection efficiencies of ice crystals have been measured and can be significantly increased by strong electric fields. A good quantitative understanding now exists of the heat and mass exchange of hailstones of simplified geometry, permitting accurate predictions to be made of the various growth regimes occuring in natural conditions. The dominant factors governing the density and structure of hailstones have also been established. However, the microphysics of the individual interactions occurring during the riming process is not well understood. None of the explanations that have been presented for the formation and multiplication of ice crystals within clouds at temperatures close to 0 °C have been rigorously explored, although electrofreezing is supported by appreciable evidence and may be responsible for the primary crystals in certain situations.

A brief description of the solar system is given which highlights those features that a plausible theory should explain. A review of theories up to 1960 sets the scene for the description of more modern work and also points out the major features for which explanations are difficult to find. The major obstacle, on which all the older theories founder, is the explanation of the distribution of angular momentum in the system whereby the planets with 0·14% of the mass possess 98% of the angular momentum. The other problem, which has been more recently emphasized by Hoyle, is the slow rotation of the Sun, which is difficult to explain in terms of it having condensed from material possessing the basic galactic rotation.

The modern theories described are:

1. Hoyle's nebula theory, which invokes the transfer of angular momentum by magnetic forces;

2. the accretion theory, which proposes that planetary material was captured by the Sun from an interstellar cloud;

3. McCrea's floccule theory, which describes the solar system as having formed from initial condensations of planetary mass;

4. the capture theory, which proposes that the Sun captured planets from a tidal filament drawn out of a light star;

5. a nebula theory proposed by Urey based on chemical evidence derived primarily from meteorites.

The statistical mechanics of long flexible chains somewhat benefit from an analogy where a chain configuration is interpreted as one path for a quantum mechanical particle. For instance: (i) the problem of a long chain weakly bound to an adsorbing surface is reminiscent of the ground state of the deuteron, where its wave function extends at distances much longer than the attractive potential; (ii) the coupling between both strands in partially denatured deoxyribonucleic acid is equivalent to a two-body scattering problem (including bound states). The mathematical principles of this correspondence are described here.

The theory of light scattering from a collection of free electrons is reviewed, and it is shown that the frequency spectrum observed at a detector is precisely that of the density fluctuations of a particular scale length in the scattering medium, the scale length being determined by the wavelength of the incident light and the geometry of the experimental arrangement. The electron density fluctuation in a plasma is calculated, and it is shown that the plasma Debye shielding distance λ_D is a critical length in the theory, the electrons behaving independently on a scale shorter than λ_D and collectively on a scale longer than λ_D. The collective behaviour is characterized by the presence of waves that can give rise to well-defined resonances in the scattered-light spectrum. The effects of differing ion and electron temperatures, current flowing in the plasma, magnetic field, and Coulomb collisions are considered briefly. Technical considerations in planning experiments to test the theory and to apply it to the diagnosis of real laboratory plasmas are discussed, with attention being given to signal-to-noise ratio, stray light, and the dispersing instrument to be used at the detector. Some representative experiments that have been carried out are reviewed.

The meaning and implications of invariance of elementary particle interactions under the operations of charge conjugation (C), parity (P), and time reversal (T) are discussed and their present experimental status reviewed. In the first section we define these invariances as relations between elements of the I matrix, as far as possible without recourse to any particular underlying dynamics. The next two sections contain the experimental support for the belief that strong interactions are invariant under C, P and T, and that electromagnetic interactions are invariant under P. The degree to which C and T have been tested in electromagnetism is also discussed. Weak interactions, and in particular the breakdown of parity invariance, are treated in the fourth section, and the last section reviews the present experimental situation with regard to the breakdown of CP invariance in the K°, K above bar ° system and outlines some possible origins of this breakdown.

The use of elastic neutron scattering to examine the magnetic structure and electronic distribution in nonmetallic materials has increased rapidly in the last few years. Unfortunately, techniques for analysing the results have not always kept pace with the complexity of the crystals. A rather confusing situation has arisen whereby two apparently unrelated approaches to the analysis problem exist. The traditional approach is to express the scattering interaction as a vector operator. For simple systems this is ideal, but it is not easy to apply to a complicated wave function involving both spin and orbital magnetization, or if the direction of magnetization is not simply related to other principal directions in the experiment. As a result, an alternative approach using tensor operators has been introduced that will handle a much wider range of systems. It is based on a rather elaborate formalism and the answers appear in a different form from the traditional method.

The main purpose of this article is to give a detailed exposition of the tensor operator approach. The fundamental theory of elastic scattering by magnetic salts is reviewed and the tensor operator approach is presented in a form that it is hoped will prove useful to those wishing to use it for specific problems. Three examples are studied in detail, namely terbium, cobalt and uranium.

In any plane of an optical tract a coherent light wave is completely described by the complex amplitude, that is to say by a two-valued function. It can, however, also be described by a one-valued or real function such as the intensity or photographic density, plus a known, coherent reference wave. This is the basis of wave-front reconstruction or holography. The history of holography is described, and some of its applications, such as holographic interferometry, contour mapping, and 3D pictures in monochrome or in natural colours. Some prospective applications are holographic panoramas and 3D projection of moving pictures. Information and codage are promising applications but are hampered by the phenomenon of `laser speckle'. Finally it is pointed out that holography is a multidimensional type of coding, which can transmit more almost independent data than the number of points contained in the hologram.