WMOD00058

• Contract/Permit #: #98-1
• Applicant: Western Kansas Groundwater Management
• Project Name: Western Kansas Weather Modification Program
• Date: 4/28/2001
• Weather Modification - Doc Type: Report

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Another atmospheric aerosol is present in the atmosphere, teaned ice nuclei (IN). These
are particles upon which, if found in condensed water droplets, enhance water droplet freezing.
Ice crystals also may foan upon IN directly from water vapor Despite the abundance of CCN,
there is a relative scarcity of IN found in most naturally occurring convective clouds. This
fundamental lack of sufficient numbers of IN causes clouds to fail to precipitate efficiently and is
thought to be the most significant reason why large, damaging hail can foan in clouds, Except in
the most complex, chaotic and unstable conditions, cloud seeding is usually able to deliver the
needed numbers of IN to correct the imbalances found in most natural cloud systems.

Clouds can be made up of unfrozen water droplets, ice crystals or their combination.
Within a convective cloud having a portion of it colder than freezing, some of the sub-freezing
water droplets remain in a liquid state, teaned "supercooled." Convective clouds often create a
condition in which both unfrozen water droplets and ice crystals simultaneously co-exist; in
Western Kansas this is critical to the foanation 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 teaned homogeneous nucleation,

Supercooled water droplets containing ice nuclei will freeze first. The speed at which
supercooled water droplets convert into ice crystals increases as clolid temperature decreases.
That is, as cloud tops grow higher above the freezing level and temperatures decrease within the
cloud, the in-cloud ice crystal production increases. A process called vapor deposition 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 which
creating a pressure gradient between them causing liquid to flow from the droplets to the ice
crystals, thereby reducing the cloud water The ice crystals 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, the unfrozen water subsequently freezing upon impact.

Accretion, or riming, occurs when unfrozen water 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
collide repetitively in a complex manner through the processes just mentioned.

When the variously-sized ice particles eventually fall out of the cloud, dropping below the
freezing level, they begin melting. If melting is not complete, then hail, graupel (2-5 mm, size
snow pellets) or snow reaches the ground as precipitation instead of rainfall.

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