BOARD00272

• Board Meeting Date: 7/28/2003
• Description: Flood Section - Weather Modification Program
• Board Meetings - Doc Type: Memo

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WM Program Background

. What is cloud seeding? The ASCE Manual of Professional Practice 81
says,. ."cloud seeding operations are selection of cloud masses that are likely to
be inefficient precipitators, the production of seeding material, its delivery and
dispersal within the cloud volume, the resulting cloud dynamical and
microphysical transitions to stimulate additional precipitation, the fallout of
precipitations and its transfer to the watershed, """, artificial ice-forming
nuclei can be produce by various methods. The most commonly used is a
seeding device or generator that burns AgI (Silver iodide) in solution with
acetone and 9ther chemicals to emit very large numbers of tiny particles,., the
generator first'vaporizes the AgI and the vapor is the rapidly cooled condensing
to form sub micron smoke particles. ".' cloud temperatures are the governing
parameter. The activity of nucleats nOlmally begins at about -4 degrees C and
increases exponentially with the decreasing cloud temperatures."

Ground based generators burn silver iodide and acetone into vapor that is carried by winds into clouds and
causes ice embryos to grow turn into snowflakes, This is the stimulation of natural cloud processes.
Generators are set close to the top of a mountain and often 5-20 miles upwind of the target area, The
effectiveness of weather modification is difficult to measure, It usually involves comparison of results in
the target area to precipitation in a "control" area, However, a 10- to 15-percent increase in snowpack
from ground based winter generators is typicaL The theoretical limit under ideal circumstances is a 25%
increase. The benefits are often economically motivated and approximately 70% of Colorado's water is
. supplied by snowmelt runoff. The primary benefit is economic but cloud seeding can also benefit: fish
and wildlife, hydroelectric power, agriculture, water supplies, salinity reduction, ski industries and water
dependent recreation, Some of the drawbacks are that it needs public acceptance/education, it is hard to
quantify results, it can be good drought relief but that is difficult task of during a drought when cloud
systems are minimal, there is potential for erosion and flooding from over seeding, and influencing our
macro climate may have unintended effects,

Weather modification is used for two basic purposes in Colorado, To Increase Precipitation, More rain
during drought, more snow for reservoir storage, more snow for earlier start on the ski season, to Reduce
Precipitation, Less hail during severe stonns, and fog suppression (no current projects),

Does it Work? The American Meteorological Society (1998) policy statement on planned and
inadvertent Weather Modification states, "There is statistical evidence that precipitation from super-
cooled orographic clouds (clouds that develop over mountains) has been seasonally increased by about
10%. The physical cause-and-effect relationships, however, have not been fully documented,
Nevertheless, the potential for such increases is supported by field measurements and numerical model
simulations," A similar statement by the American Society of Civil Engineers states that cloud seeding
can be an effective resource management tool under proper operational conditions.

The Department of Natural Resources Program: The Colorado General Assembly declared that the
State of Colorado recognizes that economic benefits can be derived for the people of the state from
weather modification. Operations, research, experimentation, and development in the field of weather
modification have therefore been encouraged,

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State Statutes (CRS 36-20-101): The CWCB has operated the Weather Modification Permitting Program
under direction from the Executive Director's Office - Department of Natural Resources since 1987, A

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