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Wallace E. Howell

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Wallace E. Howell

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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

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

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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

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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 and Manuel E. López

Abstract

With the most severe drought on record in its eighteenth month, causing severe damage and hardship in Puerto Rico, several large private concerns and government agencies interested in water decided that the immediate need for water greatly outweighed the uncertainties involved in cloud seeding, and collaborated to sponsor a program of rainfall stimulation. Operations of an emergency nature began 26 April and continued until 18 July 1965.

The result is evaluated by estimating the amount of rain that would have fallen if no seeding had been done, from the average rainfall per rainy day during a 19-year (unseeded) period. This analysis indicated an increase of 2.69 inches, equivalent to a 14 per cent increase nominally significant at the 10 per cent level.

No formal evaluation of the economic outcome is offered, but crop reports suggest that the cloud-seeding program returned value many times its cost, hence justifying the undertaking in the face of the uncertainties involved.

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