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J. A. Warburton

Abstract

Details are presented of the experimental processes involved in the concentration of silver ions from water. Monitoring procedures which lead to improvements in reliability in the techniques are described. The performance of the detection system using better quality ion-exchange resins, has been measured using the radio-isotopes Ag110 and Ba131 in dilute aqueous solutions. The lower limit of detection has been extended to ion-exchange column loadings of 4 × 10−3 gram of silver.

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J. A. Warburton

Abstract

Samples of rain and small hail have been analyzed for their silver content. The samples were collected during both seeded and unseeded storms in the project area of the 1968 Alberta Hail Studies Program. The natural background concentration of silver was determined as 5 × 10−12 gm ml−1. The precipitation from two storms, each seeded with 240 gm of AgI, contained silver up to 14 times this background value.

The circulation times for the silver iodide through the storms were determined. In one case, it was around 40 min; in the other two cases, it was around 70 min. This type of observation could be valuable in optimization of seeding techniques for hail suppression.

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J. A. Warburton

Abstract

A technique is being developed for detecting silver in very low concentrations in water. It is effective down to concentrations of 5 × 10−11 gm ml−l.

The method involves collecting and concentrating the silver ions on an ion-exchange column, precipitating the silver collected as AgI and measuring its quantity by a cold-chamber method.

The performance of the system has been measured using the radio-isotope Ag110 in the form of aqueous solutions of AgNO3. The column collects over 90 per cent of the silver ions from samples with concentrations greater than 10−11 gm ml−l. For column loadings down to 10−7 gram, the elution efficiency, using a strong cation (Ba++) as the eluting substance, is greater than 20 per cent.

When the technique was used on samples of rainwater collected in the field, no silver was detected except when the rain fell from clouds which had been seeded with AgI.

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J. A. Warburton and K. J. Heffernan

Abstract

Measurements have been made of the time lag which silver iodide particles exhibit in the nucleation of ice crystals at temperatures between −8C and −16C. The time tag is approximately exponential, the decay constants being 3.6 minutes and 1.4 minutes at −8.5C and −15.5C, respectively. The decay constant at −15.5C is less by a factor of 4.5 than that for natural ice nuclei. The observed time lag is in qualitative agreement with Fletcher's theory.

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J-P. Lacaux and J. A. Warburton

Abstract

In association with Grossversuch IV, a program designed to test the Soviet hail suppression method by seeding clouds with AgI from Oblako rockets, a complementary program was conducted by l'Observatoire du Puy-de-Dôme and the Desert Research Institute to study the diffusion of the seeding material (AgI) in the clouds, based on the analysis of silver in precipitation. This program covered the summers of 1977 and 1978, and this paper describes the results of measurements of natural background silver concentrations in unseeded precipitation. It also describes a new automatic precipitation collector, five of which were first tested in the field in 1977. A more extensive network of 15 collectors was deployed during two months of the 1978 summer.

Based on the analysis of 118 unseeded precipitation samples collected in 1977, the natural background concentration of silver was estimated as 0.9 × 10−11 g mL−1(σ = 0.6 × 10−11 g mL−1). Although the standard deviations overlap, the 1978 season results appear to indicate a lower background of 0.5 × 10−11 g mL−1 (σ = 0.3 × 10−11 g mL−1), based on the analysis of 414 rain samples. The average value for the two seasons was 0.6 × 10−11 g mL−1 with a standard deviation of 0.5 × 10−11 g mL−1. These background concentrations were found to be independent of both the length of sampling period and the precipitation intensity, averaged over the sampling periods of the collectors.

The background is sufficiently low to permit the detection of the presence of silver iodide emitted from the Soviet rockets in the precipitation. The preliminary results from one case study are presented to support this conclusion.

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G. O. Linkletter and J. A. Warburton

Abstract

Detailed analyses of three storms seeded in the National Hail Research Experiment (NHRE) during 1974 investigated the relationships between the occurrence of silver (from AgI) in precipitation and both the seeding activity and the storm histories as documented by radar, aircraft, the precipitation network and radiosondes.

