WMOD00259

• Applicant: Western Kansas Groundwater Management
• Project Name: Western Kansas Weather Modification Program
• Date: 1/1/1999
• Weather Modification - Doc Type: Report

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there is a relative scarcity of IN. In nature most convective clouds fundamentally lack sufficient
numbers of IN which causes clouds to fail to precipitate efficiently, or help form damaging hail.
Cloud seeding can usually provide the needed numbers of IN to correct the imbalances found in
most cloud systems. .

Clouds can be made up of unfrozen water droplets, ice crystals or a combination of them.
Within a convective cloud having a portion of it colder than freezing, some of the sub-freezing
water droplets remain in a liquid state, termed "supercooled water". Convective clouds often
create a condition in which both unfrozen water droplets and ice crystals co-exist simultaneously.
It is the supercooled cloud volume that is critical in Western Kansas to the formation of rain and
hail. Supercooled water can remain unfrozen to as low as -40 C (-40 F) before spontaneously
changing to ice. When such spontaneous freezing occurs, it is termed homogeneous nucleation.

Supercooled water droplets containing ice nuclei freeze first. The speed that supercooled
water droplets convert into ice crystals increases as cloud temperature decreases. That is, as cloud
topss grow higher above the freezing level, ice crystal production normally increases. A process
called vapor deposition then starts to have a significant effect within clouds when ice crystals and
supercooled water exist in the same medium. Surface pressures over ice crystals are lower than
. those over water droplets creating a pressure gradient between them that causes liquid to flow
from the droplets to the ice crystals reducing the cloud water. They continue growing rapidly
feeding on the surrounding water vapor and cloud water from nearby water droplets. Continuous
unequal movements of water droplets and ice particles inside convective clouds ensure random
collisions of ice and water droplets which promote the processes of coalescence, accretion and
aggregation to a greater or lesser extent, all of which increases ice multiplication in clouds.

Coalescence is a process in which the unfrozen water droplets collect other water droplets
by impact, freezing occurring after the impact.

Accretion, or riming, occurs when droplets freeze upon impact with cloud ice particles.

Aggregation is the process in which ice particles collect or attach to other ice particles. In
advanced stages of cloud growth, ice particles will shatter, coalesce, grow larger and repetitively
collide in a complex manner through the processes just mentioned.

When the various sizes of ice particles eventually fallout of the cloud and drop below the
freezing level, they begin melting. If melting is not complete, then hail, graupel or snow reaches
the ground as precipitation instead of rainfall.

The sizes and concentrations of all nuclei present in the atmosphere as well as their
chemical and electrical properties all combine in important ways to determine how efficiently a
cloud system can produce precipitation. Although there are massive amounts of water vapor in
the atmosphere at any time, precipitation won't occur if certain conditions required for the
formation of precipitation are absent.

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