WMOD00552

• Title: Seedability of Winter Orographic Storms in Utah
• Date: 12/1/1977
• State: UT
• Country: United States
• Weather Modification - Doc Type: Report

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3.0 SEEDING DELIVERY SYSTEM

3.1 AJrbonae SeediDg

3.1.1 Deseription of EquIpment. The airborne
, seeding component of the Wasatch Weather Modifica-
tion Project was subcontracted to Atmospherics, Inc.
The aircraft supplied to the project was a turbo-
charged Piper Aztec "C" (NI78A). This aircraft was
fully licensed for IFR (instrument night rules) nights
and equipped with deicing systems on propellers,
wings, and tail surfaces (Henderson et al., 1970).

The aircraft was permanently stationed at the'
Ogden Municipal Airport during the four-month
operational periods. It was available for operations on
a 24-hour basis. The decision to base the aircraft at
Ogden was based, in part, on considerations of air
traffic control centers at Salt Lake City and Hill Air
Force Base, night safety requirements, aircraft
service facilities, position of VORTAC installations in
relation to seeding night paths, relationship between
radar facilities and aircraft night operations, and the
'climatology at airfields throughout the Salt Lake
Valley.

In view of the high density aircraft traffic existing
in the area of the seeding tracks it was necessary in
the initial stages of project development to contact the
Federal Aviation Agency in Salt Lake City and Hill
Air Force Base near Ogden, in order to organize the
night activities in this area.

Installed on the aircraft were mounting racks of
special design for holding a variety of pyrotechnic
seeding devices. On this particular aircraft, two racks
were mounted vertically on the side of the fuselage.
These were used primarily fe:: holding and burning
pyrotechnics which did not produce large quantities of
corrosive by-products believed to react with the
aircraft skin. Two additional racks were installed in
the airstream immediately aft of the flaps and
outboard of the engine nacelles. These were used for
higher output pyrotechnics which could produce
corrosive material or heat problems in the first 10 cm,
or so, downwind from the end-burning units. Output
concentrations of all chemical constituents were such
that turbulent mixing one or two meters behind the
aircraft provides a dilution below any level which
might affect materials or life forms. The output of the
airborne seeding was approximately 120 gm h-1 of
silver iodide.

The selection of seeding tracks was based mainly
on combinations of wind speed and direction at
seeding level. Careful thought was given to safety
considerations, FAA regulations, terrain features,
commercial aircraft traffic, location of primary and
alternate airfields, radar field headquarters, location
of USU telemetry system stations, and storm types
within the total operational area.

The seeding track designations Al through C5'
related to various wind speeds and directions, are
shown in Table 2. If major changes in wind speed and
direction during any seeding period occurred, the
aircraft track selected was altered to fit the changing
weather conditions. Seeding tracks charts with
appropriate night information are depicted in Figure
6.

3.1.2 Se--l~"g Experiments. During the first
year of the airborne seeding project a number of
criteria were established to. identify an operational
period. These criteria were:

1. Cloud bases shall be no higher than 12,000
ft msl (4725 m).

2. TotBl cloud depth shall be at least 100 mb
thick. '

3. 700 mb temperature shall be -loC or colder.

4. The cloud cover shall be not less than
8/1Oths as vie~ed from Logan.

5. 700 mb wind direction shall be within the
interval from 197 degrees to 317 degrees.

6. Conditions 1 through 5 shall be forecast to
persist for not less than 8 consecutive hours.

The nearly 10,000 square mile area emcompassing
the UWRL telemetering stations is too large for
airborne seeding of winter storms by a single aircraft.
Within this rather large area a fixed target area of
1,258 square miles was chosen. This smaller experi-
mental target area allowed proper coordination
between radar facilities, air-borne operations, ground
personnel, as well as providing for a reasonable length
of time between seeding passes. The coordinates of
the four corners of the target area are:

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