WMOD00319

• Title: North American Interstate Weather Modification Council - Fiscal Year 2009 Request
• Prepared By: McClintock Associates, Inc.
• Date: 1/1/2007
• Weather Modification - Doc Type: General Information

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North American Interstate Weather Modification Council

www.natwmc.org

Cloud Seeding FAQs
Prepared by Darin Langerud, NAIWMC Member - North Dakota

Cloud seeding, often called weather modification, is not something most people know
much about, yet it is a multi-million dollar industry in the Western United States. This
document addresses the most common questions about the technology and provides the
interested reader with some ideas where more information can be found.

1. For what purposes is weather modification practiced?
Weather modification, commonly referred to as cloud seeding, is currently used to
increase precipitation from clouds, reduce hail from thunderstorms and eliminate fogs
that reduce visibility, usually at busy airports.

2. How does cloud seeding work?
Cloud seeding improves a cloud's ability to produce precipitation by adding tiny particles
called ice nuclei (which water needs to freeze) to a cloud. These nuclei help the cloud
produce precipitation by acting on supercooled liquid water (SL W), the raw material
from which precipitation is formed. These artificial ice nuclei promote freezing of SL W
at warmer temperatures than it would naturally, accelerating the precipitation process and
making more efficient use of the water in the cloud.

3. Can all clouds be seeded effectively?
No. Only clouds that meet certain criteria are suitable for seeding. Summer clouds must
possess a sustained updraft of moist air, a lack of natural ice, and grow to heights cold
enough to contain supercooled liquid water. Winter clouds must also be supercooled and
persist long enough to grow snowflakes large enough to fall to the ground. Criteria for
fog seeding are very similar to the conditions required for winter clouds.

4. What are clouds seeded with?
Clouds can be seeded with a number of different types of agents: silver iodide, liquid
propane, dry ice (solid carbon dioxide), and various salt compounds.

5. How long after seeding before a treated cloud starts to change?
Seeding effect can vary from almost immediate to around 30 minutes depending on the
seeding delivery method and the type of cloud seeded. Directly placing seeding material
in a cloud typically works more quickly than releasing the material below the cloud and
allowing the cloud to ingest it.