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- Author or Editor: C. D. Stow x
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The destructive nature of cloud-to-ground lightning strokes is well known. Loss of life and damage to buildings and other man-made structures may to a large extent be prevented by the judicial use of lightning conductors and screens but no comparable protection may be offered to expanses of agricultural crops or forests. According to Fuquay (1967) lightning is the greatest single cause of forest fires in the western United States: during the period 1946–1962, 140,000 such fires occurred causing severe losses of timber, wildlife, watershed, and recreational resources. Comparable losses occur regularly in other parts of the world. The only solution is the suppression or modification of cloud-to-ground lightning discharges. Methods of suppression are described, some of which may turn out to be practical ways of achieving this aim.
The destructive nature of cloud-to-ground lightning strokes is well known. Loss of life and damage to buildings and other man-made structures may to a large extent be prevented by the judicial use of lightning conductors and screens but no comparable protection may be offered to expanses of agricultural crops or forests. According to Fuquay (1967) lightning is the greatest single cause of forest fires in the western United States: during the period 1946–1962, 140,000 such fires occurred causing severe losses of timber, wildlife, watershed, and recreational resources. Comparable losses occur regularly in other parts of the world. The only solution is the suppression or modification of cloud-to-ground lightning discharges. Methods of suppression are described, some of which may turn out to be practical ways of achieving this aim.
Abstract
The behavior of water drops of diameter in excess of 3 mm suspended in a vertical wind tunnel is described and the vertical-position stability quantitatively and qualitatively assessed. The contribution to vertical instability caused by measured turbulence is calculated but does not fully account for the degree of instability observed. It is suggested that an inherent instability arises from the airflow constraints required to maintain a drop in the working section of the tunnel. When a drop is retained by such constraints, the artificial conditions under which experiments are performed will limit the applicability of results obtained in the wind tunnel to the atmosphere.
Abstract
The behavior of water drops of diameter in excess of 3 mm suspended in a vertical wind tunnel is described and the vertical-position stability quantitatively and qualitatively assessed. The contribution to vertical instability caused by measured turbulence is calculated but does not fully account for the degree of instability observed. It is suggested that an inherent instability arises from the airflow constraints required to maintain a drop in the working section of the tunnel. When a drop is retained by such constraints, the artificial conditions under which experiments are performed will limit the applicability of results obtained in the wind tunnel to the atmosphere.
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No abstract available.
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Abstract
Experiments showed that, as an ice specimen evaporated by exposure to a stream of dry nitrogen, a temperature gradient was created in the surface of the specimen, which became electrically charged. The charge was positive if the surface of the specimen was warmer than its interior, and negative if it was colder. A typical result was that a specimen of surface area 37 cm2 and internal temperature −20C exposed to a nitrogen stream of temperature −30C and velocity 15 cm sec−1 acquired a temperature gradient of magnitude 90C cm−1 in its surface and became negatively charged at a rate i=3.3 × l0−4 esu of current. These observations are explicable qualitatively in terms of the Latham-Mason theory of charge transfer associated with temperature gradients in ice. Calculations indicate that it cannot be determined whether quantitative agreement exists until measurements have been made of the distribution of temperature and charge within the volume of ice specimens along which temperature gradients exist. An estimate is also made of the importance of this evaporation process in generating electric charge in the atmosphere.
Abstract
Experiments showed that, as an ice specimen evaporated by exposure to a stream of dry nitrogen, a temperature gradient was created in the surface of the specimen, which became electrically charged. The charge was positive if the surface of the specimen was warmer than its interior, and negative if it was colder. A typical result was that a specimen of surface area 37 cm2 and internal temperature −20C exposed to a nitrogen stream of temperature −30C and velocity 15 cm sec−1 acquired a temperature gradient of magnitude 90C cm−1 in its surface and became negatively charged at a rate i=3.3 × l0−4 esu of current. These observations are explicable qualitatively in terms of the Latham-Mason theory of charge transfer associated with temperature gradients in ice. Calculations indicate that it cannot be determined whether quantitative agreement exists until measurements have been made of the distribution of temperature and charge within the volume of ice specimens along which temperature gradients exist. An estimate is also made of the importance of this evaporation process in generating electric charge in the atmosphere.
Abstract
The effect of individual charge-modifying processes is tested by following the development joint distributions of raindrop size and charge through a fall depth of 1 km beneath a weakly electrified warm cloud. Evaporation, ion capture and coalescence all cause the small-drop charge distribution to become narrower and any initially skewed distribution to become more symmetrical. Ion capture does not result in substantial negative charging unless the initial charge distribution is very narrow. Ion diffusion charging is found not to be significant.
Abstract
The effect of individual charge-modifying processes is tested by following the development joint distributions of raindrop size and charge through a fall depth of 1 km beneath a weakly electrified warm cloud. Evaporation, ion capture and coalescence all cause the small-drop charge distribution to become narrower and any initially skewed distribution to become more symmetrical. Ion capture does not result in substantial negative charging unless the initial charge distribution is very narrow. Ion diffusion charging is found not to be significant.
