Cloud seeding

The invention: Technique for inducing rainfall by distributing dry
ice or silver nitrate into reluctant rainclouds.
The people behind the invention:
Vincent Joseph Schaefer (1906-1993), an American chemist and
meteorologist
Irving Langmuir (1881-1957), an American physicist and
chemist who won the 1932 Nobel Prize in Chemistry
Bernard Vonnegut (1914-1997), an American physical chemist
and meteorologist
Praying for Rain
Beginning in 1943, an intense interest in the study of clouds developed
into the practice of weather “modification.” Working for
the General Electric Research Laboratory, Nobel laureate Irving
Langmuir and his assistant researcher and technician, Vincent Joseph
Schaefer, began an intensive study of precipitation and its
causes.
Past research and study had indicated two possible ways that
clouds produce rain. The first possibility is called “coalescing,” a
process by which tiny droplets of water vapor in a cloud merge after
bumping into one another and become heavier and fatter until they
drop to earth. The second possibility is the “Bergeron process” of
droplet growth, named after the Swedish meteorologist Tor Bergeron.
Bergeron’s process relates to supercooled clouds, or clouds
that are at or below freezing temperatures and yet still contain both
ice crystals and liquid water droplets. The size of the water droplets
allows the droplets to remain liquid despite freezing temperatures;
while small droplets can remain liquid only down to 4 degrees Celsius,
larger droplets may not freeze until reaching -15 degrees
Celsius. Precipitation occurs when the ice crystals become heavy
enough to fall. If the temperature at some point below the cloud is
warm enough, it will melt the ice crystals before they reach the
earth, producing rain. If the temperature remains at the freezing point, the ice crystals retain their form and fall as snow.
Schaefer used a deep-freezing unit in order to observe water
droplets in pure cloud form. In order to observe the droplets better,
Schaefer lined the chest with black velvet and concentrated a beam
of light inside. The first agent he introduced inside the supercooled
freezer was his own breath. When that failed to form the desired ice
crystals, he proceeded to try other agents. His hope was to form ice
crystals that would then cause the moisture in the surrounding air
to condense into more ice crystals, which would produce a miniature
snowfall.
He eventually achieved success when he tossed a handful of dry
ice inside and was rewarded with the long-awaited snow. The
freezer was set at the freezing point of water, 0 degrees Celsius, but
not all the particles were ice crystals, so when the dry ice was introduced
all the stray water droplets froze instantly, producing ice
crystals, or snowflakes.
Planting the First Seeds
On November 13, 1946, Schaefer took to the air over Mount
Greylock with several pounds of dry ice in order to repeat the experiment
in nature. After he had finished sprinkling, or seeding, a
supercooled cloud, he instructed the pilot to fly underneath the
cloud he had just seeded. Schaefer was greeted by the sight of snow.
By the time it reached the ground, it had melted into the first-ever
human-made rainfall.
Independently of Schaefer and Langmuir, another General Electric
scientist, Bernard Vonnegut, was also seeking a way to cause
rain. He found that silver iodide crystals, which have the same size
and shape as ice crystals, could “fool” water droplets into condensing
on them. When a certain chemical mixture containing silver iodide
is heated on a special burner called a “generator,” silver iodide
crystals appear in the smoke of the mixture. Vonnegut’s discovery
allowed seeding to occur in a way very different from seeding with
dry ice, but with the same result. Using Vonnegut’s process, the
seeding is done from the ground. The generators are placed outside
and the chemicals are mixed. As the smoke wafts upward, it carries
the newly formed silver iodide crystals with it into the clouds.
The results of the scientific experiments by Langmuir, Vonnegut,
and Schaefer were alternately hailed and rejected as legitimate.
Critics argue that the process of seeding is too complex and
would have to require more than just the addition of dry ice or silver
nitrate in order to produce rain. One of the major problems surrounding
the question of weather modification by cloud seeding is
the scarcity of knowledge about the earth’s atmosphere. Ajourney
begun about fifty years ago is still a long way from being completed.
Impact
Although the actual statistical and other proofs needed to support
cloud seeding are lacking, the discovery in 1946 by the General
Electric employees set off a wave of interest and demand for information
that far surpassed the interest generated by the discovery of
nuclear fission shortly before. The possibility of ending drought
and, in the process, hunger excited many people. The discovery also
prompted both legitimate and false “rainmakers” who used the information
gathered by Schaefer, Langmuir, and Vonnegut to set up
cloud-seeding businesses.Weather modification, in its current stage
of development, cannot be used to end worldwide drought. It does,
however, have beneficial results in some cases on the crops of
smaller farms that have been affected by drought.
In order to understand the advances made in weather modification,
new instruments are needed to record accurately the results of
further experimentation. The storm of interest—both favorable and
nonfavorable—generated by the discoveries of Schaefer, Langmuir,
and Vonnegut has had and will continue to have far-reaching effects
on many aspects of society.