The United Arab Emirates (UAE) capital of Dubai remains flooded after a deluge of rain that fell earlier this week.
By Tuesday, 142 millimetres had fallen in about 48 hours – which is about 50 per cent more than the annual total precipitation.
The Dubai International Airport is the world’s busiest for international travel and arrivals remain halted. Officials estimate it will take days to return to normal.
Several unconfirmed reports suggested cloud seeding could have played a role in the flood. But what is cloud seeding, how is it used, and is there a more likely reason for the deluge?
Clouds are made of suspended water vapour. Rain occurs when that water vapour comes together around a very small particle, called a nuclei, to form drops. This can happen around ice particles but other substances, like silver iodide, attract vapour more than ice.
Humans can spray the substance from planes or drones, or project particles from the ground using cannons, to infuse the clouds.
“When it’s in a cloud, then water molecules will see that surface and think that it’s like ice,” Canada Research Chair in Atmospheric Science Rachel Chang told Global News.
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“(Water will) start condensing or freezing on to that silver iodide particle and then basically build ice up,” she said.
Chang said the ice crystals can grow rapidly until they’re large enough to fall as snow. If the temperature is warm enough, they’ll turn to rain.
Cloud seeding was invented in the 1940s and became popular in the United States in the 1960s. Dozens of countries, including Canada, use the method, sometimes to fight forest fires or drought or to create snow or to limit the size of potentially damaging hail.
The U.S. Bureau of Reclamation spent $2.4 million last year on cloud seeding along the Colorado River.
The Associated Press reported that UAE uses salt flares for cloud seeding.
Chang said there is very little scientific evidence that cloud seeding actually works.
“The challenge is partly because no two clouds are the same. And so you can’t compare what a cloud might have done before you seeded it with what it did do after you seeded it,” she said.
One recent study found a clear precipitation pattern that mirrored seeding, but a former chief scientist at the U.S. National Oceanic and Atmospheric Administration and a University of Pennsylvania climate scientist both told The Associated Press there isn’t enough evidence for scientists to conclude it works.
Did cloud seeding cause the flood?
The UAE often uses cloud seeding to get the most water the desert country can.
Several reports, according to the Associated Press quoted meteorologists at the UAE National Center for Meteorology as saying they flew six or seven cloud-seeding flights before the rains.
Flight tracking websites showed only one such flight, according to The Associated Press.
The former NOAA chief scientist Ryan Maue told the Associated Press that the cause of the rain was “most certainly not cloud seeding.”
Weather reports spotted three low-pressure systems forming a train of storms before the rain fell.
University of Pennsylvania climate scientist Michael Mann and other scientists said many of the people pointing to cloud seeding as the cause are also climate change deniers.
— with files from Global News’ Sean Previl and The Associated Press’ Jon Gambrell, Seth Borenstein and Brittany Peterson
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