Cloud-Seeding - How Much
Progress Has Been Made?
By Roger Highfield
The Telegraph - London

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.
Rainmakers - seeding clouds the scientific way
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."


This Site Served by TheHostPros