WMOD00375

• Contract/Permit #: 0-07-81-V0144
• Project Name: South West Drought Project
• Title: Operational Weather Modification - Volume I: A Seminar Series
• Date: 9/1/1981
• State: OK
• Weather Modification - Doc Type: Report

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1. Introduction

i

The dry years of the middle 1970's have stimulated weather
modification activities (cloud seeding for the purpose of in-
creasing rainfall from convective clouds) over parts of the
Middle West and High Plains agricultural areas. These same
areas are often frequented by severe weather (excessive rains,
large hail, damaging winds and tornadoes) during most of the
crop growing season. When there is sufficient advance know-
ledge of the threat of severe weather, the field managers may
shut down cloud-seeding operations to ensure the safety of
project personnel and to avoid the charge that the seeding
caused or enhanced the severe weather. This possibility was
raised with regard to the flood-producing storm at Rapid City
in 1972 (St. Arnand et a1., 1972: Dennis et al., 1973).
Because of limitations in the data used for this study,
several basic assumptions had to be made in the development
of the suspension criteria. All June rainfall was assumed to
be convective. The seeding opportunity day was assumed to be
a day with seeding opportunities, i.e., a day with rainfall
anywhere within an operational area. Therefore, cloud seed-
ing is assumed to be much more effective in the augmentation
of natural rainfall than in the initiation of rainfall on
days in which natural rainfall would not have occurred.
Support for these assumptions is found in the 1arge-
amplitude June diurnal rainfall frequency and amount curves
for the Dodge City area (Fig. 2). Most rain fell during the
late afternoon and early nighttime hours. This period is
generally considered as the period most favorable for convec-
tive rainfall. Further, the frequency curve shows that fewer
than 20 storms occurred during the late morning (0900-1200 LST),
thus establishing that multiday storms were relatively uncommon.
Thus, given that rain occurred during a 24 h period (0700-0700),
the rainfall most likely occurred during the convective period
which is most favorable for seeding opportunities.

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