WMOD00372

• Project Name: High Plains Cooperative Program
• Title: The Design of HIPLEX-1
• Date: 4/1/1979
• Weather Modification - Doc Type: Report

IX. THE SEEDING AGENT

The seeding strategy for HIPLEX-l involves on-top seeding with a
curtain of dry ice pellets dropped from a Learjet aircraft. Placebos
will be used to minimize bias in the experiment. The physical
hypotheses and choice of response variables are not dependent upon
this seeding strategy. It does, however, provide a considerable
reduction in uncertainty in executing the seeding operation and in
distinguishing between seeded and natural cloud characteristics,
thereby facilitating the evaluation of HIPLEX-l.

Dry ice seeding has a number of significant advantages. The activity
of the seeding material, about lOll to 1012 ice crystals per gram
of dry ice, is not stronglly dependent on temperature, so the warmest
regions of the supercooled cloud can be seeded effectively. This
insures that regions of the cloud can be seeded where there is a
minimum of competition from the natural ice concentrations. Dry ice
seeding can also be directed at a specific region of the cloud. It
is effective only in the region through which it is dropped, and it
is not carried through the cloud to affect other regions. In contrast,
ice nucleus (silver iodide) seeding generally exhibits a significantly
increasing activity with decreasing temperature and would provide a
continuing activation of additional ice crystals high in the cloud.
The glaciation of the upper regions of the cloud wou"ld probably
inhibit the precipitation development by converting the cloud water
to small ice crystals that would not effectively contribute to the
growth of the larger precipitation elements. A fina') significant
advantage of dry ice seeding is the absence of cont~nination effects
and the absence of even perceived environmental effects.

See appendix D for a more detailed description of the seeding
methodology.

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