WMOD00132

• Contract/Permit #: #95-5
• Applicant: Western Kansas Groundwater
• Project Name: Kansas Weather Modification
• Date: 1/1/1995
• Weather Modification - Doc Type: Report

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There are other special kinds of atmospheric particles known
as ice nuclei (IN)---particles upon which, if found in condensed
water droplets, enhance droplet freezing. Ice crystals also may
form directly from water vapor upon ice nuclei. Despi te the
abundance of condensation nuclei found in the atmosphere, there is
a relative scarcity of ice nuclei by comparison. '

The sizes, concentrations and chemical properties of all
nuclei present in the atmosphere 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' toccur if certain conditions
required for the formation of precipitation are absent.

Clouds can be made up of unfrozen water droplets or ice
crystals or a combination of them. Within a cloud that has some
part of it colder than freezin~ some of the water droplets remain
in a liquid state and are termed "supercooled", a condition in
which both unfrozen water droplets and ice crystals are found
existing together. This supercooled cloud volume is critical to the
formation of rain and hail. Supercooled water can remain unfrozen
as low as -40 C (or -40 F) before spontaneously changing to ice;
when such freezing happens, it is called homogeneous nucleation.

Those supercooled water droplets containing ice nuclei are the
first to begin freezing. The speed that supercooled water droplets
convert, into ice crystals generally increases as cloud temperatures
decrease while rising during cloud growth. An important 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 creating a pressure gradient between them. This
gradient causes water molecules to flow from the droplets to the
crystals and causing ice crystals to grow larger at the expense of
the droplets. Eventually, the supercooled water is Used up as
crystals grow larger. This also occurs in the supercooled cloud
environment where ice crystals, once developed, continue growing
rapidly using up surrounding water vapor and cloud water from
nearby water droplets.

Once ice particles develop inside growing clouds the
continuous unequal movement of water droplets and ice particles
also insure ice-droplet collisions.' These collisions promote a
chain-reaction process called coalescence, a process in which the
unfrozen water droplets collect other water droplets by impact
before any freezing occurs. A second process, accretion, also
occurs. Accretion is a process in which water droplets grow rapidly
and ,freeze upon impact with cloud ice particles and may shatter
and/or grow larger and continue colliding repetitively in a complex
manner. As ice particles eventually fall below the freezing level
they begin to melt; if melting is not complete, hail, graupel or
snow results instead of rainfall.

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