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Wallace E. Howell
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Wallace E. Howell

Recent completion of several research projects into weather modification impacts on the environment provides an opportunity for placing this subject in a new perspective. Studies of physical and biological processes relating precipitation and ecosystem changes show relatively few discernible effects, all of them minor in nature and magnitude. Direct effects of nucleating agents no longer appear consequential. Since no acute problems have surfaced, the focus is likely to shift to possible long-term effects on ecosystems as a whole, where changes associated with natural precipitation gradients and climatic fluctuations provide a model for those to be expected from precipitation management. The weakness of environmental impacts of weather modification compared to the consequences of other human actions renders it unlikely that these impacts will be decisive within a behavioral framework.

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Wallace E. Howell

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Wallace E. Howell
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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

A simple absorption meter has been built, by using an interference filter, which compares the intensity of incident light in a water-vapor absorption band and in a nearby “window” band. Experiments with the instrument show that it is sensitive to small differences in absorption not only from a direct sunbeam but also from diffuse light scattered by the sky or reflected from clouds. Further observations on clouds are suggested as perhaps being capable of useful interpretation.

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Wallace E. Howell and Manuel E. López

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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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Manuel E. López and Wallace E. Howell

Windstorms do great damage in banana plantations near the Sierra Nevada de Santa Marta in Colombia. These plantations, which owe their location to the rainfall maximum caused by convergence in the trade winds as they flow around the mountain massif, suffer most damage from the few most violent tempests and relatively little from the frequent milder squalls, offering the prospect that even slight mitigation of severe storms would be economically rewarding. Most damage was found to be triggered by easterly waves and similar large-scale disturbances, though the windstorms themselves remained purely local. Cloud seeding to attempt reduction of windstorm intensity was undertaken during the 1956 and 1957 seasons on the hypothesis that stimulation of showers early in the diurnal build-up of instability would dissipate some of the instability and reduce insolation at the ground, thus diminishing the intensity of later convective overturning. Comparison of damages during these seeded seasons with those of preceding and subsequent seasons showed a marked reduction in the ratio of severe windstorms to mild ones during the campaign and a reduction perhaps as much 39 per cent in damages, worth several million dollars annually.

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Manuel E. López and Wallace E. Howell

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Diurnal air tides that carry Pacific maritime air eastward through passes and over lower portions of the continental divide are remarkably similar over long stretches of the Andes from northern Chile to central Colombia, embracing both desert and jungle climates on the Pacific Coast. Even where the maritime air is at its warmest, it overflows the divide as a cold current producing katabatic flows down the east slope of the range and often producing hydraulic jump phenomena in the valleys immediately to the east. The significance of this pattern for the rainfall climate of western Colombia is discussed.

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Earl G. Droessler, Wallace E. Howell, Verner E. Suomi, and Helmut Weickmann
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