Search Results
Abstract
A series of twelve releases of phloroglucinol were made into stratus clouds at temperatures of −7C to −17C.
Showers produced by dry ice seeding were used to identify particular spots in the layer clouds from which the exact locations of the phloroglucinol releases could be obtained by simple navigation. Visual observations of the cloud behavior and Formvar replicas of cloud and precipitation particles provided a means for judging the effects of the phloroglucinol.
It is concluded that phloroglucinol will induce the formation of ice in undercooled clouds. However, in these experiments, it was not nearly as effective as the dry ice in causing shower formation.
Abstract
A series of twelve releases of phloroglucinol were made into stratus clouds at temperatures of −7C to −17C.
Showers produced by dry ice seeding were used to identify particular spots in the layer clouds from which the exact locations of the phloroglucinol releases could be obtained by simple navigation. Visual observations of the cloud behavior and Formvar replicas of cloud and precipitation particles provided a means for judging the effects of the phloroglucinol.
It is concluded that phloroglucinol will induce the formation of ice in undercooled clouds. However, in these experiments, it was not nearly as effective as the dry ice in causing shower formation.
Abstract
An AN/TPS-10 radar, located at the Institute of Atmospheric Physics, University of Arizona, has been used to make extensive measurements of radar returns from cumulus clouds in the vicinity of Tucson. Data from ten days in the summer of 1955 have been analyzed with a view toward establishing the level of first formation of precipitation, day-to-day variation, average dimensions of first echo, average duration, and fraction reaching ground. Strong day-to-day variations and mountain effects are revealed. Although echoes form much more frequently over mountains than over nearby valleys, these echoes individually are less likely to produce rain at the ground.
Abstract
An AN/TPS-10 radar, located at the Institute of Atmospheric Physics, University of Arizona, has been used to make extensive measurements of radar returns from cumulus clouds in the vicinity of Tucson. Data from ten days in the summer of 1955 have been analyzed with a view toward establishing the level of first formation of precipitation, day-to-day variation, average dimensions of first echo, average duration, and fraction reaching ground. Strong day-to-day variations and mountain effects are revealed. Although echoes form much more frequently over mountains than over nearby valleys, these echoes individually are less likely to produce rain at the ground.
Abstract
Recent observations indicate that ice pellets and snow pellets are present in most convective clouds in the Central United States by the time these clouds reach top temperatures of −10C. The attendant circumstances raise the question of whether the ice plays an active role in rain development in these clouds or whether its presence is purely incidental. The ice pellets are usually preceded by the development of liquid precipitation particles large enough to produce rain by coalescence with cloud droplets. The pellet concentrations are not related to ground-level ice nuclei concentrations. Apparently the pellets form as a result of freezing of the drops, contrary to most laboratory studies of droplet freezing. Observations can be brought into harmony by invoking the droplet splintering measurements of Mason and Maybank. The presence of numerous small ice particles in these clouds at temperatures warmer than −10C casts doubt upon the value of seeding with ice nuclei for rain inducement.
Abstract
Recent observations indicate that ice pellets and snow pellets are present in most convective clouds in the Central United States by the time these clouds reach top temperatures of −10C. The attendant circumstances raise the question of whether the ice plays an active role in rain development in these clouds or whether its presence is purely incidental. The ice pellets are usually preceded by the development of liquid precipitation particles large enough to produce rain by coalescence with cloud droplets. The pellet concentrations are not related to ground-level ice nuclei concentrations. Apparently the pellets form as a result of freezing of the drops, contrary to most laboratory studies of droplet freezing. Observations can be brought into harmony by invoking the droplet splintering measurements of Mason and Maybank. The presence of numerous small ice particles in these clouds at temperatures warmer than −10C casts doubt upon the value of seeding with ice nuclei for rain inducement.
Abstract
A quantitative estimate, based upon data from the Thunderstorm Project, is made of the various water sources and sinks and the energy sources and sinks of an average thunderstorm cell. It is found that a large fraction of the water carried into the storm in vapor form is used to maintain the cold downdraft inside the storm, and that the energy restratification accompanying this downdraft is a major source of energy to the storm.
From the estimates of the water and energy budgets, the following hypothesis is tested by means of radar measurements: the net energy available (energy sources minus energy sinks) is related to the amount of development of convective rainstorms.
