WMOD00271

• Sponsor Name: North American Interstate Weather Modification Council
• Project Name: Fiscal Year 2008 Request
• Title: Fiscal Year 2008 Request
• Prepared For: NAIWMC
• Prepared By: JM McClintock Consulting, Inc
• Date: 2/24/2007
• Bill Number: HR2995 & S.517
• Weather Modification - Doc Type: General Information

I
I
I
I

,

;'\orlh Anu.'rican Interstatr \\'L'athcr ;\lodifica1ioll Council
www.nai\\lllc.Ofb\

I
I

Cloud Seeding F AQs
Prepared by Darin Langcrud, NAI\VMC Member - North Dakota

I

Cloud seeding. onen called weather modification. is nol something most people know
much ahout yet it is 3 multi-million dollar industry in the Western United Slales. This
document addresses the most common questions about the technology and provides the
interested reader with some ideas where more infonllation Can be found.

I
I
I
I
I
I
I
I
I
I
I
I

I. """T what purplJ.\'e.\' 1\ u't'ather "wdifil'atilm practiced?
Weather modification, commonly referred to as cloud seeding. is currently used 10
increase precipitation from clouds. reduce hail from thundcrstomls and c1imiOiuc fogs
that reduce visihility. usually at busy airports.

1. lI"w d"e.li dOlld _,"ef'din!: work?
Cloud seeding improves a cloud's ability to produce precipitation by adding tiny particles
called ice nuclei (which waler needs to freeze) 10 a cloud. These nuclei help the cloud
produce precipitation by acting on supercooled liquid waler (SLW). the raw material
from which precipitation is formed. These artificial ice nuclei promote freezing ofSLW
at wanner temperatures Ihan it would naturally, accelerating the precipitation process and
making more eOicienl use of the waler in the cloud.

J. Can all dOlld.'i be .lief.deJ eJfectil'f'(l'?
No. Only clouds that meet certain criteria arc 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 10 the ground. Criteria for
fog seeding are very similar to the conditions required for winter clouds.

./. What are dOlld.li .\'l!f'Jf'd with?
Clouds can be seeded with a number of different Iypes ofagenls: silver iodide. liquid
propane, dry ice (solid carbon dioxide). and various salt compounds.

5. 1I0w h"'J: aftl'T .'if'f'din!: bf10re a treatf'd dOlld .litarl.\' III dwttJ.:e?
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 bclow the cloud and
allowing Ihe cloud to ingest it.