A primer on cloud seeding

no, it did not cause the flooding in Dubai

The extraordinary rain and flooding in Dubai, combined with claims that the the UAE had been seeding clouds before the event, has led many on social media to claim that it was cloud seeding that caused the floods.

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Cloud seeding refers to attempts to manipulate the properties of clouds to produce rain or snow. It has a very checkered history and, by the end, you’ll see why I’m skeptical.

How does cloud seeding work?

Let’s begin with a surprising cloud fact: cloud droplets are not pure water. When you look at a cloud, every single cloud droplet contains a cloud condensation nucleus (CCN), a minute particle dust, pollen, soot, or even sea salt, which is essential for condensing water vapor into a droplet.

Clouds are strongly influenced by the availability of CCN. One of the most visually striking examples is the formation of “ship tracks.” These are clouds that form along the paths of ships, visible from space as stark, linear patterns across the oceans. They form when ships’ diesel exhaust, which is full of particles, serves as additional CCN and leads to the generation of clouds.

The lines on this plot are ship tracks — clouds formed when particle-laden ship exhaust acts as cloud nuclei, leading to cloud formation. Link

OK, back to cloud seeding. Imagine the following situation: a cloud full of liquid particles that are supercooled, meaning the temperature of the water in the cloud droplets is below 0°C. There are several different factors that can prevent these particles from spontaneously converting to ice, but I won’t go over them here — just recognize this is possible. In this schematic, the water droplets are blue:

Now let’s introduce some ice nucleating particles (INPs) into this cloud. INPs are typically complex structures like mineral dust or biological particles, which are effective at catalyzing the deposition of water vapor directly as ice, bypassing the liquid phase under colder conditions. These INPs start growing as ice particles, which are represented by the gray hexagons:

From Wikipedia, here’s what happens next:

The Wegener–Bergeron–Findeisen process is a process of ice crystal growth that occurs in mixed phase clouds (containing a mixture of supercooled water and ice) ... This is a subsaturated environment for liquid water but a supersaturated environment for ice resulting in rapid evaporation of liquid water and rapid ice crystal growth through vapor deposition. If the number density of ice is small compared to liquid water, the ice crystals can grow large enough to fall out of the cloud, melting into rain drops if lower level temperatures are warm enough.

In other words, the ice particles start growing by pulling water vapor out of the air, which dries the air, leading to evaporation of the liquid droplets. After a bit of time, this is what the cloud looks like:

Eventually, the ice particles grow large enough to fall out of the atmosphere and, lo and behold, cloud seeding has caused it to rain or snow!

There are various ways to seed clouds, such as having planes fly through clouds and drop INPs or release INPs from ground-based dispensers and let atmospheric currents carry them into the clouds.

Watch this super cool video showing a cloud seeding demonstration from the group that invented it:

Does cloud seeding actually work? Yes!

There was a scientific experiment a few years ago to measure how much additional snowfall could be created by seeding mountain-wave clouds. Mountain-wave clouds — clouds formed when air cools as it flows over mountains — are some of the most amenable to cloud seeding because the air cools rapidly as it ascends, leading to large numbers of supercooled liquid droplets. Adding INPs to this air can generate ice particles that fall out of the cloud.

The results of this experiment showed that it did work, but the enhancement was modest.

Is cloud seeding a scam? Also Yes!

Cloud seeding works by redistributing water in clouds that already contain a lot of water. This means that it only works in very specific situations. It cannot, for example, work if there are not already water-rich clouds in the atmosphere.

Many people around the world, desperate for rainfall, and not familiar with the principles of cloud physics, are vulnerable to exploitation by cloud seeders who overstate the effectiveness of their methods or promise unrealistic outcomes. Unscrupulous cloud seeders can make a good living taking advantage of these people by offering them hope and solutions when mainstream meteorologists tell them there is no hope. Occasionally, natural rainfall might coincidentally follow a cloud seeding attempt, allowing these operators to claim success and further perpetuate their deceptive practices.

In 2011, my colleague and Texas State Climatologist, John Nielsen-Gammon, was recruited to verify the outcomes of a cloud-seeding experiment. It was a dismal failure that produced no rain. He wrote about his experiences and findings on his blog (archived here), where he concluded:

By now my working hypothesis is that the rainmakers are not dishonest or trying to fool anybody, they simply don’t understand how the atmosphere works and have fooled themselves into thinking they can do something about it. Strange things happen with the weather all the time, and if the right things happen it’s easy to start thinking that you’ve managed to do something. My parting words to the investment coordinator are: “If you wait long enough, it will rain anywhere.”

Given all of this background and history, most meteorologists I know have come to view cloud seeding skeptically — my default position is that it’s always a scam. While I am always open to changing my stance in light of new and compelling evidence, such evidence has not yet appeared.

Thus, it’s probably not surprising that I’m skeptical about the impact of cloud seeding on the floods in Dubai. My skepticism is quite general, but many other meteorologists have weighed in with specific objections to the connection between this particular event and cloud seeding.

On the other hand, there’s always climate change. One of the oldest and most robust predictions of climate science is that a warming world will experience more intense precipitation events. This expectation stems from the basic physics and has been observationally validated, so we can be confident that this really happening.

Events like the recent flooding in Dubai can be viewed as consistent with these predictions. However, before we conclude that climate change definintely contributed, detailed attribution studies are essential to determine precisely how much of an impact climate change had on any specific weather events.

In the end, though, it’s far more likely that climate change played a role than cloud seeding.

Related resources

Read John Nielsen-Gammon’s write-up about his work verifying a cloud-seeding experiment. The original blog is not available, but this page contains links to an archive.

Good write-up from DRI about cloud seeding

Much of the research supporting cloud seeding is crap

Washington Post on cloud seeding & Dubai

NewScientist on cloud seeding & Dubai