ALBERT

Notes: Summary Working on the hypothesis that atmospheric ice-forming nuclei are largely of terrestrial origin, the nucleating ability of various types of soil particles and mineral dusts has been investigated. Of the thirty substances tested, twenty-one, mainly silicate minerals of the clay and mica groups, were found to produce ice crystals in supercooled clouds and also on supercooled soap films at temperatures of − 18° C, or above, and of these, ten were active above − 12° C. The most abundant of these is kaolinite with a threshold temperature of − 9° C. Ten natural substances, again mainly silicates, were found to become more efficient ice nuclei having once been involved in ice-crystal formation, i.e. they could be pre-activated or «trained». Thus, ice crystals grown on kaolinite nuclei, which are initially active at −9° C, when evaporated and warmed to near 0° C in a dry atmosphere, leave behind nuclei which are thereafter effective at − 4° C. Particles of montmorillonite, another important constituent of some clays, and which are initially inactive even at −25° C, may be pre-activated to serve as ice nuclei at temperatures as high as −10° C. It is suggested that although such particles can initially form ice crystals only at cirrus levels, when the ice crystals evaporate they will leave behind some «trained» nuclei which may later seed lower clouds at temperatures only a few degrees below 0° C. On this hypothesis, the fact that efficient nuclei are occasionally more abundant at higher levels would not necessarily imply that they originate from outer space. Indeed, in view of our tests on products of stony meteorites, produced both by grinding and vaporization, which show them to be ineffective at temperatures above − 17° C, it seems likely that atmospheric ice nuclei are produced mainly at the earth's surface, the clay minerals, particularly kaolinite, being a major source. Although a good deal of work has been carried out in different laboratories on the ice-nucleating ability of a wide variety of inorganic compounds, there has been little agreement in the results. Careful tests carried out in our laboratory have revealed a number of reasons for this. Spurious results may be obtained because of the presence, in the air or the chemicals, of small traces of silver or free iodine, leading to the formation of silver iodide: if all such trace impurities are removed, many of the substances that have been claimed to provide efficient ice nuclei are found to be quite ineffective. It is dangerous to infer that all twinkling particles in a water cloud are ice crystals since particles of some seeding agents glitter even at positive temperatures. The threshold temperature of a nucleant will depend upon the criterion adopted for the onset of nucleation, i.e. upon the fraction of the total number of particles of seeding agent which are activated; this, in turn, will depend upon the fraction of particles which happen to possess suitable crystallographic faces for nucleation. Much may also depend upon the manner in which the test is performed. Since some nucleating materials produce ice crystals only after a delay of 30 seconds or more, they may appear to be ineffective if tested in the transient cloud of an expansion chamber but highly effective if allowed to remain in an ice-supersaturated atmosphere for a minute or more. Again, we have found that the efficiency of some nuclei is governed by the supersaturation as well as the temperature of the environment, and the supersaturation regimes in expansion, diffusion, and mixing-cloud chamber may be widely different. Highly soluble particles, although able to act as «sublimation» nuclei in atmospheres super-saturated relative to ice but sub-saturated relative to water, on entering a water cloud go quickly into solution and lose their nucleating ability. Inorganic substances which definitely nucleate a supercooled water cloud in a mixing-cloud chamber at temperatures of −15° C and above are: AgI (−4° C), PbI2 (−6° C), CuS (−6° C), Ag2S (−8° C), Ag2O (−9° C), HgI2 (−8° C), V2O5 (−14° C), Cu2I2 (−15° C), the figures in brackets indicating the threshold temperatures at which about one particle in 104 becomes active as an ice nucleus. Cadmium iodide (−12° C), ammonium fluoride (−9° C) and iodine (−14° C) are examples of salts which will act as sublimation nuclei in an ice-supersaturated atmosphere and will nucleate a supercooled soap film, but which are ineffective in a water cloud because of their solubility. Although the most efficient nucleating agents tend to be hexagonal in structure, there are some striking exceptions e.g. Ag2S, Ag2O, HgI2, but in most cases, we have been able to find a low-index crystal surface on which the ice lattice could grow with a misfit of only a few per cent. In an attempt to investigate the nucleation mechanism in more detail, we have studied the growth of ice on single crystals of various nucleating agents. Perfect orientation of ice crystals has so far been observed on the basal faces of silver iodide, lead iodide, cupric sulphide, cadmium iodide, and freshly-cleaved mica, on the (001) plane of iodine, and on the (010) plane of mercuric iodide.

