- Imagine you could head off storms, prevent floods and
mudslides, or deliver a much-needed downpour to a region that is cracked
and parched by drought. Indeed, imagine a day when nations in arid regions
fight over the water vapour that passes overhead in clouds.
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- Rainmakers - seeding clouds the scientific way
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- Millions of pounds are already being spent on "cloud
seeding" worldwide, notably in dry areas, yet weather scientists still
look furtive if you ask them about rainmaking: despite decades of anecdotal
evidence that it works, most feel that they lack statistically significant
data to prove that it is possible to create an artificial downpour.
History is littered with attempts to alter weather patterns by shamans,
witches and rain dances but the first suggestion that rainmaking might
indeed be feasible came in the Forties as a result of Project Cirrus, a
study of rain and snow formation.
Conducted by General Electric, America, the project was led by Nobel laureate
Irving Langmuir, who believed, wrongly, that seeding in New Mexico could
trigger rain in New York. But he did make important advances in understanding
how to make drops fall.
First you need clouds. They form when warm moist air is cooled, either
when the air is pushed up by flowing over a mountain, when a wedge of cold
air noses under the warm air in a frontal system, or when convection patterns
are turned by the Sun's heat.
The air cools as it rises and its capacity for water vapour decreases.
The excess vapour must condense as cloud droplets but these droplets need
to form on something, such as a dust mote or particle. If the air contains
many such "nuclei", the cloud consists of many tiny drops that
can remain in a liquid state even if the cloud grows well above the altitude
at which water normally freezes.
Working with Vincent Schaefer, Langmuir found that pellets of solid carbon
dioxide, or dry ice, could make supercooled water droplets freeze in a
supercooled cloud to form ice crystals that grew into snow. The rainmakers
also used silver iodide crystals, which have a similar structure to frozen
water, to create nuclei for water drops to condense upon.
Since a pioneering experiment by Mr Schaefer in November 1946, there have
been many cloud seeding attempts. The original so-called "glaciogenic"
methods have been complemented by using salt particles to seed raindrops.
Optimism soared in the Fifties and Sixties that drought could be ended
by artificial downpours.
This scientific hubris could be glimpsed once again last year, when the
BBC ran a story claiming that a supposed 1952 RAF rainmaking experiment
in Bedfordshire had caused a torrent in Lynmouth, Devon, that claimed 35
lives. The story was dismissed by experts: cloud seeding, if it works,
only affects local rain clouds.
Rainmakers admit that most attempts conducted worldwide to date have been
inconclusive because of lack of follow-up research and because it has been
so tricky to disentangle the effects of seeding from rain that would have
fallen in any event.
Indeed, it was only in January of last year that a team from the influential
National Centre for Atmospheric Research in Boulder, Colorado, felt that
the field was coming of age after following up a groundbreaking effort
conducted in South Africa between 1990 and 1997 by Dr Deon Terblanche and
the late Dr Graeme Mather of the South African Weather Service.
Crucially, the South Africans did not use silver iodide, the effects of
which are controversial. Instead, they used water-attracting (hygroscopic)
salts to build bigger droplets and create rain more efficiently. Between
1997 and 2000, another programme took place in the Limpopo province where,
according to Dr Terblanche, they recorded a "possible" 108 per
cent increase in rainfall between seeded and unseeded storms.
The complementary American study, which came in the wake of years of drought
in northern Mexico, saw NCAR researchers soar into the Mexican rain clouds
in a twin-engine turboprop airplane equipped with wing-mounted racks carrying
24 of the South African flares and instruments to study cloud physics.
The flares spewed salted smoke into the moisture-rich up-draughts entering
the clouds from below. Tiny particles of hygroscopic salt (a mixture of
sodium, magnesium, and potassium chlorides) attracted and absorbed the
surrounding water vapour. As the cloud's water vapour was sucked into the
particles, larger droplets formed, which then grew faster and fell out
as rain.
The three-year randomised experiment in the northern Mexican state of Coahuila
suggested that rainfall from seeded clouds lasted longer, the rainfall
area was larger, and overall precipitation was higher (sometimes even doubled)
than similar non-seeded clouds.
In short, they obtained similar results to the South African study - "something
quite rare in this field," said Dr Terblanche. "We were very
encouraged," added NCAR's Dr Roelof Bruintjes.
Although the results were statistically significant in many cases, the
number of cases remains marginal for such analyses. Funding for a planned
fourth year of data gathering dried up when the Mexican drought ended.
This left the total number of cases of rainfall at 94, compared with 127
in South Africa.
"Our sample size is marginal," said Dr Bruintjes, "though
they do point in the same direction as those in South Africa."
The South Africans, who wrapped up their experiments in 2000, are now reviewing
the data and hope to present a cost-benefit analysis to their government
later this year. One issue, said Dr Terblanche, is that the effect has
only been well-documented in storm clouds, so some question whether this
would be enough to make an impact on rainfall over a large area.
Dr Bruintjes has started another project in the mountain range between
the United Arab Emirates and Oman. He is also awaiting the go-ahead for
additional research in Mexico to improve the statistics and, with luck,
produce results that stand up to scientific scrutiny. However, there is
another problem: scientists cannot fully explain how seeding works. "We
must be able to explain the responses of the cloud to the seeding procedure
before we can claim success," said Dr Bruintjes.
Significantly, however, support for cloud seeding has come from another
line of research on pollution which, paradoxically, reveals how particles
can prevent cloud water from condensing into raindrops and snowflakes.
Prof Daniel Rosenfeld of the Hebrew University of Jerusalem used satellite
images and measurements of "pollution tracks" downstream from
major urban areas and air pollution sources such as power plants, lead
smelters, and oil refineries in Turkey, Canada and Australia. He found
that polluted clouds can shut down precipitation because they contain abnormally
small water droplets.
The droplets' small size is caused by pollution particles that act as seeding
sites around which cloud moisture condenses. Approximately one million
small droplets must collide and coalesce in order to make a precipitation-sized
drop - that is, one large enough to fall below the cloud base and reach
the ground before evaporating. But in polluted clouds there are too many
small droplets and not enough larger ones. These small droplets float around
with low probability of bumping into each other and merging into raindrops.
"Clean air makes clouds that precipitate fast and efficiently, whereas
polluted air makes clouds with a large number of small drops that are slow
to coalesce into raindrops," he said.
Because pollution is significant in many regions, Prof Rosenfeld's findings
suggest that human activities may be disturbing rainfall patterns on a
global scale. But he argues that his work actually backs the case for cloud
seeding.
Unlike the pollution aerosols, if cloud-seeding aerosols "are made
to be larger with more soluble material, they can have the opposite effect,
that is, enhance rainfall," said Prof Rosenfeld, who is carrying out
a hygroscopic cloud seeding experiment in Israel, using concentrated Dead
Sea brine.
"The great demonstrated sensitivity of rain-forming processes in clouds
to pollution particles indeed lends credibility to the possibility that
we can boost rainfall by cloud seeding," he said. "This may lead
the way to a renewal of the serious scientific efforts in cloud seeding,
this time on a more solid scientific ground."
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- http://www.telegraph.co.uk/connected/main.jhtml?view=DETAILS&grid=P8&
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