Abstract
A quantitative estimate, based upon data from the Thunderstorm Project, is made of the various water sources and sinks and the energy sources and sinks of an average thunderstorm cell. It is found that a large fraction of the water carried into the storm in vapor form is used to maintain the cold downdraft inside the storm, and that the energy restratification accompanying this downdraft is a major source of energy to the storm.
From the estimates of the water and energy budgets, the following hypothesis is tested by means of radar measurements: the net energy available (energy sources minus energy sinks) is related to the amount of development of convective rainstorms.
Abstract
Data from Thunderstorm Project observations in Florida, derived mainly from airplane traverses through thunderstorms, are analyzed in order to obtain a description of thunderstorm structure and circulation. Thunderstorms are normally found to consist of several more-or-less independent convective systems or “cells”. Each cell goes through a life cycle represented by three fairly distinct stages-the cumulus stage, the mature stage, and the dissipating stage.
In the cumulus stage the cell is formed from an updraft of air which, as in the other stages, “entrains” air from the environment. In this stage no rain has yet reached the ground. In the mature stage, rain is occurring and a large part of the cell consists of a downdraft which characterizes the rain area. The updraft continues in a portion of the cell in the low and intermediate levels and in all parts of the top levels. In the dissipating stage, downdrafts are present throughout, although weak upward motion is apparent in the upper parts.
The thunderstorm of 9 July 1946 is taken as typical of the structure and vertical currents that have been described. Radar photographs and data from the surface micronetwork are used in substantiation of the airplane findings. Strong horizontal divergence at the surface in the downdraft and rain areas and convergence under the updrafts are shown.
From the abundant upper-air wind and temperature-humidity soundings made with balloons in and around the thunderstorms, data from the various sources are combined to study the thermodynamics involved in the circulation. The entrainment of air into the cells has an important effect which by actual measurement and introduction of reasonable values of entrainment satisfactorily accounts for the observed air currents. With entrainment, the updraft lapse rate approaches that of the environment. It is then demonstrated that it is possible for falling rain to “trigger” a downdraft of cold air that reaches and spreads out over the surface of the earth as the cold core of the rain area.
Abstract
Data from Thunderstorm Project observations in Florida, derived mainly from airplane traverses through thunderstorms, are analyzed in order to obtain a description of thunderstorm structure and circulation. Thunderstorms are normally found to consist of several more-or-less independent convective systems or “cells”. Each cell goes through a life cycle represented by three fairly distinct stages-the cumulus stage, the mature stage, and the dissipating stage.
In the cumulus stage the cell is formed from an updraft of air which, as in the other stages, “entrains” air from the environment. In this stage no rain has yet reached the ground. In the mature stage, rain is occurring and a large part of the cell consists of a downdraft which characterizes the rain area. The updraft continues in a portion of the cell in the low and intermediate levels and in all parts of the top levels. In the dissipating stage, downdrafts are present throughout, although weak upward motion is apparent in the upper parts.
The thunderstorm of 9 July 1946 is taken as typical of the structure and vertical currents that have been described. Radar photographs and data from the surface micronetwork are used in substantiation of the airplane findings. Strong horizontal divergence at the surface in the downdraft and rain areas and convergence under the updrafts are shown.
From the abundant upper-air wind and temperature-humidity soundings made with balloons in and around the thunderstorms, data from the various sources are combined to study the thermodynamics involved in the circulation. The entrainment of air into the cells has an important effect which by actual measurement and introduction of reasonable values of entrainment satisfactorily accounts for the observed air currents. With entrainment, the updraft lapse rate approaches that of the environment. It is then demonstrated that it is possible for falling rain to “trigger” a downdraft of cold air that reaches and spreads out over the surface of the earth as the cold core of the rain area.
Abstract
Observations of precipitation and cloud-top height in the central United States and in the Caribbean area, obtained from radar-equipped airplanes, have been analyzed in terms of the fraction of clouds of a given height which contains precipitation. These data are compared with observations of a similar type taken by Braham, Reynolds and Harrell in New Mexico. It is concluded that the condensation-coalescence process can account for the formation of precipitation in convective clouds in all three regions but in varying proportions of clouds.
Abstract
Observations of precipitation and cloud-top height in the central United States and in the Caribbean area, obtained from radar-equipped airplanes, have been analyzed in terms of the fraction of clouds of a given height which contains precipitation. These data are compared with observations of a similar type taken by Braham, Reynolds and Harrell in New Mexico. It is concluded that the condensation-coalescence process can account for the formation of precipitation in convective clouds in all three regions but in varying proportions of clouds.