Notes: Summary Our present knowledge concerning the abundance, size, physical properties, origin and nature of condensation nuclei is reviewed and critically assessed.

Notes: Summary The number and size of salt particles produced by the bursting of air bubbles in sea water has been measured. Bubbles of diameters varying between 1/2 and 2 mm each produced about 300 nuclei the sizes of which, under the electron microscope, were mainly between 0.1 μ and 0.5 μ diameter. They appeared to consist mainly of sodium chloride, the smallest ones containing only 10−15 g of salt. These results, together with measurements of the size distribution of salt nuclei collected over the oceans in areas of spray formation, indicate that the total concentrations of salt nuclei over the oceans in winds of up to 15 m sec−1 probably do not exceed 100 cm−3. The corresponding rate of production of salt nuclei at the sea surface is estimated to be 1000 cm−2 sec−1. It is therefore inferred that sea spray contributes perhaps only one-fifth of the nuclei involved in cloud formation, the majority being the products of combustion, either natural or man-made.

Notes: Summary A critical survey is given of the physical processes responsible for the release of precipitation such as drizzle, rain, snow, and hail in middle and low latitudes. By suitable analyses of long series of airplane observations of characteristic states in layer and shower clouds indications are derived for the type of the efficient physical processes. Investigations carried out at the Imperial College during the last years are discussed in a brief report: studies on the growth of precipitation elements in cloud models and theoretical conditions for the formation of showers are compared with both visual and radar observations.

Notes: Summary This progress report summarises the researches of the cloud physics group at Imperial College during the last four years 1949–1952. It describes work carried out on the following topics: 1. the spontaneous condensation of water vapour; 2. measurements on the concentration and size distribution of sea-salt nuclei over the N. Atlantic and their contribution to atmospheric opacity; 3. laboratory and theoretical studies on the supercooling and freezing of water and aqueous solutions; 4. experimental and theoretical researches on ice crystal growth from the vapour; 5. theoretical computations on the growth of water drops and ice particles in both layer and shower type clouds; 6. studies of the form, structure and development of high-level ice clouds and their formation relative to orographic features; 7. the construction of instruments for precipitation studies.

Notes: Zusammenfassung In «naiven» Mäusen werden Verhaltensveränderungen nach Injektion kleiner Mengen von Gehirnmaterial aus Saccharin bevorzugenden und röntgenbestrahlten Mäusen beobachtet. Es trat Vermeidung der Saccharinlösung ein, und zwar ähnlich wie sonst in «trainierten», Saccharin bevorzugenden Tieren nach der Bestrahlung.

Notes: Abstract The linear additivity of free energies of ionization of substituted acids relative to the unsubstituted parent acid is described in terms of the Hammett relationship and as the weighted sums of microscopic free energies for benzenetricarboxylic acids.

Notes: Abstract Thermodynamic parameters for the successive ionizations of a number of monomeric polycarboxylic acids have been determined from a temperature-dependence study of the ionization constants calculated from potentiometric data obtained using cells without liquid junction. Specifically the acids are: 1,2,3-propanetri-; 1,2,3,4-butanetetra-; 1,3-benzenedi-; 1,2,3-, 1,2,4-, and 1,3,5-benzenetri-; 1,2,4,5-benzenetetra-; and 1,2,3,4,5,6-benzenehexacarboxylic acids. The data are critically discussed in terms of the common models of ionization and of reactivity effects of substituents on ionization for monomeric and polymeric polybasic acids.

Notes: Abstract The early work of Stern has been reviewed briefly along with some of our experiments carried out later with similar geometrical arrangements in order to highlight his successes at the time and the future developments of his findings and predictions. The double crystal technique, developed by Stern has been used for energy analysis of He beams and results are shown for scattering from a rough reconstructed (001) InSb surface and Xe adsorbed layers. Surface phonons were not observed in the case of InSb probably as a result of the rough surface.