WMOD00510

• Title: The Practicability of Dry Ice for On-Top Seeding of Convective Clouds
• Date: 1/1/1978
• Weather Modification - Doc Type: Report

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Reprinted from JOURNAL OF ApPLIED METEOROLOGY, Vol. 17, No.1, January 1978
American Meteorological Society
Printed in U.S.A.

The Practicability of Dry Ice for On-Top Seeding of Convective Clouds

EDMOND W. HOLROYD III,1 ARLIN B. SUPERl AND BERNARD A. SILVERMAN2

Bureau of Reclamation, U. S. Department of the Interior
(Manuscript received 22 July 1977, in final form 23 September 1977)

ABSTRACT

Dry ice is shown to be an attractive agent for on-top seeding of convective clouds. A modest payload of
small dry ice pellets can effectively seed dozens of clouds, depending on cloud volumes encountered and
crystal concentrations desired. A dry ice pellet size of about 7 mm diameter is suggested for efficient use of
seeding agent when dropped from the -lOoC level.
Supercooled convective clouds that were seeded on-top with dry ice were investigated to determine em-
pirical nucleation effectiveness values. The clouds were repeatedly penetrated to measure the resulting ice
crystal concentrations. The experiments gave conservative effectiveness values of 2 to 5 X 1011 crystals per
gram of dry ice, but with possible error bars extending an order of magnitude to each side of those values.
A well-documented experiment giving effectiveness values twice as large is discussed in detail.

1. Introduction

This paper preSents some new estimates of the
effectiveness of dry ice as a cloud seeding agent and
considers the practicability of using dry ice for convec-
tive cloud seeding in view of these and other estimates
of its effectiveness. Dry ice-solid carbon dioxide at a
temperature of about' - 80oC-was used by Schaefer
(1946) in his initial experiments that started the modern
era of weather modification. It was used in many
cloud seeding experiments during the late 1940's and
early 1950's. However, silver iodide (AgI) became a
much more common seeding agent because of its higher
effectiveness per unit mass and, more importantly, the
logistical ease and reduced costs associated with AgI
seeding near cloud base or from the ground.
Schaefer (1976) has recently urged a revival in the
use of dry ice in cloud seeding. The practicability of
using dryice is obviously determined to a large degree
by what its effectiveness actually is, i.e., the number
of ice crystals resulting per gram of material used.
Schaefer states that it ofl'ers economy and superior
effectiveness (relative to AgI) at all temperatures
below ooc. He indicates that in air at -160C and at
least supersaturated with respect to water, more than
1014 ice embryos can be formed per gram of dry ice.
Other investigators have reported values, usually
based on laboratory work, which generally range from
1010 to 1011 crystals per gram (Weickmann, 1957; Eadie
and Mee, 1963; Fukuta, 1965; Allee et al., 1972). The
latter work was with actual clouds, but ice crystal
measurements were limited. More recent laboratory

I Miles City, Mont. 5930l.
2 Denver, Colo. 80225.

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work by Fukuta et al. (1971) indicates an effectiveness
value of 8 X 1011 crystals per gram from - 2 to-11 oc.

2. Dry ice seeding experiments

Several experiments have recently been conducted to
provide better estimates of dry ice effectiveness in
actual clouds, especially summertime cumuliform
clouds. These experiments,' inspired by the work of
Allee et al. (1972), were conducted by one of the
authors (Holroyd) while working with the Common-
wealth Scientific and Industrial Research Organization
(CSIRO) in Australia and by all authors through the
U. S. Bureau of Reclamation's High Plains Cooperative
Program (HIPLEX).
In all seeding experiments, the dry ice pellets were
dropped out a hole in the bottom of the airplane in as
continuous a fashion as practical. A ha,nd scoop of
known volume was used and the number of scoops
recorded. The dry ice mass data are therefore
approxi,mate.

a. Dry ice pellet sizes and mass

The dry ice used in both the American and Australian
experiments was obtained commercially in the form of
cylindrical pellets. They were transported from the
factory to the research site in various containers over
distances on the order of 1000 km. During storage the
pellets slowly sublimed. Therefore, there is a potential
for size changes from the time of manufacture to the
time of use in seeding.
To observe any size spectrum changes during storage,
a 13.5 kg sample of dry ice was placed in a cardboard
box, transported for 3 h, and then kept in the box in a

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