ABU DHABI (NYTIMES) - Iranian officials have worried for years that other nations have been depriving them of one of their vital water sources. But it was not an upstream dam that they were worrying about or an aquifer being bled dry.

In 2018, amid a searing drought and rising temperatures, some senior officials concluded that someone was stealing their water from the clouds.

"Both Israel and another country are working to make Iranian clouds not rain," Brigadier-General Gholam Reza Jalali, a senior official in the country's powerful Revolutionary Guard, said in a 2018 speech.

The unnamed country was the United Arab Emirates, which had begun an ambitious cloud-seeding programme, injecting chemicals into clouds to try to force precipitation.

Iran's suspicions are not surprising, given its tense relations with most Persian Gulf nations, but the real purpose of these efforts is not to steal water but simply to make it rain on parched lands.

As the Middle East and North Africa dry up, countries in the region have embarked on a race to develop the chemicals and techniques that they hope will enable them to squeeze rain drops out of clouds that would otherwise float fruitlessly overhead.

With 12 of the 19 regional countries averaging less than 10 inches (25cm) of rainfall a year, a decline of 20 per cent over the past 30 years, their governments are desperate for any increment of fresh water, and cloud seeding is seen by many as a quick way to tackle the problem.

And as wealthy countries like the UAE pump hundreds of millions of dollars into the effort, other nations are joining the race, trying to ensure that they do not miss out on their fair share of rainfall before others drain the heavens dry - despite serious questions about whether the technique generates enough rainfall to be worth the effort and expense.

Morocco and Ethiopia have cloud-seeding programmes, as does Iran. Saudi Arabia just started a large-scale programme, and a half-dozen other Middle Eastern and North African countries are considering it.

China has the most ambitious programme worldwide, with the aim of either stimulating rain or halting hail across half the country. It is trying to force clouds to rain over the Yangtze River, which is running dry in some spots.

While cloud seeding has been around for 75 years, experts say the science has yet to be proven. And they are especially dismissive of worries about one country draining clouds dry at the expense of others downwind.

The life span of a cloud, in particular the type of cumulus clouds most likely to produce rain, is rarely more than a couple of hours, atmospheric scientists say. Occasionally, clouds can last longer but rarely long enough to reach another country, even in the Persian Gulf, where seven countries are jammed close together.

But several Middle Eastern countries have brushed aside the experts' doubts and are pushing ahead with plans to wring any moisture they can from otherwise stingy clouds.

Today, the unquestioned regional leader is the UAE. As early as the 1990s, the country's ruling family recognised that maintaining a plentiful supply of water would be as important as the nation's huge oil and gas reserves in sustaining its status as the financial and business capital of the Persian Gulf.

While there had been enough water to sustain the tiny country's population in 1960, when there were fewer than 100,000 people, by 2020 the population had ballooned to nearly 10 million.

And the demand for water soared as well. UAE residents now use roughly 147 gallons per person a day, compared with the world average of 47 gallons, according to a 2021 research paper funded by the UAE.

Currently, that demand is being met by desalination plants. Each facility, however, costs US$1 billion (S$1.4 billion) or more to build and requires prodigious amounts of energy to run, especially when compared with cloud seeding, said Mr Abdulla Al Mandous, director of the National Centre of Meteorology and Seismology in the UAE and the leader of its cloud-seeding programme.

After 20 years of research and experimentation, the centre runs its cloud-seeding programme with near military protocols. Nine pilots rotate on standby, ready to bolt into the sky as soon as meteorologists focusing on the country's mountainous regions spot a promising weather formation - ideally, the types of clouds that can build to heights of as much as 40,000 feet.

The UAE uses two seeding substances: the traditional material made of silver iodide and a newly patented substance developed at Khalifa University in Abu Dhabi that uses nanotechnology that researchers there say is better adapted to the hot, dry conditions in the Persian Gulf.

The pilots inject the seeding materials into the base of the cloud, allowing it to be lofted tens of thousands of feet by powerful updrafts.

And then, in theory, the seeding material, made up of hygroscopic (water attracting) molecules, bonds to the water vapour particles that make up a cloud.

That combined particle is a little bigger and in turn attracts more water vapour particles until they form droplets, which eventually become heavy enough to fall as rain - with no appreciable environmental effect from the seeding materials, scientists say.

That is in theory. But many in the scientific community doubt the efficacy of cloud seeding altogether. A major stumbling block for many atmospheric scientists is the difficulty, perhaps the impossibility, of documenting net increases in rainfall.

"The problem is that once you seed, you can't tell if the cloud would have rained anyway," said Dr Alan Robock, an atmospheric scientist at Rutgers University and an expert in evaluating climate engineering strategies.

Another problem is that the tall cumulus clouds most common in summer in the UAE and nearby areas can be so turbulent that it is difficult to determine if the seeding has any effect, said Dr Roy Rasmussen, a senior scientist and an expert in cloud physics at the National Centre for Atmospheric Research in Boulder, Colorado.

Israel, a pioneer in cloud seeding, halted its programme in 2021 after 50 years because it seemed to yield at best only marginal gains in precipitation. It was "not economically efficient," said Dr Pinhas Alpert, an emeritus professor at the University of Tel Aviv who did one of the most comprehensive studies of the programme.

Despite the difficulties of gathering data on the efficacy of cloud seeding, Mr Al Mandous said the UAE's methods were yielding at least a 5 per cent increase in rain annually - and almost certainly far more. But he acknowledged the need for data covering many more years to satisfy the scientific community.