WMOD00518

• Project Name: Project Skywater
• Title: Cloud Seeding Principles and Techniques
• Date: 1/1/1971
• Weather Modification - Doc Type: Report

precipitation. Of even greater significance in the case of
supercooled cumulus clouds are the dynamic effects
due to ice-phase seeding. When natural environmental
conditions are right, cumulus clouds can be stimulated
to grow larger and last longer. The introduction of
silver iodide into the supercooled part of the cloud
causes the cloud droplets to freeze into ice crystals.
This conversion process, multiplied millions of times
over within the cloud releases a large amount of heat,
called the heat of fusion. This phenomena makes the
cloud more buoyant, and causes it to grow larger, and
thereby to process more water longer and more
efficiently then would have occurred naturally.

Seeding Winter Storm Systems

Cold-cloud seeding of winter orographic storms is
fairly well established and understood. Clouds form as
moist air is lifted and cooled during its west-to-east
course across the mountains. Left to nature's devices,
many are highly inefficient precipitators, content to
keep aloft more than 90 percent of their moisture
burden.
By treating certain of these cold clouds with silver
iodide, their precipitation efficiency can be greatly
improved. As has been already stated, the microscopic
crystals of silver iodide act as artificial ice nuclei that
form ice crystals which attract moisture from
surrounding droplets and grow large enough to fall to
the ground as snow.

Treating Cumulus Clouds

The modification of cumulus clouds is considerably
more complex. These large, billowy giants form when
warm, moist air fairly close to the earth's surface is
lifted and cooled to the point where water vapor
condenses into cloud droplets. Lifting can be induced
by the heating of the earth's surface by the sun, or by
the passage of a cold or a warm front through a given
area.
The "life span" of most cumulus clouds generally is
about 20 to 30 minute~. They yield little precipitation,

because nature provides too little time for the cloud
droplets to coalesce into drops large enough to fall.
Two seeding techniques are used to promote rainfall
from these clouds: Silver iodide (ice-phase) seeding,
and hygroscopic seeding.

Temperature is Critical

Silver iodide seeding is used when the cloud-top
temperature of the cumulus formation is 230 to 260 F
or colder, or when the cloud gives indications that it
will shortly reach that temperature level in the
atmosphere. The primary objective here is to initiate
the dynamic heat release which will stimulate cloud
growth. If cloud updrafts are slow, the objective then is
the same as for mountain winter storms. That is, to
increase the number of ice crystals in the supercooled
part of the cloud to the optimum concentration for the
given conditions. The ice crystals will grow rapidly at
the expense of the cloud droplets, and become large
and heavy enough to fall through the cloud and melt
into raindrops.

Use of Hygroscopic Materials

In hygroscopic seeding, such materials as common
salt and a mixture of ammonium nitrate-urea may be
used. They have the unique ability to absorb water
vapor from the surrounding air. Finely ground particles
of hygroscopic material, released into the updraft
beneath a growing cloud, will grow large enough by
condensation, then by coalescence with cloud droplets
to produce rainfall.
This process is regarded as particularly effective in
clouds with strong updrafts that might otherwise be
inefficient precipitators. Left alone, these clouds would
blow much of the water out of the top where an ice
crystal (cirrus) cloud forms and holds aloft much of
the water that has been processed. Hygroscopic seeding
usually forms large drops much lower in the cloud, and
at warmer temperature levels where there is the
greatest supply of moisture. The result is that more of
the cloud's water content reaches the ground as rain.

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