Cloud seeding
Paul Holper
July 2001
Cloud seeding is a way of trying to artificially generate
additional rainfall from clouds. It may involve attempting to produce
rain when none would normally fall or it may be working to increase precipitation
over a particular area.
Clouds and rain
Air always contains moisture. Whenever air cools, water
vapour may condense into tiny droplets of liquid. Clouds are made up of
millions of these water droplets.
Before these tiny droplets can form raindrops, snowflakes or hailstones,
they have to join with millions of others if they are to become heavy
enough to fall to the ground. They will only do this if particles are
present in the atmosphere. These particles are called cloud nuclei and
may be dust, salt from evaporated sea spray, sand or other material from
forest fires, volcanic eruptions and pollution.
Under the cold conditions in clouds, droplets of water can form small
ice crystals on the surfaces of the cloud nuclei. Water vapour in the
cloud then freezes directly onto the surface of these crystals, which
become heavier and eventually fall.
Cloud seeding
The particles that scientists add to clouds during seeding
mimic the structure of ice and serve as additional nuclei for crystal
formation. Condensation and freezing of water release a large amount of
heat that makes clouds more buoyant and may double their size and height.
As clouds grow taller, they draw in more moist air that can add to the
rain formed.
Clouds can be seeded in a variety of ways. Researchers can seed cold
clouds with silver iodide particles, which have a crystal structure similar
to that of ice particles. Water can deposit on the silver iodide particles,
coat them with ice and keep growing as if the entire particle were a natural
ice particle.
Cold clouds may also be seeded with dry ice pellets, which cool the nearby
air far below 0°C. Cloud droplets in the cooled air freeze and form
ice particles that can grow as more water freezes on their surface.
Another way of seeding clouds is via a process known as hygroscopic seeding.
This involves using flares to generate smoke full of salt. The salt particles
act as nuclei that generate large water drops that can readily develop
into raindrops.
Seeding using silver iodide burners, dry ice pellets and hygroscopic
flares is done from a plane. Clouds may also be seeded from the ground
using silver iodide generators.
Australian cloud seeding experiments
Cloud seeding experiments began in Australia just a year
after the world’s first laboratory trials in the USA.
From 1947 to 1952, CSIRO scientists used Royal Australian Air Force aircraft
to drop dry ice into the tops of cumulus clouds. The method worked reliably
with clouds that were very cold, producing rain that would not have otherwise
fallen.
CSIRO carried out similar trials from 1953 to 1956 in South Australia,
Queensland and other States. Experiments used both ground-based and airborne
silver iodide generators.
During the late 1950s and early 1960s, CSIRO performed cloud seeding
in the Snowy Mountains, on the York Peninsular in South Australia, in
the New England district of New South Wales, and in the Warragamba catchment
area west of Sydney.
Of these four experiments, only the one conducted in the Snowy Mountains
produced statistically significant rainfall increases over the entire
experiment.
In the late 1960s, the Governments of Victoria, New South Wales, Queensland,
South Australia and Western Australia seeded clouds. Results were either
inconclusive or controversial.
On the other hand, CSIRO’s activities in Tasmania in the 1960s were
successful. Seeding over the Hydro-Electricity Commission catchment area
on the Central Plateau achieved rainfall increases as high as 30% in autumn.
The Tasmanian experiments were so successful that the Commission has
regularly undertaken seeding ever since in mountainous parts of the State.
CSIRO also conducted cloud seeding experiments in Emerald, Queensland
(1972-1975), and in Western Victoria (1979-1980). The Western Australian
Government ran a study in 1980-1982 examining the viability of seeding
in the northern wheat belt. None of these activities found that seeding
would be an economical, reliable way of increasing rainfall.
During the late 1980s, CSIRO Atmospheric Research acted as scientific
advisors to Melbourne Water in a cloud seeding assessment conducted over
the Baw Baw plateau. This is a major water catchment area east of Melbourne.
The experiment generated no statistical increase in rainfall.
Does cloud seeding work?
CSIRO has shown that in Australia cloud seeding is effective
only in a limited number of weather conditions. Cloud seeding will never
break droughts; cloudless skies will never produce rain. In fact, many
types of clouds cannot be successfully seeded. Cloud seeding is most likely
to be effective when used on cumulus or stratiform clouds in air forced
up over mountains.
Seeding is unlikely to be effective during winter and spring over the
inland plains of southern and eastern Australia. It is also likely to
fail in summer over eastern and north-eastern Australia plains and immediately
to the north of Perth.
In the tropics, the high rainfall variability makes proof of increased
rainfall from cloud seeding extremely difficult.
Based on over 50 years’ experience with cloud seeding, CSIRO has
established procedures for undertaking a cloud seeding experiment. These
rigorous guidelines ensure that at the conclusion of seeding operations
there will be a clear-cut answer to whether or not the activity was successful.
In other words, has the seeding netted a statistically significant increase
in rain over the catchment? If the answer is no, there is no point in
persevering.
It may be worth again attempting rainfall enhancement experiments in
areas where past efforts have failed, but proper planning needs to be
done first, along with rigorous independent evaluations. |
