Cloudy schemes to make it rain

Even though cloud-seeding techniques have been used since at least the middle of the 20th century, it has proved difficult to say definitively whether they increase rainfall but research is beginning to highlight its effectiveness.

Cloud seeding operations in the UAE. Photo courtesy National Centre for Meteorology and Seismology
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Praying for rain has long been a tradition in the UAE, as in other parts of the Middle East.

Rain is vital for resupplying aquifers and inland oases, and directly irrigating agricultural lands. So it is no surprise that prayers for rain are considered so important that people are given time off work to do so.

Although these prayers were first made more than 1,000 years ago, a complementary approach to promoting rainfall has much more recent origins.

For more than a decade, the UAE has looked to technology to spark downpours through cloud seeding.

The National Centre of Meteorology and Seismology runs dozens of cloud seeding missions a year, using salt particles released from aircraft to promote droplet formation.

And the UAE is helping to advance cloud seeding technology by funding domestic and international research projects.

One recipient of funds from the UAE Research Programme for Rain Enhancement Science is Dr Linda Zou and her research team at the Masdar Institute of Science and Technology, who recently filed an application with the United States Patent and Trademark Office.

The researchers have developed a way of applying a thin titanium dioxide coating to salt crystals to enhance condensation.

Such particles are thought to be especially effective at promoting rainfall around convective or convection clouds, which usually form in rising columns of air.

Although cloud-seeding techniques have been used since at least the middle of the 20th century, it has proved difficult to say definitively whether they increase rainfall.

After all, while downpours may follow cloud-seeding missions, they could just as easily have happened without human intervention.

In the past there have been warnings that developing countries desperate for rain could be handing over large sums of money to commercial cloud seeding operations despite a lack of evidence that the methods work.

Among the researchers who are investigating the efficacy of the techniques is Joshua Aikins of the University of Colorado Boulder in the United States.

He is involved in tests in a mountainous region of Idaho, in north-western US, in which silver iodide particles are released from flares from aircraft or ground-based generators.

The research, supported by the Idaho Power Company, uses radar information to determine whether rain is enhanced.

“I think there’s a lot of evidence in the literature that’s more theoretical that silver iodide should be good seeding material. But seeing enhancement on a radar screen that wouldn’t have been there without cloud seeding, it’s kind of the missing picture,” says Mr Aikins. Many countries have invested in cloud seeding, including China, which has spent tens of millions of dollars on such programmes.

The world’s most populous nation has also tried to manipulate the elements to ensure clear skies during important national celebrations.

Recent work in Jordan, which faces serious water shortages, is thought to have resulted in more rain.

In Britain, projects have analysed whether cloud seeding could help to lower the temperatures of the sea surface, which could make hurricanes less powerful by starving them of energy.

Perhaps more outlandish is the suggestion that cloud seeding could limit global warming.

Among those interested in producing clouds that could reflect sunlight is Stephen Salter, emeritus professor of engineering design at the University of Edinburgh.

Developing methods advanced by an academic at the University of Manchester, Prof Salter has created designs for wind-powered ships that shoot jets of water into the sky, producing a mist of water droplets.

“I’ve been designing ships. I’ve been working on these as an engineer for about 10 years,” he says.

The spray these ships shoot into the air would help to make clouds more reflective because the salt particles act as condensation nuclei, around which water droplets form.

They cause clouds to be made of more small water drops, rather than fewer large drops. And as a result, the clouds are better at reflecting solar energy.

Prof Salter says clouds can be made to be about 5 percentage points more reflective.

Several hundred vessels, each costing about £2.5 million (Dh11.7m), could be enough to help cancel the rise in temperature caused by carbon dioxide emitted since the industrial revolution, Prof Salter has calculated.

He feels that using these ships would be preferable to geoengineering, which involves sending sulphate particles into the atmosphere.

As the water sprays on the ships can be easily switched on and off, the method is more responsive to changing conditions, says Prof Salter.

The use of spray ships remains on the drawing board and has yet to gain funding, although Prof Salter and others remain optimistic that the technology could work, given the chance.

With cloud seeding, the early evidence from the work in Idaho appears to be positive.

“For some of the data we’ve collected so far, there could be some enhancement through aircraft,” Mr Aikins says.

But the ground-based silver iodide generators appear to be less likely to produce rain, perhaps because when particles are released from below they cannot descend through a cloud and increase in size.

Results from the project, which was launched in January, could be published officially this year.