How a silver crystal can turn clouds into rain

In a new study, researchers at TU Wien mapped how silver iodide starts ice at the atom scale. The scientists discovered that only one side of a silver crystal has the right atomic structure to trigger ice formation, revealing how these tiny particles help clouds release rain.

The key number is 24.8 degrees Fahrenheit, a temperature where silver iodide can start freezing in supercooled clouds. That is far warmer than pure water can freeze on its own without a solid trigger.

Why a tiny surface change matters

The work was led by Jan Balajka, a physicist at Vienna University of Technology. His research focuses on atomically precise surfaces and the first steps of ice growth.

When clouds freeze on particles, scientists call that ice nucleation, the process where the first stable ice cluster forms. It often happens on dust or salt, but silver iodide is especially active.

Cloud seeding adds tiny silver iodide particles to clouds from planes to nudge ice formation.

“Cloud seeding is a decades-old approach to modifying weather that uses a range of supporting technologies for research and operations,” stated the U.S. Government Accountability Office in a 2024 technology assessment.

Two faces, two outcomes

At the surface, atoms can reshuffle into new patterns called surface reconstruction, atoms rearranging at the top layer to lower energy.

The team showed that the silver terminated face keeps a hexagonal pattern that matches ice, while the iodine side rebuilds into a rectangular grid that breaks the match.

The experiments used noncontact atomic force microscopy (NAFM), a method that images atoms by sensing tiny forces without touching. They ran at cryogenic temperatures to lock atoms in place.

“These findings highlight the decisive role of surface atomic structure and indicate that the Ag-terminated basal plane is primarily responsible for efficient ice nucleation on AgI,” wrote Balajka. That line captures why one face works and the opposite face stalls growth.

Crystal formation and cloud seeding

In clouds, freezing often begins by heterogeneous nucleation, ice beginning on a solid surface rather than in pure water. Because only the silver side helps, the mix of faces on each airborne particle will influence how many crystals form.

For decades, lab work pointed to a close lattice match between hexagonal ice and silver iodide. An older X-ray analysis reported the closest match then seen for a nucleant of this type.

That match story is now sharper. The new results show that the atomic pattern at the very surface, not the deeper crystal, sets the template that water follows.

The policy context is not simple. A federal assessment reported that nine U.S. states run programs and that added precipitation estimates range from 0 to 20 percent.

This does not mean seeding always works. It means the cloud type, the seeding delivery, and the exact particle surfaces together decide the outcome in the sky.

What the models say

The Vienna team paired imaging with density functional theory (DFT), a quantum method that calculates how electrons bind atoms. That combination identified the most stable surface patterns and how water organized into an ice-like sheet.

Separate simulation work adds a related constraint on how ice starts on this material. One recent mechanism concluded that at least four molecular layers of water must build up on silver iodide before freezing begins.

This push and pull between surface order and water layering explains part of seeding’s mixed record. If a particle exposes too much of the iodine face, or if humidity is low, ordered growth can lag even when particles are present.

The practical takeaway is clear for materials design. To build better ice starters, tune surfaces that hold the right atomic spacing and encourage ordered water layers.

Future research on crystals and rain

Lab conditions differ from windy, sunlit clouds. The study used ultraclean crystals and low temperatures to isolate the first steps.

Field particles are smaller and rough. Many facets expose both faces, so the fraction of silver terminated area will vary from particle to particle.

That reality matters for real flights. A batch rich in iodine facing facets could be less efficient even if the bulk material is the same.

The research also points to practical tweaks. Manufacturers could aim for particle treatments that stabilize the silver terminated surface or mask the iodine face.

There is a broader benefit for climate models. Knowing which faces seed ice best helps tune how models convert cloud droplets into ice and snow.

Better physics at the atomic scale makes better weather predictions at regional scales. It helps policymakers decide when seeding is worth attempting and where.

The study is published in the journal Science Advances.

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