/pdf/aerosol-cloud-precipitation-interactions-part-1-the-nature-26b8hz0z7e.pdf

Aerosol cloud precipitation interactions. Part 1. The nature and sources of cloud-active aerosols

Several mechanisms can extend the equilibrium domain of a liquid phase into the solid region of the normal phase diagram. The causes of premelting, which include surface melting, interface curvature and substrate disorder, occur in all types of substances, including H2O. In the case of H2O, premelting can have important environmental consequences, among which are the heaving of frozen ground, breakdown of rock and concrete, sintering of snow, flow of glaciers, scavenging of atmospheric trace gases by snow and ice, and the electrification of thunderclouds. The article reviews the basic mechanisms of premelting and discusses their roles in the environmental phenomena. The principal results of numerous studies are reviewed, and trends in current research are outlined.

http://iopscience.iop.org/article/10.1088/0034-4885/58/1/003/pdf

The premelting of ice and its environmental consequences

Galactic cosmic rays could influence Earth's cloudiness by creating aerosol particles that prompt cloud formation. That possible effect looks to be smaller than thought, but the story won't end there.

https://science.sciencemag.org/content/sci/298/5599/1732.full.pdf

Cosmic rays, clouds and climate

The theory of homogeneous nucleation of liquid drops in supersaturated vapours is reviewed. A new kinetic treatment which accounts for the heating of the growing clusters due to the latent heat of condensation is presented, and the irreversible thermodynamics of non-isothermal nucleation discussed. It is found that growing clusters are generally colder than the surrounding vapour during their sub-critical growth period. Time dependent nucleation is discussed and a simple estimate for the time-lag in establishing the steady state is given. The nucleation in cloud chambers expanding with constant speed is discussed in detail, and the number of droplets formed and their final mean size is calculated as a function of the terminal expansion ratio. Re-evaporation of nucleated clusters is discussed. Numerical results are given for a number of typical situations in experiments on water-vapour. These results for the nucleation rate as corrected by Lothe and Pound predict much lower ‘critical’ expansion ra...

Homogeneous nucleation and growth of droplets in vapours

There are quantum statistical contributions to the free energy of formation of embryos and critical nuclei in transformations to condensed phases. They arise from consideration of the absolute entropy of the embryos or nuclei. In the case of homogeneous nucleation of water droplets from vapor the factor of increase in equilibrium concentration of critical nuclei is about 1017, resulting in a discrepancy of corresponding magnitude between the Becker‐Doering equation and the critical supersaturation measurements of Wilson and Powell. It is suggested that some of this disagreement may be removed by treating the problem as a transient rather than a steady‐state situation and considering non‐accommodation in the smaller size classes. Also there is an appreciable quantum statistical effect in homogeneous nucleation of crystals from the supercooled liquid and a related effect in nucleation from the vapor of crystals on substrates.

