WMOD00312 ‎(2)‎

• Title: Cloud Seeding Feasibility for the Shasta-Trinity Watershed
• Date: 12/1/1993
• Weather Modification - Doc Type: Report

growth process is called accretion, or riming, and can produce snow pellets (also called soft
hail or graupel) with rather large fall speeds. Aggregation is yet another growth process for
ice crystals in which both natural and artificially produced crystals may chain together into
large snowflakes, increasing fall speeds and allowing fallout to the mountain.

Snowflakes can settle to the ground if they grow large enough and fall far enough before
being carried downwind from the mountain barrier. Typically, downward motion of the
airstream to the lee of the barrier will evaporate the cloud droplets and sublimate the ice
crystals. Cloud seeding thus involves a race to get snowflakes to the ground before the zone
of downward motion is reached. Dennis (1980) presents a more detailed discussion of the
physical processes involved in the formation of snowfall in winter orographic clouds.

The conceptual model describing events following seeding of a winter orographic cloud has
not changed markedly since Ludlam (1955) wrote his classic article on the subject. Super and
Heimbach (1983) restated similar ideas in different form and presented them as general
statements which could be made about artificial seeding. They noted that, for cloud seeding
to increase snowfall from winter clouds over mountainous terrain, several critical links in a
chain of physical events must exist. First, seeding material must be successfully and reliably
produced. Second, this material must be transported into a cloud region that has supercooled
water or ice supersaturations in excess of what can be converted to ice by naturally occurring
ice crystals. Third, the seeding material must have dispersed sufficiently upon reaching this
region so that the desired concentration range of ice crystals affect a significant cloud volume.
With AgI, seeding requires, fourth, a temperature low enough for substantial nucleation to
occur. Fifth, once ice crystals form, they must remain in an environment suitable for growth.
Fallout must occur prior to crystals being carried beyond the mountain barrier where
downslope motion, cloud evaporation, and ice crystal sublimation typically exist.

As shown on figure 2.1 below, AgI does not approach activity levels considered necessary to
seed clouds (1011 nuclei/g) until the temperature is below -5 oC. Other agents that seed by
chilling the air, such as CO2 (dry ice) or liquid propane, reach this threshold just below the
freezing point. Selecting the right seeding agent requires a determination ofthe temperature
within the SLW region and the time available for crystal growth.

10"

10" _ 30 Mola Agl+Perchlorata
(Demott et al.. 1983)
E + 2% Agl+NH,1
Cl
..... (Demott at aI., 1983)
W 10"
...J
0 DRY ICE
::l .
Z (Horn et aI., 1982)
PROPANE
0
10" (Hicks and Vali. 1973)
A SNOMAX
(Ward and Demott. 1989)
10"

o -5 -10 -15 -20
TEMPERATURE (OC)

Figure 2.1. - Activity curve of various seeding agents vs. temperature as available in the published literature.

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