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C. D. Stow

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.

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J. Latham and C. D. Stow

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.

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S. Latham and C. D. Stow

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No abstract available.

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C. D. Stow and K. Jones

Abstract

A disdrometer for the measurement of the co-spectra of raindrop size and charge is described which can evaluate to a significant extent spurious data caused by unavoidable drop overlap within the charge detector volume. The sizes of individual drops in the range 0.1–2.5 mm radius and charges within the limits of magnitude 0.1–10 pC could be determined. The use of two size detectors enabled the measurement of drop velocity and the detection of drop overlap within the charge-sensitive volume to be made. Non-ideal data which arise from natural conditions, measuring sites, and from fundamental or unavoidable deficiencies in disdrometer design, could be tested and monitored automatically using a central processor under software control: TIME OUT, when a drop failed to pass through both size detectors; COINCIDENCE, caused by simultaneous occupation of both size detector volumes; SIZE MISMATCH, when the separate size measurements did not agree within limits predetermined by software; HIGH or LOW VELOCITY, when the actual drop velocity was not close to the terminal value expected from size measurement. The self-evaluating disdrometer cannot be designed to provide the minimum error content possible but offers the advantage of assessing the proportion of spurious data present; it is argued that this may be preferable to the situation in which the error content is lower but unknown. The performance of the instrument was assessed using individual drops generated in the laboratory and by exposure to natural rain falling through an aperture into a chamber shielded from wind and associated turbulence. The latter test was made at a non-ideal site in order to demonstrate the ability of the disdrometer to provide information on invalid data so that raw co-spectra may be corrected. In the preliminary tests described a substantial proportion of the drops possessed fall speeds significantly below their expected terminal velocity, in some cases as much as 30% less, and not more than 20% of the drops detected satisfied all criteria for acceptance. Further, an examination of drop arrival rates showed that not all data could be fitted to a Poisson-type distribution, either because of rapid changes in the mean arrival rate or on account of the clustering of drops. The potential seriousness of the drop overlap problem, which is fundamental to all methods of measurements, is emphasized in the trial analysis: uncertainties in the exact form of the size distribution, particularly for drops in the radius range 0.1–0.5 mm, render the design of any instruments of fixed entrance aperture size dubious; the co-spectra must be expected to show appreciable distortion unless data associated with drop overlap, particularly within the charge-sensitive volume, are excluded. Some improvements in the current disdrometer design are suggested.

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M. C. Woodward and C. D. Stow

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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S. G. Bradley and C. D. Stow

Abstract

A model for raindrop interactions incorporating satellite droplet production is used to determine drop size distributions at the ground. Assuming an exponential distribution at cloud base and using known meteorological conditions, the theoretical drop size distribution agrees well with distributions observed in the field; both predicted and observed distributions have reduced numbers of drops of radius near 0.35 mm. A time series analysis is facilitated by using a running-mean technique on each part of observed drop size distributions. A cross correlation is then performed between the time-series for large, medium and small drop concentrations. It is found that the smallest drops are associated in time with drops having a high satellite droplet production rate. On the other hand, the time delay at ground level between changes in large-drop concentration and small-drop concentration can be interpreted in terms of fallspeed differences. This allows the effective altitude of the active rain-producing region to be determined.

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J. G. Hosking and C. D. Stow

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.

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S. G. Bradley and C. D. Stow

Abstract

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S. G. Bradley and C. D. Stow

Abstract

An apparatus is described which enables reliable measurement of precipitation charge and size to be made. Design criteria are thoroughly treated and general solutions are provided to all aspects of the problem. The theories allow the performance of the apparatus in relation to drop splashing, drop overlap, and finite sampling time to be estimated with confidence. Full constructional details and performance of the apparatus and associated data storage equipment operating under strict laboratory conditions is given.

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S. G. Bradley and C. D. Stow

Abstract

Measurements of the size and charge on precipitation particles obtained using the apparatus described in Part I are given. Size data are found most frequencly to fit a log-normal distribution, and charge data a normal distribution; bi-modal distributions of charge and size data are also evident. Changes in the distribution parameters during the passage of frontal systems are calculated and correlations between drop size and drop charge examined for evidence of a systematic charging mechanism. Correlations between size and charge data and atmospheric potential gradient and prevailing meteorological conditions are also discussed.

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