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- Author or Editor: K. O. L. F. Jayaweera x
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Abstract
The growth rates, and the masses to which the crystals grow at different times, are calculated for various temperatures using the electrostatic analogy and assuming that the crystals follow the experimentally observed growth modes. The additional contribution to growth due to ventilation of the falling crystal is calculated. For small plate-like crystals and for all sizes of columns it is comparable with the growth by diffusion alone, while for larger plate-like crystals ventilation predominates.
Peaks in the growth rate occur at temperatures of about −5 and −15C as a consequence of the high axial ratio of the crystals characteristic of these temperatures (needles and dendrites, respectively). At other temperatures, calculated growth rates do not differ widely from those obtained by assuming a spherical ice particle.
Abstract
The growth rates, and the masses to which the crystals grow at different times, are calculated for various temperatures using the electrostatic analogy and assuming that the crystals follow the experimentally observed growth modes. The additional contribution to growth due to ventilation of the falling crystal is calculated. For small plate-like crystals and for all sizes of columns it is comparable with the growth by diffusion alone, while for larger plate-like crystals ventilation predominates.
Peaks in the growth rate occur at temperatures of about −5 and −15C as a consequence of the high axial ratio of the crystals characteristic of these temperatures (needles and dendrites, respectively). At other temperatures, calculated growth rates do not differ widely from those obtained by assuming a spherical ice particle.
Abstract
Measurements of terminal velocities of metal models of plate-like ice crystals in a tank containing various concentrations of glycerol in water suggest that the terminal velocities of a given model can be predicted from that of a disc made of the same material having the same mass and thickness. The terminal velocities of the models are less than that of the corresponding disc by a factor determined by the shape of the model. This factor is a maximum at 25% for the star-shaped model. These results are independent of the material of the disc or liquid and cover a Reynolds number range of at least 0.5 to 200; they are therefore applicable to plate-like ice crystals falling in air.
Abstract
Measurements of terminal velocities of metal models of plate-like ice crystals in a tank containing various concentrations of glycerol in water suggest that the terminal velocities of a given model can be predicted from that of a disc made of the same material having the same mass and thickness. The terminal velocities of the models are less than that of the corresponding disc by a factor determined by the shape of the model. This factor is a maximum at 25% for the star-shaped model. These results are independent of the material of the disc or liquid and cover a Reynolds number range of at least 0.5 to 200; they are therefore applicable to plate-like ice crystals falling in air.
Abstract
Single unrimed columnar ice crystals (>200 µm in length) from shallow layer clouds were collected in silicone oil, photographed under a microscope, and melted to determine their mass. These ice crystals were representative of those growing in a water sub-saturated environment in the temperature range −4 to −10C. The axial lengths of the crystals were related by the expression D = 6.3 L 0.437, where D and L are the lengths of the minor and major axes, expressed in micrometers, and have a nearly constant density of 0.3 gm cm−3. The habit of the ice crystals was very similar to those obtained in the water-droplet-free environment of laboratory diffusion chambers and was elementary or bundles of sheaths. Present observations suggest that bundles originate by the growth of secondary ice crystals on the prism face of the crystal. These secondary ice crystals then grow on the surface of the parent crystal and may grow at a rate faster than, and at the expense of, the parent crystal. The results of these observations were compared with those made on ice crystals growing in clouds supersaturated with respect to water.
Abstract
Single unrimed columnar ice crystals (>200 µm in length) from shallow layer clouds were collected in silicone oil, photographed under a microscope, and melted to determine their mass. These ice crystals were representative of those growing in a water sub-saturated environment in the temperature range −4 to −10C. The axial lengths of the crystals were related by the expression D = 6.3 L 0.437, where D and L are the lengths of the minor and major axes, expressed in micrometers, and have a nearly constant density of 0.3 gm cm−3. The habit of the ice crystals was very similar to those obtained in the water-droplet-free environment of laboratory diffusion chambers and was elementary or bundles of sheaths. Present observations suggest that bundles originate by the growth of secondary ice crystals on the prism face of the crystal. These secondary ice crystals then grow on the surface of the parent crystal and may grow at a rate faster than, and at the expense of, the parent crystal. The results of these observations were compared with those made on ice crystals growing in clouds supersaturated with respect to water.
Abstract
During the Alaskan cold season, the extreme low temperatures that prevail in the interior valleys are significantly modified by valley wind episodes. The infrared sensor on the NOAA polar orbiting satellite clearly detects this warming and delineates the exact area covered by valley winds. Satellite IR images were used in conjunction with surface and upper air maps, radiosonde data, and pilot reports to analyze two valley wind episodes during early 1975. Clear weather allowed excellent satellite viewing and strong temperature contrasts, because of strong radiational cooling in non-windy valley locations. In both cases a high-amplitude 500 mb ridge was moving into the interior from the west with a strong surface high centered cast of the windy area. The strength of the surface wind correlated well with the strength of the surface pressure gradient parallel to the valleys. The wind originated in the narrow sections of the valleys and consistently gusted to 45 kt in the Tanana Valley. The ability of the satellite to view these areas of warming and associated turbulence will alert forecasters to valley wind episodes where there are no conventional surface observations or pilot reports.