Although the results show that the AgI was broadly dispersed when weak, poorly organized storms were seeded, it appears that the seeding agent was confined to only limited regions of the more vigorous storms which had well-defined internal circulation patterns. In several cases, it has been possible to use radar data to identify a specific cell within a storm which produced “seeded precipitation,” and thereby roughly define the pathways taken by the AgI aerosol after release from the aircraft.

Analyses have been performed on precipitation from 18 storms seeded by NHRE in the 1973 and 1974 seasons in order to determine the proportion of samples which contained “seeding” silver. In 1973, 50% of the samples contained silver significantly above the background concentration of 10−11 g ml−1. In 1974, the figure was 70%. However, by comparing the observed concentrations with those expected from theoretical predictions, the proportions of samples containing enough silver to represent a significant seeding effect became very low (probably <10%).

If the silver content of the precipitation is taken as an index of targetting effectiveness for modification of hail, then its “spotty” distributions, both temporally and spatially, suggest that there may be serious difficulties in assessing hail mass modification based on a statistical comparison of seed versus no-seed days.

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J. A. Warburton and L. G. Young

Abstract

Thermal neutron activation methods have been applied to the determination of silver content in samples of hail, rain and snow. Hail and rain samples were collected in South Dakota in regions where AgI cloud seeding was being conducted; the snow was collected in the eastern Sierra Nevada in an area where no such seeding was being conducted. No silver was detected in the snow samples analyzed, indicating concentrations on the average less than 2.5 × 10−11 gm ml−1. Eighty percent of the hail and rain samples analyzed contained measurable quantities of silver up to 70 times the minimum detectable amount. Some of these latter samples were collected in nominally unseeded areas, and although the chemical form in which the silver entered the precipitation is unknown—thereby casting doubt on its consequence—the observations raise important questions which deserve answers, particularly as they may affect statistical evaluations of weather modification experiments.

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T. E. Hoffer and J. A. Warburton

Abstract

The observed size of ice crystals obtained by sampling in Formvar is compared to the theoretical size an ice crystal should attain when growing by diffusion. The variables which are not fixed by environmental conditions are adjusted for the best fit. For reasonable values of these parameters the agreement between observation and calculation is good, except in the case where phloroglucinol was the nucleating agent.

The sizes of the ice crystals sampled after the same apparent time and under the same growth conditions varied over a range of 4 to 1; this range is discussed.

The observations, all in natural cloud conditions, are not in complete accord with diffusional growth. The differences are discussed with respect to weather modification.

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J. A. Warburton and L. G. Young

Abstract

Laboratory techniques have been developed for concentrating and isolating silver from aqueous samples. When these concentrated samples are activated in a thermal neutron flux of 4 × 1012n cm−2 sec−1, quantitative measurements down to 10−9 gm masses of silver can be made. When 1-liter samples of water are used, the lower limit of detection of silver in the sample is about 3 × 10−12 gm cm−3 concentration. Typical Ag concentrations being observed in snow collected in the western United States range from this lower limit to 10−8 gm cm−3, the majority being between 10−9 and 10−11. The lower limit does not therefore appear to be a serious factor in silver determinations in snow collected at ground stations. The system described is a non-destructive one, the accuracy of measurement using γ-ray spectrometry being 1, 2, 10 and 80% for masses of silver 10−6, 10−7, 10−8 and 10−9 gm, respectively. By using a 24-sec half-life radioisotope for the Ag determination, the cost of activation and turn-around time on samples is kept to a minimum. Data analysis is computerized and rapid.

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J. A. Warburton and C. T. Maher

Abstract

Measurements were made of the concentration of silver in precipitation samples collected in areas of Australia where cloud seeding with silver iodide has occurred. Silver was detected in 41 of 63 ‘seeded’ samples and in three of 23 ‘unseeded’ ones. Values of concentration of the silver were between 3 × 10−11 gm ml−1. There is some evidence that silver is detected more frequently when the precipitation falls from seeded clouds which have top temperatures <−15C.

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