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Thomas J. Henderson

Abstract

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Thomas J. Henderson

Abstract

For 30 years the field of weather modification has been struggling through a labyrinth of scientific, legal, societal, economic, legislative and operational paths which have produced a continuing controversy on the propriety of operations, the credibility of stated results, and the priorities of research. Such is the nature of our quest for knowledge and the ultimate application of new ideas. Weather modification is unique in that it deals with infinitely changing atmospheric parameters which are more difficult to predict and measure than the unknowns in most other disciplines. If a subject is poorly understood, it is always poorly explained and in the end more rigorously questioned by the uninformed. The present status of weather modification should be neither surprising nor discouraging to the point of rejection.

This paper explores some of the major problems at the operational level and discusses the interactions between applications and the other facets of our society which influence those engaged in operations. Some specific suggestions are presented on how we might find our way from the labyrinth to a clear path in the future.

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Thomas J. Henderson

Abstract

In 1954 a cloud seeding program designed to increase rainfall and snowpack was initiated over the water-shed of the Kings River in the Sierra Range of California. The project has been funded by the Kings River Conservation District, Fresno, California, and operated continuously each season during the 7-month October–April periods. At the end of the first three-year period, a multiple regression analysis was developed utilizing the unregulated historic flow of the Kings River and the flow of adjacent rivers presumed to be unaltered by cloud seeding activities. This statistical analysis has been applied to the flow of the rivers. During the ten-year seeded period 1954–1964, the analysis shows an apparent increase in flow amounting to 6 per cent of the total predicted by the recession analysis. This apparent increase is significant at the 0.005 level.

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Guy G. Goyer
,
Lewis O. Grant
, and
Thomas J. Henderson

Abstract

Weathereord, a 40-grain detonating fuse containing about 20 per cent of silver iodide, has been evaluated as a cloud seeding nuclei generator in the laboratory, in the field, and in aircraft cloud seeding. Comparative data on the output efficiencies of several types of silver iodide generators are presented and show that, as a dispersal system, Weathercord provides in unit time and unit volume the highest concentration of nuclei available from any known source. The tests in supercooled logs at Yellowstone National Park and two test cases of the seeding from aircraft of orographic cumuli are also described. Although of a preliminary nature these field tests suggest the effectiveness of the large concentrations of AgI nuclei, generated by Weathercord, in modifying relatively thin supercooled clouds.

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William L. Woodley
,
Thomas J. Henderson
,
Bernard Vonnegut
,
Glenn Gordon
,
Robert Breidenthal
, and
Shirley M. Holle

Abstract

This paper presents the results of studies of aircraft-produced ice particles (APIPs) in supercooled fog over Mono Lake, California. The King Air 200T cloud physics aircraft of the University of Wyoming and three other aircraft (a Piper Aztec, a Cessna 421-C, and a T-28) were involved in the tests. The King Air served as the monitoring aircraft when the other aircraft were tested and as both the test and monitoring aircraft when it was tested.

The studies demonstrated that the King Air produces APIPs. The ice crystals, in concentrations up to several hundred per liter, are initially quite small and of almost uniform size, and they grow to larger nearly uniform sizes with time. APIPs production is most likely at low ambient temperatures and high power settings, and when the gear and flaps are extended.

APIPs were not detected from the other aircraft. The Piper Aztec and Cessna 421 aircraft were tested on days on which an APIPs signature was produced by the King Air. The T-28 aircraft was tested when the fog-top temperature was greater than − 6°C and neither the T-28 nor the King Air produced APIPs under these conditions.

Homogeneous nucleation appears to be responsible for the observed APIPs signature, although the exact mechanism for nucleation is not known. In addition, there is the suggestion that a weaker APIPs signature may be generated by heterogeneous nucleation, when the cooling in the prop-tip vortex falls short of that thought necessary for homogeneous nucleation (i.e., ∼ − 39°C).

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William L. Woodley
,
Glenn Gordon
,
Thomas J. Henderson
,
Bernard Vonnegut
,
Daniel Rosenfeld
, and
Andrew Detwiler

Abstract

This paper presents new results from studies of aircraft-produced ice particles (APIPs) in supercooled fog and clouds. Nine aircraft, including a Beech King Air 200T cloud physics aircraft, a Piper Aztec, a Cessna 421-C, two North American T-28s, an Aero Commander, a Piper Navajo, a Beech Turbo Baron, and a second four-bladed King Air were involved in the tests. The instrumented King Air served as the monitoring aircraft for trails of ice particles created, or not created, when the other aircraft were flown through clouds at various temperatures and served as both the test and monitoring aircraft when it itself was tested. In some cases sulfur hexafluoride (SF6) gas was released by the test aircraft during its test run and was detected by the King Air during its monitoring passes to confirm the location of the test aircraft wake. Ambient temperatures for the tests ranged between −5° and −12°C. The results confirm earlier published results and provide further insights into the APIPs phenomenon. The King Air at ambient temperatures less than −8°C can produce APIPs readily. The Piper Aztec and the Aero Commander also produced APIPs under the test conditions in which they were flown. The Cessna 421, Piper Navajo, and Beech Turbo Baron did not. The APIPs production potential of a T-28 is still indeterminate because a limited range of conditions was tested. Homogeneous nucleation in the adiabatically cooled regions where air is expanding around the rapidly rotating propeller tips is the cause of APIPs. An equation involving the propeller efficiency, engine thrust, and true airspeed of the aircraft is used along with the published thrust characteristics of the propellers to predict when the aircraft will produce APIPs. In most cases the predictions agree well with the field tests. Of all of the aircraft tested, the Piper Aztec, despite its small size and low horsepower, was predicted to be the most prolific producer of APIPs, and this was confirmed in field tests. The APIPs, when they are created, appear in aircraft wakes in concentrations up to several hundred per liter, which are initially very small and almost uniform in size but grow to larger nearly uniform sizes with time. APIPs production is most likely at low ambient temperatures when an aircraft is flown at maximum power with the gear and flaps extended, resulting in a relatively low airspeed under high-drag conditions. It is predicted that APIPs production of an aircraft can be decreased or eliminated altogether by using a propeller with a larger number of propeller blades, such that the engine thrust is distributed over more blades, thereby decreasing the cooling on each blade. Plans to test this hypothesis using three- and four-bladed King Airs as the test aircraft never came to fruition because of unsatisfactory weather conditions. It is likely that APIPs have confounded the results of some past cloud microphysical investigations, especially those in which repeat passes were made through individual clouds under heavy icing conditions by aircraft known now to be APIPs producers. Aircraft flying under such conditions are forced to use high power settings to overcome the drag of a heavy ice load. These are the conditions that field tests demonstrate are most conducive to the production of APIPs. In these situations, APIPs may have led investigators to conclude that there was more rapid development of ice, and higher concentrations of ice particles in clouds, than actually was the case.

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