WMOD00097

• Applicant: Western Kansas Groundwater Management
• Project Name: Cloud Seeding
• Date: 5/1/1989
• Weather Modification - Doc Type: Report

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water vapor in it supplying additional moisture to the precipitation-
making process. This allows the cloud to. "process" more water over a
longer period of time and to do so more efficiently than if unseeded,

Although silver iodide produces the dynamic effect very well, it
is produced more rapidly by dry ice. On the WKWM Program dry ice is
dispensed through an opening in an aircraft capable of flying well
above the freezing level---into the supercooled region of the clouds.
The dry ice dispenser carries about 200 lbs and has been designed to
automatically release dry ice pellets at a rate of 5 pounds per minute
of flight. Dry ice dropped into supercooled clouds immediately
converts the supercooled water drops into ice crystals. Silver iodide
only starts becoming increasingly effective at temperatures below
-5C---which translates into approximately 1,500 feet to 3,000 feet
above the freezing level. In this respect, dry ice is much quicker and
more effective than silver iodide in producing ice crystals in the
supercooled part of the cloud, even down to the freezing level,
However, relatively large quantities of dry ice must be dropped to
produce the same number of ice nuclei generated by a given mass of
silver iodide---approximately 1000 - 2000 grams dry ice to. one gram
silver iodide.

Dispensing silver iodide, using the cloud top delivery technique,
is more expensive .than using dry ice. Silver iodide first has to be
manufactured into an ejectable cartridge that can be expelled from a
rack mounted to an aircraft. Manufacturing costs of ejectable
cartridges, or flares, is much greater than dry ice when compared to
an equivalent amount of ice crystal production.

The following cloud systems and variations of them are most
responsible for producing rain and hail here in Western Kansas:

(1) air-mass

(2) multiple cell

(3) squall line

Air-mass storms begin as isolated cloud systems that develop a
well-organized cloud base usually with an easily delineated inflow
area protruding below its base. Specific cloud movements may vary
widely depending upon the height individual clouds reach as well as
the windspeed and direction variability with height. Terrain effects
probably play some role despite Kansas' relative flatness.

If we draw a line through the center of the precipitation area of
an air-mass storm in the direction of storm movement and another line
at right angles to it, also through the center of the precipitation
area, updrafts found upwind in the cloud are termed "trailing" or
"back side"; downwind, ahead of that line, is the "front side".
Although updrafts are sometimes found on the front side part of an
air-mass storm, it is not considered to be of primary importance to
the formation of precipitation. Normally, the important updrafts for
these storms are found below cloud base at some distance behind the
precipitation area, usually along the trailing edge of the storm. A
typical air-mass storm and seeding methods are shown in Fig. 3.

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