James E. McDonald

DIRECT ABSORPTION OF SOLAR RADIATION BY ATMOSPHERIC WATER VAPOR

Abstract A review of Fowles solar-absorption studies reveals that no pressure corrections must be applied to the familiar Fowle band-absorptivity curves since these were obtained by methods that automatically corrected them to sea-level pressures. Using these absorptivities, along with recent solar-energy-distribution data, total insolational absorptivities are obtained. These are fitted (with relative accuracy of about 1 per cent over the range 0.3 to 8.0, precipitable water) to an exponential function similar to that used by Mugge and Moller to obtain working relations for use in calculating daily heating amounts due to insolational absorption by water vapor. It is found that variation of the pressure exponent from zero to unity in trial pressure-correction laws yields only a six-per-cent variation in total columnar absorption for a concrete case. Estimates accurate to within about ten per cent with respect to the Fowle absorption data are attainable for levels below about 400 mb in humid regions. Howe...

THE SHAPE AND AERODYNAMICS OF LARGE RAINDROPS

Abstract The physical factors which might be expected to control the shape of large raindrops are surface tension, hydrostatic pressures, external aerodynamic pressure, electrostatic charge, and internal circulation. Each of these is examined quantitatively, and it is concluded that under most conditions only the first three play important roles in producing the deformation characteristic of large raindrops. By analysis of high-speed photographs of water drops falling at terminal velocity, the distribution of aerodynamic pressures is deduced and is shown to imply that separation in the airflow about a raindrop has very significant effects on drop shape. The surface integral of the vertical components of the deduced aerodynamic pressures is found to be in reasonable agreement with the drop weight. The effect of boundary-layer separation on a number of physical processes occurring at the surface of falling drops is noted briefly.

HOMOGENEOUS NUCLEATION OF SUPERCOOLED WATER DROPS

Abstract The experimentally observed crystallization of supercooled water near −40C is examined in terms of the theory of homogeneous nucleation. The thermodynamic and molecular-kinetic nature, of the nucleation process is outlined to show why supercooling in natural clouds can occur so frequently. Past efforts to explain the −40C transition are examined critically, and are found to contain a number of significant errors. Because the theoretical nucleation rates are extremely sensitive to the numerical value of the specific surface free energy of a water-ice interface, particular attention is devoted to the refinement of previous estimates of this parameter. It is shown that both Krastanows and Masons estimates were inaccurate, and that in the latters approach, neglect of the distortion energy of the surface layer of ice led to a marked under-estimate of the nucleation efficiency which was concealed by the effects of several counteracting errors. Difficulties lying in the way of a direct calculation of...

ABSORPTION OF ATMOSPHERIC RADIATION BY WATER FILMS AND WATER CLOUDS

Abstract Spectrally averaged infrared absorptivities of thin water films are computed using recent spectrometric results. A water film 5 µ thick irradiated at normal incidence with 0C blackbody radiation is found to have a total absorptivity of 0.49, while, for thicknesses of 10, 50, and 100 µ, the absorptivities are 0.67, 0.92, and 0.95, respectively. Spectrally averaged reflectivity at normal incidence for bulk water is 0.04. Change in the shape of the blackbody emission curve with a change in temperature from 0C to 40C produces negligibly small effect on the spectrally averaged absorptivity and reflectivity. A simple bulk-water method yields infrared cloud absorption half-depths of 17 m for 0.25 g m−3 (and 5 m for 1 g m−3) liquid water content. Only 0.5 per cent is transmitted deeper than 400 m for 0.25 g m−3 or deeper than 100 m for 1 g m−3. Evidence for concluding that Mie-theory effects will tend to reduce these absorption depths to still lower values is presented.

AN INVESTIGATION OF THE MELANDER EFFECT

Abstract Melanders experimental results on the apparent production of salt nuclei by evaporation of salt solutions are examined both theoretically and experimentally. It is concluded theoretically that the rate of evaporative escape of molecular sodium chloride or other salts from sea water is too low by a factor of 1018 to account for the formation of the nuclei active in cloud condensation. It is shown experimentally that the rate of collection of salt particles of diameter in excess of 0.05 micron over evaporating salt solution is too low by a factor of at least 106 to account for the observed rate of atmospheric condensation. Therefore, there appears to be no basis for accepting Wrights suggestion that Melanders work provides an answer to Simpsons long-standing objection to the sea-salt hypothesis of condensation nuclei.

REMARKS ON "SEA SALT IN A TROPICAL STORM"

Abstract : Marshall and Palmer (1948) have shown that, for rains of a given intensity, there is a definite distribution curve of number of raindrops of a particular range of diameter. The writer has indicated here that the chlorimity of rains also varies with rain intensity. Recently obtained data, concerning atmospheric sea salt, are presented in the form of distribution curves. These curves show the number of sea-salt particles sampled at different altitudes, of a given weight range, plotted against the weight. A computation is made, using a salt-particle distribution curve obtained at cloud levels, in which water is added to each particle until it reaches an assumed chlorimity for a given rain intensity. Each particle is thereby increased in size and becomes a drop of a new weight. The distribution curves of these computed drops are compared to the observed distribution curves of Marshall and Palmer, for various rain intensities and are found to be remarkably similar. This result implies that in the process of growth, the droplets containing each salt particle grow to raindrop size through coalescence with much more numerous and relatively non- saline cloud droplets. The numbers of droplets in cumulus clouds over the sea are compared to the numbers of condensation nuclei in the sub-cloud layer and to the number of larger sea-salt particles. A method of sampling the large sparcely-distributed salt particles in the atmosphere is described briefly.