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Wallace E. Howell

Abstract

Aspects of the finding, by simulation, that historical/operational target/control evaluations of weather modification projects are radical appear questionable. Their form is simulated but not their content. The radical tendency has already been discounted in reviewing bodies. Means are available to reduce the degree of radicalism. Attention to these possibilities may well lead to cost-effective new information from past operational data.

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Wallace E. Howell

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No abstract available.

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Wallace E. Howell

Abstract

During the last ten 28-day cycles of Langmuir's periodic seeding experiment, beginning 15 October 1950, silver iodide smoke generators were operated one day each week from 1600 to 2400 LT on odd-numbered cycles and from 0800 to 1600 LT on even-numbered cycles, to see if daytime inactivation of the smoke would affect the outcome. However, the differences between nighttime-seeded and daytime-seeded cycles went unanalyzed. Analysis of previously published data now shows that the weekly periodicity of precipitation was greater during nighttime-seeded than during daytime-seeded cycles, especially in the block of analysis regions east-northeastward from the seeding site at Alamogordo, New Mexico. The same was true with respect to periodicity of temperature at the 70 kPa level over Omaha, Nebraska. Between nighttime-seeded and nonperiodic-seeded cycles there was a greater difference than between daytime-seeded and nonperiodic-seeded cycles. Whether these differences were caused by seeding remains unsettled. No accepted theory supports the hypothesis of effect, but if it were true it would be of immense importance.

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Wallace E. Howell

Abstract

A short account is given of seven cloud-seeding programs conducted in the summer and autumn of 1964 for drought relief in the Northeast, and evaluations of six of them by target-control regressions of normalized monthly data are presented. The results indicate increases varying from one to sixty per cent, averaging twenty-five per cent, nominally significant at the one per cent level. Circumstances such as non-randomization that compromise the evaluation are discussed. Note is also taken of an indicated fourteen per cent rainfall increase in 1964 in a hail-suppression target area where operations were recently suspended on account of a state law prohibiting cloud seeding on the grounds that it contributes to drought.

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Wallace E. Howell

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Wallace E. Howell

Abstract

A program of cloud seeding for rain stimulation has been carried on since September 1951, interrupted only by flood periods and the winter dry seasons, on the headwaters of the Rios Moche, Chicama and jequetepeque and adjacent parts of the continental divide between about 7S and 8S, using mainly silver iodide seeding from the ground. The climate is one of winter drought and summer convective rain resulting from complex interactions between the Pacific marine layer and the overlying easterlies. Compared with seasonal averages for twelve seasons before seeding began, twelve seeded seasons indicate a rainfall increase estimated at from 8 to 15 per cent. Conventional tests show significance at about the 2 per cent level but are rendered inconclusive by lack of prescribed experimental design and the fact that practical application is the primary mission of the program with experimental design having been subordinated. Criteria establishing the economic break-even point for continuance of the program are, however, amply exceeded.

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Wallace E. Howell

Abstract

Recent studies of precipitation, aircraft icing, and visibility through fog have focussed attention on the physical constitution of clouds, a subject to which knowledge of the drop-size spectrum and its origin would be an important contribution. The drop-size spectrum resulting when air containing condensation nuclei is uniformly cooled may be computed, leading to a differential equation for the growth of a cloud drop which cannot be integrated analytically. A numerical method of integration is therefore employed.

Three integrations of drop growth under simulated natural conditions are described and compared with data for natural clouds, leading to the following conclusions:

The computed drop-size spectra agree with observation regarding the relatively uniform clouds most frequently observed. Convergence and mixing of fine-grained turbulence are suggested as influences broadening the distribution in the less uniform clouds. Even for the latter, the computed spectra agree with observation on the shape of the spectrum curve. It is not clear what conditions of cooling most favor a broad spectrum. The computed mean drop sizes agree very well with observation, indicating that growth by collision is not ordinarily significant in uniform clouds.

The concentration of drops is determined primarily by the rate of cooling during the initial stage of condensation. It depends, as a rule, only slightly on the concentration of condensation nuclei. With continued uniform cooling, the drop concentration diminishes slightly toward a fixed constant value. It can be increased, once the cloud has formed, only through a great increase in the rate of cooling.

Supersaturation during cloud formation ordinarily reaches about 0.1 per cent and can surpass 1 per cent only under extreme circumstances. Subsaturations in descending air currents have the same order of magnitude. Observations of lower relative humidities in clouds, if real, must indicate clear spaces.

Operation of the equation of growth on nuclei of modal sizes proposed by Dessens cannot explain the modal drop sizes reported by Köhler.

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Wallace E. Howell

Abstract

Occurrences and quantity of rainfall in three areas in central Cuba are related to accompanying large-scale weather patterns, to winds at three levels in the atmosphere, and to the lapse rates. The relationships are described, with comments as to causes of them.

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Wallace E. Howell

Abstract

No Abstract Available

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Wallace E. Howell

Abstract

No abstact available.

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