WMOD00234

• Contract/Permit #: #93-5
• Applicant: Western Kansas Groundwater
• Project Name: Kansas Weather Modification
• Date: 1/1/1993
• Weather Modification - Doc Type: Report

There are two natural cloud types which produce all rain and
other forms of precipitation: "warm clouds" and "cold clouds".
Rain from these clouds are also referred to as "warm cloud rain"
and "cold cloud rain".

A "warm" cloud, is one in which temperatures are never below
freezing and does not produce ice crystals in its volume. This
cloud is generally characterized by the relatively slow growth of
cloud water droplets which eventually attain sufficient size and
weight to fall from the cloud then collect other cloud droplets,
scavenging them along their paths downward. Although this type or
cloud occasionally appears in Western Kansas, it doesn't playa
dominant role in producing precipitation here. Note: Large size
warm-rain drops can be important embryo sources in the production
of hail when they merge into sub-freezing clouds which are not of
the warm-rain variety, get carried aloft rapidly by updrafts where
they freeze, grow larger and fall as hail, despite some melting.

Of primary importance to Western Kansas is the "cold" cloud.
Cold clouds, having temperatures below freezing in some part of it,
produce the supercooled water droplets and ice crystals which
ini tiate the precipitation process and is most responsible for
producing significant precipitation in Western Kansas.

The prevailing hypothesis under which the WKWM Program ~ail
suppression portion operates is that hailstones grow to large sizes
because there are too few ice crystals formed naturally in clouds
during vigorous thunderstorm growth, thereby allowing relatively
abundant supercooled water to collect upon relatively fewer numbers
of ice particles or other hail embryos. Often, those particles grow
into hailstones too large to melt before impact at the ground.
Current theory is that by increasing the concentrations of ice
crystals within ice crystal-deficient clouds, hailstones will be
prevented from growing to a size large enough to damage property
and crops. Crop type, stage of crop growth and hail size all play
an important role in determining crop damage severity. There are
other theories about hail development and movement within clouds
because hail doesn't always form exactly the same way, every time,
in all clouds. Most long-term cloud seeding programs around the
world use the same hypothesis and have shown varying degrees of
success in both reducing crop damage and increasing rainfall.

The seeding agents used on the WKWM Program are silver iodide
and dry ice---both are delivered by aircraft. Silver iodide ice
nuclei are vaporized in cloud base updrafts, whereas, dry ice is
dropped into growing clouds at temperature levels of -10C to -12C,
about 20,000 feet in mid-summer. Over the years, the results from
programs like ours and cloud physics research have been applied to
the WKWM Program keeping it reasonably state-of-the-art.

To produce high numbers of ice nuclei from silver iodide, it
must first be vaporized. On the ~lM Program a combustion process

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