Reconsiderations of Nucleation Theory

The method and apparatus used in the measurements are substantially those described in a paper "On Some Determinations of the Sign and Magnitude of Electric Discharges in Lightning Flashes." The induced charge on an exposed earthed conductor (test-plate or sphere) is used as a measure of the electric field. The testplate virtually forms part of a flat portion of the earth’s surface, and the vertical electric force or potential gradient at ground level is equal (in electrostatic measure) to 4 π Q/A, where Q is the charge on its exposed surface and A is its area. The charge Q on the earth-connected sphere of radius R, when exposed at a height h , great compared with R, is a measure of the potential at that height; the zero potential of the sphere being the resultant of the undisturbed atmospheric potential V at the height h and of the potential Q/R due to the charge on the sphere, so that Q/R = - V. The earthed conductors can be shielded from the earth’s field: the test-plate by means of an earth-connected cover, the sphere by lowering it into a conducting case resting on the ground. The quantity of electricity which flows to earth through the connecting wire on exposing or shielding the test-plate or sphere, is measured by a special type of capillary electrometer in which the readings indicate the total quantity of electricity which has traversed the instrument ; the sign and magnitude of the charge on the exposed conductor, and thus of the potential gradient, at the beginning and end of an exposure are thus determined. The sign and magnitude of sudden changes of potential gradient which occur while the conductor is exposed are indicated by the direction and magnitude of the resulting displacements of the electrometer meniscus. The total flow of electricity between the atmosphere and the test-plate or sphere during an exposure is also measured —being given by the difference between the electrometer readings before and after the exposure. The principal improvement introduced has been the provision of apparatus for giving a photographic trace of the electrometer readings; rapid changes in the field occupying less than one-tenth of a second are in this way recorded. In the observations described in the previous paper the sphere was supported in a manner which did not admit of absolute measurements being made, as the charge measured included that on the upper part of the support as well as that on the sphere itself; in these earlier measurements therefore the sphere was standardised by comparison with the test-plate. The method of supporting the sphere is now such that the charge on the sphere alone is measured, while the disturbing effect of the earthed supporting rod is small, and thus the potential at the level of the earthconnected sphere can be calculated from the charge upon it. The new method of mounting the sphere is shown in fig. 1.

/pdf/investigations-on-lightning-discharges-and-on-the-electric-1lzgjw238h.pdf

Investigations on Lightning Discharges and on the Electric Field of Thunderstorms

The behaviour of air saturated with aqueous vapour and allowed to expand suddenly, has been investigated by Coulier, Aitken, Kiessling, and R. v. Helmholtz. As is well known, if the moist air has been previously freed from “dust,” no condensation takes place except on the walls of the vessel, even if the expansion be sufficient to produce considerable supersaturation. For convenience, the term “dust” is here used to include all nuclei which can be removed either by filtering or by repeatedly forming a cloud by expansion and allowing it to settle. What is the limit, if such exists, to the degree of supersaturation which can be attained without condensation taking place throughout the moist air, is a question of considerable meteorological as well as purely physical interest. It was primarily with the object of finding an answer to this question that the experiments to be described were undertaken, such experimental evidence as already existed on the subject being of a very incomplete and contradictory character.

Condensation of Water Vapour in the Presence of Dust-Free Air and Other Gases

As is well illustrated by photographs of their tracks, the majority of β-particles of given initial velocity bave approximately the same range, measured along the path. When the track shows a visible branch, indicating that an exceptionally close encounter has resulted in the ejection of an electron with kinetic energy amounting to an appreciable fraction of that of tbe primary particle, the main track is naturally of less tban tbe average range. A diminution of range may also be expected to follow such of the nuclear deflections as result in the emission of X radiation; this decrease in range indeed affords a possible method of measuring the energy of the radiation emitted in individuai cases and so testing theories such as that of Kramers as to the nature and origin of the continuous X-ray spectrum.

The Acceleration of β-particles in Strong Electric Fields such as those of Thunderclouds

Some thundercloud problems

The Sign and magnitude of the electric transferred from the atmosphere to the earth in lightning flashes are of interest, not only in themselves, but also as having to be taken into account in any attempt to test whether there is a balance in the interchange of electrical charges between the atmosphere and the surface of the earth. As is well known, the evidence at present available appears to show that on the whole an excess of positive electricity passes from the atmosphere to the earth as a result of conduction by ions and convection by charged raindrops. I am not aware of the existence of any data on which more than the roughest estimate of the magnitude of lightning discharges could be based. Schuster, starting with two determinations by Pockels of the maximum current, measured by means of its magnetising effect, and taking 1/1000 second as the duration of a flash, estimates the quantity brought down by a discharge as about 10 coulombs.

Charles Thomson Rees Wilson

Papers

Investigations on Lightning Discharges and on the Electric Field of Thunderstorms

Condensation of Water Vapour in the Presence of Dust-Free Air and Other Gases

The Acceleration of β-particles in Strong Electric Fields such as those of Thunderclouds

Some thundercloud problems

On some determinations of the sign and magnitude of electric discharges in lightning flashes