Note · Climate and energy
What is cloud seeding? Can we really make it rain?
Cloud seeding sprays particles such as silver iodide into clouds that are already there, to help their water fall as rain or snow. It can add a little, not make storms.
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Contents · 6 sections
Cloud seeding is a way of nudging clouds that already exist to drop more of their water, by adding tiny particles, usually silver iodide or salt, that vapor and droplets can freeze or collect on. It is released from aircraft, rockets or burners on the ground. Seeding can't create clouds out of clear sky, and the best measurements show it adds modest amounts of rain or snow under the right conditions, not floods.
In short
- Seeding works only on suitable clouds: it gives their water something to freeze or gather on, so more of it grows heavy enough to fall.
- The clearest evidence, from radar studies of winter mountain clouds, shows real but small gains, measured in tenths of a millimeter.
- Seeding cannot make a storm, and no major flood has been shown to have been caused by it.
How does cloud seeding work?
A cloud is a crowd of droplets too small to fall. A typical cloud droplet is about a hundredth of a millimeter across; a raindrop holds roughly a million times as much water. Most clouds never rain because their droplets never grow that big. Seeding tries to speed up one of the two ways droplets grow.
The first is through ice. Many clouds are supercooled: their droplets stay liquid well below 0°C because pure water needs something to freeze onto, and below about −40°C it freezes on its own. Silver iodide crystals have a shape close to that of ice, so supercooled water freezes on them. Once ice crystals form among the droplets, they grow at the droplets' expense, because water vapor settles more readily on ice than on liquid water. The crystals get heavy and fall as snow, or melt into rain on the way down. This is called glaciogenic seeding, and according to the Wikipedia summary of the research, it works best in clouds between about −20°C and −7°C.
The second is through salt. In warm clouds, hygroscopic seeding releases salt particles that attract water, so a few droplets grow large, collide with others and merge into drops heavy enough to fall. The United Arab Emirates uses this approach on summer convective clouds.
The method dates to 1946. That July, Vincent Schaefer at General Electric found that dry ice turned supercooled cloud in a freezer into ice crystals, and later that year his colleague Bernard Vonnegut showed that silver iodide did the same. On 13 November 1946, a plane dropped dry ice into a cloud near Mount Greylock, Massachusetts, and snow fell from it.
What happens in one seeding operation?
Take a winter operation of the kind run over the mountains of Idaho, where utilities seed storms to add to the snowpack that feeds rivers and reservoirs in spring.
- Forecast. Meteorologists look for a storm with supercooled liquid water in its clouds and winds that will carry seeding material over the target mountains. Clouds that are already snowing efficiently, or too warm, are left alone.
- Release. An aircraft flies back and forth upwind of the range, burning flares that release silver iodide smoke, or ground generators on the slopes burn a silver iodide solution that rises into the cloud.
- Freezing. Downwind of the release, ice crystals begin to form on the particles within minutes and grow as they drift.
- Snowfall. About half an hour or more later, the crystals fall out over the mountains as snow.
- Spring. The extra snow melts with the rest and runs into the river system.
In 2017 a research project called SNOWIE tracked that chain in Idaho with radar for the first time, by seeding in zigzag lines where little snow was falling naturally, so that the seeded snow showed up as a zigzag on the radar. The study, published in 2020, found that gauges on the ground measured 0.05 to 0.3 mm of extra snowfall (counted as melted water) as the seeded snow passed over. Added up over the area, the three cases produced about 120,000 cubic meters of water from 20 minutes of seeding, 240,000 from 86 minutes and 340,000 from 24 minutes.
Put that in pictures. An Olympic swimming pool holds about 2,500 cubic meters, so the best case made about 136 pools of water. That is real water, but 0.3 mm is about the thickness of three sheets of paper, and a single ordinary winter storm can leave tens of millimeters of water across the same mountains. Seeding, at its best, is a small addition to a storm, not a storm.
Does cloud seeding actually work?
