University looks to make cloud seeding more effective
After years of frustration and delay, research could improve rain success rates, writes Daniel Bardsley
Cloud seeding in the UAE is as popular as it has ever been in the two decades since it began. Last year, the National Centre of Meteorology flew 242 missions.
Much of the pioneering work carried out in the US in the mid20th century involved injecting thousands of silver iodide particles into the atmosphere. These particles mimic the actions of ice nuclei, particles around which ice crystals form, and so promote rain.
Silver iodide is still commonly used in colder conditions, while in warmer temperatures salt, or sodium chloride, particles are often preferred.
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 rain generated by a cloud, the weather bureau’s figures show, while in clearer conditions the increase is about 35 per cent.
Attempts have been made to mix salt with other substances, including other salts and polymers that attract water, to increase effectiveness but improvements have been modest.
Ken Carslaw, a professor of atmospheric science at the University of Leeds in the UK, says 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 of what’s intended,” says Prof Carslaw, who researches how particles in the atmosphere behave and what effect they have on clouds.
“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 Khalifa University could lead to a change in the efficiency of cloud seeding. This is especially in cases where salt particles are used.
Last year 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 Nano, Prof Zou and her co-researchers have published full details of how they have used nanotechnology to produce these particles, and how much more effective they appear to be at adsorbing water vapour.
Adsorption – not to be confused with absorption – involves the adhesion of 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 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 cautions that this is very different from an actual cloud seeding operation, she describes the results as “very positive and promising”.
A key trait is the way in which the titanium dioxide coating and the salt particle that it surrounds act with one another to adsorb more water vapour.
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.
As a result of this synergy in the cloud chamber experiments, this structure adsorbed many times as much water vapour as pure sodium chloride.
And water turned to liquid at a much lower relative humidity than when salt particles were used. The new particles also 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 says. “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 nanostructured particles are more likely to accelerate droplet growth and trigger rainfall, Prof Zou says.
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 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 but we’re in the process of planning for the field trials,” says Prof Zou, whose project is financially supported by the UAE Research Programme for Rain Enhancement Science.
“We’re now addressing the challenge of the scale-up, to produce the particles in larger quantity.”
So after many years in which cloud seeding has not shown great progress in efficiency, the new capabilities of scientists to operate at the nano-assisted level could provide a much sought-after breakthrough.