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Seeding Tests on Supercooled Stratus Using Vertical Fall Pyrotechnics

Joe L. SutherlandNorth American Weather Consultants, Salt Lake City, UT 84117

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John R. ThompsonNorth American Weather Consultants, Salt Lake City, UT 84117

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Don A. GriffithNorth American Weather Consultants, Salt Lake City, UT 84117

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Bruce KunkelAir Force Geophysics Laboratory, Hanscom AFB, MA 01731

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Abstract

In Michigan in early 1977, an experiment was conducted to test the ability of silver iodide (AgI) ice nucleus curtains generated by vertical-fall pyrotechnics to produce clearings in supercooled stratus. A second objective of the experiment was to determine how well a clearing could be targeted over a preselected ground location. Previous stratus clearing tests had primarily involved curtains of dry ice particles or horizontal lines of AgI nuclei. Silver iodide pyrotechnics were chosen because of their logistical advantages over dry ice.

Results of the Michigan testing were favorable. Clearings were produced in cloud decks up to 1400 m thick and as warm as −8°C. In thicker cloud decks, glaciation occurred only to a depth equal to the fall distance of the pyrotechnics. There were indications of “overseeding” from the relatively poor visibility through the cleared area that likely was caused by high ice-crystal concentrations. Targeting was successful when accurate wind data were available.

Abstract

In Michigan in early 1977, an experiment was conducted to test the ability of silver iodide (AgI) ice nucleus curtains generated by vertical-fall pyrotechnics to produce clearings in supercooled stratus. A second objective of the experiment was to determine how well a clearing could be targeted over a preselected ground location. Previous stratus clearing tests had primarily involved curtains of dry ice particles or horizontal lines of AgI nuclei. Silver iodide pyrotechnics were chosen because of their logistical advantages over dry ice.

Results of the Michigan testing were favorable. Clearings were produced in cloud decks up to 1400 m thick and as warm as −8°C. In thicker cloud decks, glaciation occurred only to a depth equal to the fall distance of the pyrotechnics. There were indications of “overseeding” from the relatively poor visibility through the cleared area that likely was caused by high ice-crystal concentrations. Targeting was successful when accurate wind data were available.

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