It does something, but how much is hard to show. Weather varies so much from day to day that a 10% change in rain is easily lost in the noise, and the only reliable test is a randomized experiment that seeds some storms and not others, chosen by chance, over many years. Few programs have done that.
| Where | What it seeds | What the evidence shows |
|---|---|---|
| Idaho and Wyoming, US | Winter storms over mountains, to add snowpack | Radar studies (SNOWIE, 2020) saw seeded snow directly; the Wyoming pilot project estimated at most about a 3% seasonal gain |
| Israel | Winter rain clouds over the north | Early experiments in the 1960s and 1970s suggested gains; later analysis found little or none, and the national program was stopped in 2021 as ineffective and expensive |
| United Arab Emirates | Summer convective clouds, with salt | Large program since the late 1990s; gains are estimated by the operator, not from long randomized trials |
| China | Rain and hail suppression across many provinces | The world's largest program; published randomized evaluations are few |
| Australia | Rain over Tasmania and snow in the Snowy Mountains | Of the national trials begun in 1947, only the Snowy Mountains trial gave a statistically significant increase |
In December 2024, the US Government Accountability Office reviewed the research and found that studies estimated between 0% and 20% more precipitation from seeding, from a handful of recent studies, and that measuring the effect remains difficult. It counted nine US states using seeding and others that had banned or considered banning it.
Still argued
Researchers and operators disagree on how much seeding adds over a whole season. Operators report seasonal gains of a few percent to about 10%: the Sacramento Municipal Utility District, which has seeded since 1969, reports 3% to 10% more snowpack. Many atmospheric scientists accept that seeding can increase snowfall from suitable winter mountain clouds, as SNOWIE showed, but say gains from summer storms are unproven, because convective clouds are too variable to measure against. The GAO's range of 0% to 20% captures the spread rather than settling it.
Did cloud seeding cause the Dubai or Texas floods?
No evidence shows that it did. On 16 April 2024, a storm dropped record rain on the United Arab Emirates and flooded Dubai's roads and airport. Social media blamed the country's seeding program. The storm was part of a large weather system that forecasters had tracked across the Gulf, and the UAE's National Center of Meteorology told CNBC that it had not seeded during the event, saying it does not seed during extreme weather and that once a severe thunderstorm is present it is too late to.
On 4 July 2025, flash floods in central Texas, worst along the Guadalupe River, killed more than 100 people by PolitiFact's count three days later, a toll that rose further as the missing were found. A company called Rainmaker had seeded two clouds over south-central Texas on 2 July. PolitiFact rated the claim that it caused the floods false: the seeded clouds had dissipated more than a day before, and meteorologists traced the rain to moisture from the remnants of Tropical Storm Barry meeting the Hill Country's terrain, with rain falling at 4 to 6 inches an hour at times.
The physics backs the fact-checks. A storm like either one moves billions of cubic meters of water; the best-measured seeding added hundreds of thousands. Warming does make heavy downpours more likely, because warmer air holds more water, which is part of climate change, not seeding.
Is cloud seeding safe, and is it geoengineering?
Silver iodide is used in small amounts, and studies of seeded areas have found silver levels far below those that harm people or wildlife, though some ecologists want more monitoring in sensitive places. Seeding is sometimes called geoengineering, but it is local and short-lived: it changes one storm over one valley for an hour, not the planet's temperature. It also needs clouds, so it can't end a drought, when suitable clouds are what's missing. Large-scale weather patterns such as El Niño decide how many come.
Scientists once tried to weaken storms the same way. From 1962 to 1983, the US government's Project Stormfury seeded hurricanes, hoping to shrink their eyewalls, and gave up when it became clear that hurricanes held too little supercooled water for seeding to matter and changed that way on their own.
Check yourself
- Why does silver iodide make a supercooled cloud drop snow, and why does that need the cloud to be below freezing?
- What did the SNOWIE radar study show that earlier gauge statistics couldn't?
- Why can't seeding have caused a flood like the one in Texas in 2025?
Sources
- Quantifying snowfall from orographic cloud seeding — Friedrich and others, PNAS, 2020
- Cloud seeding technology: assessing effectiveness and other challenges — U.S. Government Accountability Office, 2024
- UAE denies cloud seeding took place before severe Dubai floods — CNBC, 2024
- Naturally occurring rainfall caused deadly Texas floods, not cloud seeding — PolitiFact, 2025
- Cloud seeding — Wikipedia