WMOD00372

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

IV. THE EXPERIMENTAL UNIT

The experimental unit for HIPLEX-1 is the small, semi-isolated
cumulus congest us cloud. It is the only cloud type with a pbten-
tial for precipitation in the Miles City area that is well enough
understood at the present time to permit a physically meaningful,
randomized statistical-physical experiment of the type described in
the above-stated objectives.

The small, semi-isolated cumulus congestus clouds are defined by the
following general characteristics:

1. As described in the Glossary of Meteorology (1959), "A strongly
sprouting cumulus species with generally sharp outlines and, some-
times, with~ great vertical development; it is character~zed by
its cauliflower or tower aspect, of large size." '

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2. Visually distinct and separable from other cumulus congestus
by distances ranging from one to several diameters. Separation
may extend throughout the cloud depth or the cloud may rise up
from a continuous, low-level stratiform layer at its base.

3. Cloud diameters from 2 to 8 km.

4. Cloud depths from 2.5 to 4.5 km.

The small cumulus congest us clouds generally produce no more than
light precipitation at the ground naturally. Usually, natural
ice-crystal concentrations are very low in these clouds in their
growing stages. They are often confined below a relatively strong
inversion and/or a dry layer at about 5 to 6 km m.s.l. Additional
characteristics of cumulus congestus clouds near Miles City, Montana,
are listed in table 1.

Although only small-to-moderate amounts of precipitation are expected
to result from seeding, the increase over the natural case should be
significant, and the additional precipitation, when produced on an
operational basis, could be agriculturally important. More important,
however, is the contribution the knowledge gained through this experi-
ment will make in building the scientific basis for working with more
complex clouds that provide major amounts of precipitation.

The number of days that relatively isolated cumulus congestus clouds
were observed during the afternoon and early evening near Miles City
for the 3 years 1976-77-78 is given in table 2. Observations indicated
that clouds were unsuitable for HIPLEX-1 purposes on about half of
those days. However, three clouds per day could be sampled adequately,

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