WMOD00289 ‎(3)‎

• Title: Augmentation of Rainfall from Summer Cumulus Clouds
• Date: 2/5/1983
• Weather Modification - Doc Type: Report

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Agricultural Water Management, 7 (1983) 3-14
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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AUGMENTATION OF RAINFALL FROM SUMMER CUMULUS CLOUDS

A.S. DENNIS

Division of Atmospheric Resources Research, Bureau of Reclamation, U.S. Department
of the Interior, P.O. Box 25007, Denver, CO 80225 (U.S.A.)

(Accepted 5 February 1983)

ABSTRACT

Dennis, A.S., 1983. Augmentation of rainfall from summer cumulus clouds. Agric. Water
Manage., 7: 3-14.

The latest round of field experiments r~lated to augmentation of rainfall from summer
cumulus clouds over the Great Plains is described briefly. HIPLEX (High Plains Coopera-
tive Program) was designed to reduce scientific uncertainty associated with attempts to
increase rainfall by cloud seeding in the High Plains. Field programs were conducted at
three sites. A randomized experiment, HIPLEX-1, was set up in Montana to test a physi-
cal hypothesis linking dry ice seeding to rainfall from cloud base.
HIPLEX has confirmed the importance of ice processes to precipitation formation in
cumulus clouds of the High Plains and the dominance of mesoscale convergence/diverg-
ence fields on convective cloud development. HIPLEX-1 clearly showed increases in cloud
ice concentrations associated with dry ice seeding 2 and 5 min after treatment time. Some
changes in the subsequent steps in the physical hypothesis linking seeding to increased
rainfall at cloud base are indicated but cannot be firmly established with the present
data set.

INTRODUCTION

Weather modification has been used as a weapon against drought in the
Great Plains of the United States for over 30 years. Many of the projects
have involved attempts to increase rainfall by seeding summer cumulus
clouds. Despite frustration because of the lack of clearcut evidence of rain-
fall increases, the attempts have continued because of the perception that
even small increases in rainfall would reduce significantly the impact of
drought.
It is sometimes asked whether or not enough clouds exist during droughts
to make seeding them practical. Certainly one could limit the impact of a
drought by seeding the marginal conditions that exisi before and after the
drought and around the edges of the drought-strickEm area. Furthermore,
climatological data indicate that, even in the middle of a severe drought,
enough clouds exist to justify attempts at seeding them (Huff and Semonin,
1975).

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