It is around two decades since cloud seeding began in the UAE, and this technique to promote precipitation is now as popular as ever.
Last year, the National Centre of Meteorology (NCM) ran no fewer than 242 missions.
Cloud seeding is used in many other countries, among them the United States, where pioneering research was carried out in the middle of the 20th century.
Much of the work then involved injecting thousands of tiny silver iodide particles into the atmosphere, often by using a plane loaded with flares. These particles mimic the actions of ice nuclei – particles around which ice crystals form – and so promote precipitation, which might fall as snowflakes or rain.
Silver iodide is still commonly used in colder conditions, while in warmer temperatures, salt (sodium chloride) particles are often preferred.
Over the years there has been little change in the particles used and the efficiency of cloud seeding has not shown significant improvement.
While some researchers remain unsure whether cloud seeding works at all, in hazy conditions there is likely to be a 10 to 15 per cent increase in the rainfall generated by a cloud, according to NCM figures, while in clearer conditions the increase is about 35 per cent.
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Attempts have been made to mix salt with other substances, including other salts and polymers that also attract water, to increase effectiveness. However, improvements have been modest.
And there are many challenges linked to cloud seeding aside from what particles are used. Professor Ken Carslaw, a professor of atmospheric science at the University of Leeds in the United Kingdom, said there is also the issue of when and where in a cloud the seeding is carried out.
“You have to seed in the right place and the right time. If not, you get even the opposite effect from what’s intended,” said Prof Carslaw, who researches how particles in the atmosphere behave and what effect they have on clouds.
“In some cloud systems, you can get fairly predictable results, but that’s not the case in all situations … Our ability to measure the cloud and understand how it will respond is incomplete; that makes cloud seeding unpredictable. We don’t even know how a cloud will evolve naturally.”
Although there is much about cloud seeding that remains to be learnt, recent work at Abu Dhabi’s Masdar Institute, part of Khalifa University of Science and Technology, could lead to a step change in the efficiency of cloud seeding. This is especially in cases where salt particles are used.
Last year, as was reported, Professor Linda Zou, a professor of chemical and environmental engineering at the university, applied for a patent for the use of a titanium dioxide coating on salt particles.
Now, in a paper in the journal American Chemical Society (ACS) Nano, Prof Zou and her co-researchers have published full details of how they have used nanotechnology – science at the nano scale – to produce these particles, and how much more effective they appear to be at adsorbing water vapour than pure salt (adsorption – not to be confused with absorption – involves the adhesion of the water molecules to the particles).
High levels of water vapour adsorption are essential if particles are to successfully seed clouds, but on their own, without a coating, salt particles only work at very high humidity conditions and tend to be ineffective when relative humidity is less than 75 per cent.
The research involved tests in a cloud chamber, which is a three-dimensional environment in which condensation and water droplet formation caused by the seeding material can be assessed. While Prof Zou cautioned that this is very different from an actual cloud seeding operation, she nonetheless described the results as “very positive and promising”.
A key characteristic is the way in which the titanium dioxide coating or shell, and the salt particle that it surrounds, act in synergy with one another to adsorb more water vapour, which turns into larger droplets.
The titanium dioxide shell is a hydrophilic (water-loving) surface, and when it adsorbs water vapour it increases the relative humidity around the salt particle, and this can be turned into drops of liquid.
As a result of this synergistic effect, in the cloud chamber experiments, this structure absorbed many times as much water vapour as pure sodium chloride. Also, water turned to liquid at a much lower relative humidity than when normal salt particles were used. Thirdly, the new particles created much larger water droplets. Prof Zou is not surprised at the positive findings.
“I designed that [particle] because I expected the nanostructure would enhance the function, but I am glad to see such significant and very positive improvement,” she said.
“We found the total numbers of large water droplets that are likely to form rainfall have increased 290 per cent.”
The larger water droplets formed by the new nanostructured particles are, said Prof Zou, more likely to accelerate droplet growth and trigger rainfall.
Numerical modelling is being carried out to predict how effective the particles will be in atmospheric conditions, and it will also be crucial to see if the lab results can be replicated in actual cloud seeding operations. Field trials have yet to happen, but planning and preparation is being carried out by the National Centre of Meteorology.
“We cannot tell the time [when trials will happen] … but we’re in the process of planning for the field trials,” said Prof Zou, whose project is financially supported by the UAE Research Programme for Rain Enhancement Science.
“We are now addressing the challeng of the scale-up, [to produce the particles in] larger quantity.”
So, after many years in which cloud seeding operations have not shown significant progress in efficiency, the new capabilities of scientists to operate at the nano-assisted level could provide a much sought after breakthrough.
– The study’s other authors are Dr Yanlong Tai, Haoran Liang (a PhD student), Dr Nabil El Hadri, Dr Steve Griffiths and Dr Mustapha Jouiad of Khalifa University, and Dr Ali Abshaev and Dr Buzgigit Huchinaev of the High-Mountain Geophysical Institute in Russia.