Abstract
As a means of studying ice nucleation, snow and ice pellets collected from the tops of clouds were melted and refrozen in order to determine their freezing temperatures. In all cases where a definite cloud top temperature could be assigned, the melted ice pellets froze at a temperature colder than that of the cloud top, indicating that these pellets did not originate through the heterogeneous freezing of cloud drops. Essentially no difference was indicated in the freezing temperatures of ice pellets collected on seeded and non-seeded days. A firm statement on this point could not be made as the number of observations is limited, and it is not certain that the seeding agent had been ingested into the cloud being studied.
Abstract
As a means of studying ice nucleation, snow and ice pellets collected from the tops of clouds were melted and refrozen in order to determine their freezing temperatures. In all cases where a definite cloud top temperature could be assigned, the melted ice pellets froze at a temperature colder than that of the cloud top, indicating that these pellets did not originate through the heterogeneous freezing of cloud drops. Essentially no difference was indicated in the freezing temperatures of ice pellets collected on seeded and non-seeded days. A firm statement on this point could not be made as the number of observations is limited, and it is not certain that the seeding agent had been ingested into the cloud being studied.
Abstract
The concentrations of salt particles over central Illinois are being monitored through the use of a sodium flame-counter and an impactor. The number of large particles as determined by the impactor has been found to average about two per cubic meter, and high concentrations of the magnitude found over oceans have never been detected. It is concluded that these large salt particles are not found over mid-continents in sufficient numbers to initiate precipitation through a process of condensation and coalescence unless the chain-reaction process is also operative. There is no evidence that these large particles are necessarily carried inland from an oceanic source region.
Abstract
The concentrations of salt particles over central Illinois are being monitored through the use of a sodium flame-counter and an impactor. The number of large particles as determined by the impactor has been found to average about two per cubic meter, and high concentrations of the magnitude found over oceans have never been detected. It is concluded that these large salt particles are not found over mid-continents in sufficient numbers to initiate precipitation through a process of condensation and coalescence unless the chain-reaction process is also operative. There is no evidence that these large particles are necessarily carried inland from an oceanic source region.
Abstract
Precipitation particle measurements from the upper levels of cumulus congestus clouds are analyzed with regard to general cloud characteristics, liquid water content, and precipitation water content as related to the theoretical radar reflectivity. Conclusions are: (1) the majority of the cumulus congestus clouds examined, whose tops exceed 14,000 ft, contained precipitation particles (250-microns diameter) in the upper levels sometime during their life cycle, (2) particle concentrations in excess of 1000 per m3 were found in about 20 per cent of the clouds examined, (3) the relationship Z=1.6×10−2M1.46 for radar reflectivity is applicable for cumulus congestus in the early stages of precipitation development.
Abstract
Precipitation particle measurements from the upper levels of cumulus congestus clouds are analyzed with regard to general cloud characteristics, liquid water content, and precipitation water content as related to the theoretical radar reflectivity. Conclusions are: (1) the majority of the cumulus congestus clouds examined, whose tops exceed 14,000 ft, contained precipitation particles (250-microns diameter) in the upper levels sometime during their life cycle, (2) particle concentrations in excess of 1000 per m3 were found in about 20 per cent of the clouds examined, (3) the relationship Z=1.6×10−2M1.46 for radar reflectivity is applicable for cumulus congestus in the early stages of precipitation development.
Abstract
Precipitation particles in the 100- to 6.50-µ-diam range were sampled in a large number of tropical convective clouds. These samples permit one to trace the development of precipitation in these clouds. Liquid-water-content measurements were made simultaneously with some of the particle measurements. From these data, it is shown that the large concentrations of large drops are associated with low liquid-water contents and, conversely, that the large values of liquid water are associated with small numbers of droplets greater than 150 µ in diam. The computed relationship between radar reflectivity, water-content, and median-volume diameter is very similar to that which has been reported for other cloud types.
Abstract
Precipitation particles in the 100- to 6.50-µ-diam range were sampled in a large number of tropical convective clouds. These samples permit one to trace the development of precipitation in these clouds. Liquid-water-content measurements were made simultaneously with some of the particle measurements. From these data, it is shown that the large concentrations of large drops are associated with low liquid-water contents and, conversely, that the large values of liquid water are associated with small numbers of droplets greater than 150 µ in diam. The computed relationship between radar reflectivity, water-content, and median-volume diameter is very similar to that which has been reported for other cloud types.
