Title: Artificial Rainmaking
Full Citation
Permanent Link: http://ufdc.ufl.edu/WL00002917/00001
 Material Information
Title: Artificial Rainmaking
Physical Description: Book
Language: English
Publisher: Fla Engineering and Industrial Experiment Station
Spatial Coverage: North America -- United States of America -- Florida
Abstract: Richard Hamann's Collection - Artificial Rainmaking
General Note: Box 12, Folder 1 ( Materials and Reports on Florida's Water Resources - 1945 - 1957 ), Item 31
Funding: Digitized by the Legal Technology Institute in the Levin College of Law at the University of Florida.
 Record Information
Bibliographic ID: WL00002917
Volume ID: VID00001
Source Institution: Levin College of Law, University of Florida
Holding Location: Levin College of Law, University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Full Text

Artificial Rainmaking


It has already been pointed out that substantial
rain can fall only from those places in the atmosphere
where large masses of warm, moist air undergo a rising
motion associated with the large-scale dynamics of
atmospheric motion. These atmospheric regions are
generally associated with barometric lows-storms in
the ordinary sense-or with systems of thermal con-
vection such as summer thundershowers.
From considerations of the quantity of moist air
entering a disturbance and the cooling it undergoes, it
is not difficult to compute the maximum amount of
rainfall that could ensue; and such computations have
proved very useful in predicting the maximum rain-
fall for design of flood control structures and similar
purposes. However, it is found that the actual rainfall
very seldom even approaches the theoretical maximum.
It appears that of the moisture supplied to the active
portion of the storm a very large part is lost by being
spread out sidewise and mixing with drier air sur-
rounding the storm. In this way the rain-producing
potential and the heat energy of the system are rapidly
dissipated. Every time a typical cumulus cloud is
seen to push its tower into the sky, the mixing of
moist air with dry can be observed easily. Storms in
General, then, are very inefficient as rain-producing
mechanisms, precipitating actually only a few per

*W. E. Howell Associates, Inc., Cambridge, Mass.

cent of the total quantity of water involved in the
storm circulation. It is this gap between actual and
potential rainfall that can be exploited by artificial
rainmaking. That hope is the greater because the
heat released by rain production supplies no small
portion of the storm's energy, and adding to it could
have more than local consequences.

Natural and Artificial Rainmaking Mechanisms

When warm, moist air rises, it expands and cools,
and the moisture starts to condense as cloud. But,
ordinarily, this cloud is composed of water droplets
too small to fall-so small that several hundred million
of them would be needed to fill a teaspoon. These
tiny droplets have so little independent motion that
they seldom collide with one another. The result is
that the cloud is colloidally stable and can endure
for a long time-a matter of many hours-without any
of the droplets falling to the ground. In the mean-
time, the edges of the cloud are continually mixing
with drier air; and unless the rising motion continues
unabated, the cloud eventually dissipates. The great
majority of clouds go through their life cycle and
dissipate without producing a single drop of rain.
Before rain can fall, some mechanism must act to
change the cloud from one of many minute particles,
too small to fall, into one of far fewer, far larger
particles. Two such mechanisms are found in nature,


and to each corresponds a means of stimulating rain-
fall artificially.
If a cloud, or a part of it, reaches a level where
its temperature falls below freezing, the droplets re-
main liquid, but in a supercooled state, for freezing
is initiated only by the action of ice-forming nuclei
or extreme low temperature. In the natural atmo-
sphere there are ice-forming nuclei present, in con-
centrations that vary widely from a few per cubic meter
to as many as ten million per cubic meter, mostly active
at temperature around 0F. Those nuclei that be-
come active form the seeds for ice crystal formation;
once started, the ice crystals grow rapidly at the ex-
pense of the surrounding cloud droplets and soon be-
come large enough to fall.
This mechanism is often inefficient in nature be-
cause there may not be enough ice-forming nuclei
present and because their activity is delayed until
the temperature of the cloud falls far below freezing,
while dissipation of the cloud's rain-producing po-
tential proceeds.
Artificial stimulation is achieved by seeding the

cloud with an adequate number of artificial ice-forming
nuclei. The substance most widely used for this pur-
pose is silver iodide, dispersed in the form of a smoke
of extremely small crystals. Released near the ground
into the warm moist air that forms the water source
for the rain, the silver iodide smoke is carried upward
by the same ascent of air that forms the clouds. With-
in the clouds, the smoke particles become active at
about 260F., long before the natural nuclei that may
be present, and initiate the ice-crystal growth that
leads to rain. In this way, rain falls from clouds that
might never have produced rain naturally, and else-
where the rainfall starts sooner, before so much dilu-
tion of the rainfall potential has occurred. Thus
more rain falls, and more heat is released into the as-
cending cloud system, strengthening the system.
Let us turn now to the second natural rainmaking
mechanism. It has been pointed out that the cloud,
before it reaches the freezing level, is colloidally stable,
and that if undisturbed it "ages" rather slowly. A
cloud formed from particularly warm, moist air ages
faster, for it contains more droplets, closer together.

