WMOD00267

• Applicant: Arlin B. Super and James A. Heimbach Jr.
• Sponsor Name: Colorado Water Conservation Board and US Bureau of Reclamation
• Project Name: Literature Review/Scientific Study
• Title: Feasibility of Snowpack Enhancement from Colorado Winter Mountain Clouds: Emphasis on Supercooled Liquid Water and Seeding with Silver Iodide and Propane
• Prepared For: Coloado Water Conservatoin Board
• Prepared By: USBR
• Date: 9/30/2005
• State: CO
• Weather Modification - Doc Type: Scientific Study

The second fundamental topic considers the practicalities of seeding SL W cloud over
mountains. The most common method of valley releases of silver iodide (AgI) is examined, as
well as high elevation ground releases of both AgI and propane, and aircraft seeding. The aircraft
approach is ruled out for a number of reasons to be discussed.

The final topic concerns expansion of liquid propane to chill cloudy air below -40oC. This
produces vast numbers of embryonic ice crystals by homogeneous nucleation. This is a method
of high elevation ground seeding with an agent and mechanism different than AgI seeding. It is
given emphasis in this report because the method is not widely known, but recent results from a
randomized propane seeding experiment in Utah are very encouraging. Propane seeding may
provide an adjunct or alternative to AgI seeding in Colorado.

This report is primarily based on published articles in the peer-reviewed scientific literature.
However, some references are given to internal project reports where the work has special
relevance but was never published in the open literature. Most articles are based on observations
from either Colorado or Utah orographic clouds. Such clouds are believed similar in the
adjoining states with mountains at similar latitudes. Considerable field research related to winter
orographic seeding has been conducted in each state.

At the risk of oversimplification, there appear to be three general schools of opinion among
people with at least a casual interest in winter orographic cloud seeding. Many people, including
a large number of meteorologists, hold the view that this type of cloud seeding has not yet been
demonstrated as successful in increasing seasonal snowfall, and that more basic research is
needed before application will have a sound scientific basis. However, it should be recognized
that relatively few meteorologists can presently be considered to be experts in this field. Because
of very limited federal research funding for over decade now, researchers and graduate students
have needed to move on to other fields. Several scientists who were experts in winter orographic
cloud seeding have retired. The views of National Weather Service forecasters or their managers
are often quoted by the local media, yet such meteorologists are unlikely to have even a passing
knowledge of weather modification. This lack of knowledge can sometimes be the cause of
misinformation cited by local and national media.

Another "camp" of people, including many water users and commercial cloud seeders, holds
the view that this type of cloud seeding has been sufficiently well established for widespread
application. Some have held this view for several decades as evidenced by the long duration of a
few operational projects. The main evidence supporting this optimistic view seems to be based
on claims by commercial operators, and the large body of internal project reports they have
amassed over decades, almost all suggesting positive results from seeding. But such reports are
often subject to potential unconscious bias (and possibly deliberate in some cases) because future
company earnings will likely be affected. Moreover, many internal reports have shortcomings in
application of statistics, with widespread use of the historical target-control regression method,
long known to have serious potential flaws, but one of few options in the absence of
randomization. One common but incorrect view is that seeding must work if so many reports
have shown positive results, even if relatively few have undergone in-depth peer review. And, as
evidenced by several published articles which seriously challenged earlier experimental
suggestions once held up as "proof," even peer review does not guarantee solid results. Those
who maintain that the large number of internally reported "success stories" must mean that winter
orographic seeding is on a solid footing would do well to ponder why experimental randomized
programs have seldom been able to verify similar results.

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