Abstract
Raindrop collisional breakup is included in a model of joint size-charge distribution development. Consistent with experimental observations the model includes a finite filament (joining separating drops) that upon rupture produces charged satellite droplets. It is shown that for positive fields, large numbers of small drops carry negative charge, leading to an increasingly negative space charge with fall depth. The model also suggests that larger separated charge exists on the major drop products than previously estimated.
Abstract
Raindrop collisional breakup is included in a model of joint size-charge distribution development. Consistent with experimental observations the model includes a finite filament (joining separating drops) that upon rupture produces charged satellite droplets. It is shown that for positive fields, large numbers of small drops carry negative charge, leading to an increasingly negative space charge with fall depth. The model also suggests that larger separated charge exists on the major drop products than previously estimated.
Abstract
No abstract available.
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No abstract available.
Abstract
Typical results of field measurements of the fallspeeds of natural raindrops are presented and instrumental effects investigated by numerically simulating the measurement processes. Deviations between the measured fallspeeds and those expected on the basis of theory and windtunnel results are smaller than previous investigators have found, probably because of the improved instrumentation used. The random component of these deviations is consistent with estimated experimental error, but on average drops are found to fall about 5%–10% slower than expected, and this mean deviation increased with drop size; these data are consistent with the existence of local updrafts and downdrafts. The effects of the observed fallspeed fluctuations on estimates of other quantities at ground level (e.g., drop radius) are considered and, in some cases, shown to be significant.
Abstract
Typical results of field measurements of the fallspeeds of natural raindrops are presented and instrumental effects investigated by numerically simulating the measurement processes. Deviations between the measured fallspeeds and those expected on the basis of theory and windtunnel results are smaller than previous investigators have found, probably because of the improved instrumentation used. The random component of these deviations is consistent with estimated experimental error, but on average drops are found to fall about 5%–10% slower than expected, and this mean deviation increased with drop size; these data are consistent with the existence of local updrafts and downdrafts. The effects of the observed fallspeed fluctuations on estimates of other quantities at ground level (e.g., drop radius) are considered and, in some cases, shown to be significant.
Abstract
Data derived from an experimental investigation of the drop collision problem are described in which the numbers of satellite droplets per collision N g was found. 1026 separate collisions were observed of which 266 resulted in coalescence without breakup, and at least 405 produced the so-called satellite droplets. The value of N g varied from zero to seven, with average values N¯ g proving to be a linear function of the collision speed U, where N¯ g could be predicted by the relationship N¯ g=3.5 (U−1) if U>1 m s−1. For impact speeds of less than 1 m s−1, negligible number of satellites were produced. It proved impossible to correlate N¯ g with either the impact parameter or the drop charge, both of which could be accurately controlled, but there appeared to be a dependence of N¯ g on both a dimensionless parameter of rotational energy and a dimensionless parameter of electrostatic energy for a given collision. The statistical nature of the data is emphasized in relation to computations on raindrop distributions.
Abstract
Data derived from an experimental investigation of the drop collision problem are described in which the numbers of satellite droplets per collision N g was found. 1026 separate collisions were observed of which 266 resulted in coalescence without breakup, and at least 405 produced the so-called satellite droplets. The value of N g varied from zero to seven, with average values N¯ g proving to be a linear function of the collision speed U, where N¯ g could be predicted by the relationship N¯ g=3.5 (U−1) if U>1 m s−1. For impact speeds of less than 1 m s−1, negligible number of satellites were produced. It proved impossible to correlate N¯ g with either the impact parameter or the drop charge, both of which could be accurately controlled, but there appeared to be a dependence of N¯ g on both a dimensionless parameter of rotational energy and a dimensionless parameter of electrostatic energy for a given collision. The statistical nature of the data is emphasized in relation to computations on raindrop distributions.
Abstract
Examination of the arrival-rate distribution of raindrops from disturbed, post-cold-front air masses using the statistic k, the ratio of the variance of the drop rate to the mean, shows strong deviations from Poisson behavior toward clustering of drops. Clustering occurs predominantly for drops smaller than about 0.25 mm radius and correlates strongly with the small-drop arrival rate and, to a lesser extent, with rainfall intensity but does not appear to be caused by drop splashing, breakup or local turbulence. There is evidence to suggest that, if rapid intensity fluctuations are causing clustering, these fluctuations occur with a characteristic period of less than 10 s.
Abstract
Examination of the arrival-rate distribution of raindrops from disturbed, post-cold-front air masses using the statistic k, the ratio of the variance of the drop rate to the mean, shows strong deviations from Poisson behavior toward clustering of drops. Clustering occurs predominantly for drops smaller than about 0.25 mm radius and correlates strongly with the small-drop arrival rate and, to a lesser extent, with rainfall intensity but does not appear to be caused by drop splashing, breakup or local turbulence. There is evidence to suggest that, if rapid intensity fluctuations are causing clustering, these fluctuations occur with a characteristic period of less than 10 s.