Abstract
During the Alaskan cold season, the extreme low temperatures that prevail in the interior valleys are significantly modified by valley wind episodes. The infrared sensor on the NOAA polar orbiting satellite clearly detects this warming and delineates the exact area covered by valley winds. Satellite IR images were used in conjunction with surface and upper air maps, radiosonde data, and pilot reports to analyze two valley wind episodes during early 1975. Clear weather allowed excellent satellite viewing and strong temperature contrasts, because of strong radiational cooling in non-windy valley locations. In both cases a high-amplitude 500 mb ridge was moving into the interior from the west with a strong surface high centered cast of the windy area. The strength of the surface wind correlated well with the strength of the surface pressure gradient parallel to the valleys. The wind originated in the narrow sections of the valleys and consistently gusted to 45 kt in the Tanana Valley. The ability of the satellite to view these areas of warming and associated turbulence will alert forecasters to valley wind episodes where there are no conventional surface observations or pilot reports.
Abstract
The very high resolution radiometer imagery from the NOAA-3 satellite is used to obtain the spatial and temporal distribution of thunderstorms in Alaska. Although the observations presented here are confined to only one summer, they show 1) the capability of NOAA-3 very high resolution radiometer imagery for thunderstorm studies, and 2) that the widespread thunderstorm development is the result of the interaction of large-scale motion with topography of certain areas in the interior of Alaska.
Abstract
The very high resolution radiometer imagery from the NOAA-3 satellite is used to obtain the spatial and temporal distribution of thunderstorms in Alaska. Although the observations presented here are confined to only one summer, they show 1) the capability of NOAA-3 very high resolution radiometer imagery for thunderstorm studies, and 2) that the widespread thunderstorm development is the result of the interaction of large-scale motion with topography of certain areas in the interior of Alaska.
Abstract
The results of the cloud seeding experiment in New England, Australia, have been stratified to show different effects of seeding on different daily rainfall totals. No effect was detected on days with an area average rainfall of less than 0.1 inch, which contributed about 21% to the total rainfall. Days with between 0.1 and 0.5 inch, contributing 45% to the total rain, showed increases of 10–20%. The 0.5–1.0 inch class, contributing about 22%, showed an apparent 10% decrease. There were too few cases to give a meaningful result for heavier falls.
Abstract
The results of the cloud seeding experiment in New England, Australia, have been stratified to show different effects of seeding on different daily rainfall totals. No effect was detected on days with an area average rainfall of less than 0.1 inch, which contributed about 21% to the total rainfall. Days with between 0.1 and 0.5 inch, contributing 45% to the total rain, showed increases of 10–20%. The 0.5–1.0 inch class, contributing about 22%, showed an apparent 10% decrease. There were too few cases to give a meaningful result for heavier falls.
Abstract
The ice-nucleating properties of cloud-seeding aerosols produced by burning a solution af AgI and NaI in acetone have been studied in a laboratory cloud chamber. The ice-forming ability of these mixed aerosols is not influenced as markedly by the supersaturation in the test cloud as is the case with “pure” AgI aerosols. This is ascribed to the hygroscopic nature of the particles which promotes drop formation at low supersaturations and allows the AgI to nucleate by a freezing mechanism. The relationship between nucleus size and nucleation temperature has been investigated and is compared with theory. Fletcher's theory, as recently modified to take account of the soluble component in mixed AgI aerosols,does not explain the rapid decrease in nucleation temperature with decreasing particle size and the wide size range of nuclei active at a particular temperature. A theory which assumes that nucleating sites of widely differing efficiencies are randonly distributed over the surface of the nucleating material is favored.
Abstract
The ice-nucleating properties of cloud-seeding aerosols produced by burning a solution af AgI and NaI in acetone have been studied in a laboratory cloud chamber. The ice-forming ability of these mixed aerosols is not influenced as markedly by the supersaturation in the test cloud as is the case with “pure” AgI aerosols. This is ascribed to the hygroscopic nature of the particles which promotes drop formation at low supersaturations and allows the AgI to nucleate by a freezing mechanism. The relationship between nucleus size and nucleation temperature has been investigated and is compared with theory. Fletcher's theory, as recently modified to take account of the soluble component in mixed AgI aerosols,does not explain the rapid decrease in nucleation temperature with decreasing particle size and the wide size range of nuclei active at a particular temperature. A theory which assumes that nucleating sites of widely differing efficiencies are randonly distributed over the surface of the nucleating material is favored.