Figure l.-Escatawpa Watershed Project-per cent of normal
rainfall at the end of September 1954.


The aging is further greatly accelerated if there a few
relatively large salt particles present-particles of
100 micromicrograms or larger-for these particles
form droplets big enough to start growing, slowly at
first, by collision. "Giant" salt particles of this sort
have been identified in maritime air. Rain from this
origin is favored in the maritime tropics, where the
warmth, humidity, and salt occur together. Rain
produced in this manner is generally showery.
Whenever "warm" clouds are sufficiently water-
rich to contain significant quantities of free water,
stimulation can be effected by seeding the cloud with
salt particles or liquid spray of the proper particle
size. The particles must be large enough to fall and
collide with the cloud droplets so as to start growing,
but large enough to be carried up by the rising air
currents and dispersed throughout the cloud. If the
cloud is too small, seeding causes it to dissipate, pro-
ducing rain so light it does not reach the ground;
but experiments on larger clouds have shown a highly
successful outcome.
While success in the use of these stimulation tech-

Figure 3.-Escatawpa Watershed Project-per cent of normal
rainfall at the end of November 1954.

niques requires close and continuous analysis of the
meteorological situation in the target area, and skillful
exploitation of the opportunities that nature presents,
it does not depend upon secret or mysterious powers.
Nor are the consequences without their limitations.
Rain cannot be made from a clear sky, but only when
suitable clouds are present. Analysis of a large number
of commercial rain stimulation projects has shown
that the results from month to month are quite vari-
able, like the weather itself. About 5 or 10 per cent
of the time no effect is apparent; about as frequently,
the rainfall is apparently doubled, or more. The most
frequent result is rainfall 20 to 40 per cent more than
expected naturally, and the average increase over
many projects is very close to 35 per cent.
It doubtless will be of especial interest to residents
of Florida to see the results of some recent operations
nearby. The following sets of figures illustrate the
results from a project on the Escatawpa watershed
in Alabama and Mississippi, and on the Black and
Pee Dee watersheds in South Carolina. The maps
show the percentage of normal rainfall in each target

Figure 4.-Escatawpa Watershed Project-per cent of normal
rainfall at the end of December 1954.

Figure 7.

Figure 5.

and its surroundings. The successive figures accumu-
late the results; the reason for drawing them thus is
that individual monthly maps tend to emphasize
erratic short-term fluctuations that even out over the
longer period to reveal a more meaningful pattern.
Because of the obvious disturbing influence of hurri-
cane "Hazel" on the South Carolina targets, the
October map for that project was redrawn omitting
the hurricane rainfall (see Fig. 8).

Figure 8.

Figure 6 Figure 9.

hi -

Figure 10.

The evaluation diagram for each project (Figs.
5 and 11) show the target rainfall in relation to the
rainfall of a nearby control area, outside most of the
seeding influence, for the current seeded season and
for the same season of ten previous years-as many
years as homogeneous rainfall records were available
for the analysis. The regression line, representing the
most probable relationship between target and control
rainfall, is arrived at by the method of least squares.
It will be noted that in the Escatawpa watershed a
rainfall of 15.59 inches was received, 62 per cent more
than the expected amount of 9.61 inches. The likeli-
hood of an increase this large occurring by chance is
only one in twenty. In the South Carolina target (the
whole rest of the state being used as control) the rain-
fall was 13.34 inches, 53 per cent more than the ex-
pected amount of 8.70 inches, an increase that would
occur by chance only one in one hundred trials.

Control rinfo//

Figure 11.

Rainfall increases of this magnitude are, of course,
of the greatest importance to industry and agriculture
wherever additional water has productive value. Aside
from the widespread scientific interest that has been
aroused, not entirely free from controversy, rain stimu-
lation is being used increasingly by hydroelectric in-
terests, large-scale industrial water consumers such as
pulp and paper companies, and by industrialized
agriculture such as sugar producers. As an industrial
technique it is still in its infancy; far broader appli-
cation of it may be expected in the future.

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