Fire and ice: Volcanic ash plays a surprising role in cloud formation

From grounding airliners to fertilizing distant oceans, volcanic eruptions are known for dramatic effects. Yet one influence has remained elusive: the way ash lofted into the upper atmosphere changes the delicate ice clouds that blanket the planet’s high altitudes.

A new investigation led by Lawrence Livermore National Laboratory (LLNL) closes that gap. It reveals that volcanic ash grains from powerful eruptions do more than just drift. They actively reshape the microphysical structure of cirrus clouds, altering how Earth absorbs and releases heat.

Volcanic ash and cloud formation

The research team analyzed ten years of observations captured by NASA’s CloudSat satellite radar and its companion CALIPSO lidar. These instruments measure ice crystal size and count, and map the height of ash-filled plumes inside clouds.

The experts compared months before and after three well-documented mid-latitude eruptions – Kasatochi in Alaska (2008), Sarychev in Russia’s Kuril Islands (2009), and Calbuco in Chile (2015). This comparison allowed the scientists to tease out the fingerprint of volcanic particles on cirrus formation.

“Our research helps close a significant knowledge gap about whether and how volcanic eruptions influence cloud formation,” said lead author Lin Lin, a scientist at LLNL. Other aerosols like dust or pollution rise slowly or stay low, making their effects harder to isolate.

Cloudy chaos was expected

Before looking at the data, the team expected to see cloudy scenes packed with many small ice particles, assuming that the extra ash grains would offer more surfaces on which ice could form.

“At the beginning of the study, we did expect clouds affected by volcanic eruptions to look different from natural clouds, but not in the way we ultimately found,” Lin said.

She and her colleagues were surprised to discover that ash-rich plumes produced the opposite pattern: the satellite record showed cirrus composed of fewer but markedly larger crystals. Clouds also became more frequent in the altitude band where the ash circulated.

Volcanic ash changes ice

Why would adding particles decrease the number of ice shards? The answer lies in how ice nucleates.

In a pristine, extremely cold parcel of air, liquid droplets can freeze spontaneously once temperatures drop below about –38 °C, a process called homogeneous nucleation. Volcanic ash changes the script by providing gritty mineral surfaces.

These surfaces encourage water molecules to align into an ice lattice at warmer temperatures and lower humidity through heterogeneous nucleation.

Because that freezing starts earlier, just a handful of ash grains can swoop up most of the available water vapor, starving the air of moisture before additional crystals can form. The result is a small population of oversized, slowly settling ice particles.

“The results completely overturned our original expectations,” Lin explained. “We anticipated that volcanic aerosols would lead to an increase in the number of ice crystals in clouds. But to our surprise, the data showed the opposite.”

She added that adapting the analysis to match the unexpected outcome – rather than clinging to the initial theory – was a challenge and a joy. “Letting go of our initial idea and developing a new explanation based on unexpected findings was both the hardest and most rewarding part of the process.”

Volcanic ash alters Earth’s climate

Cirrus clouds are thin, wispy blankets that let in most incoming sunlight yet trap infrared radiation heading back to space, exerting a net warming influence. Clouds with fewer, bigger crystals tend to be more transparent to infrared heat, potentially cooling the climate.

Volcanic ash may thin cirrus clouds, offsetting some short-term warming from sunlight-reflecting sulphate aerosols released during eruptions. Determining which influence dominates will require climate models to track ash as a distinct type of ice-nucleating particle – something many current models do not yet do.

The findings matter beyond volcanic episodes. Ash grains share chemical similarities with desert dust, a common aerosol in the upper troposphere. If dust creates sparse-crystal cirrus, it may subtly shift the greenhouse effect over dusty areas like the Sahara outflow.

Cloud data helps flight safety

Understanding how ash reshapes cirrus also has practical payoffs. Aviation maps focus on ash mass, but cloud changes can hinder satellite detection and long-range plume tracking.

For climate researchers, the study supplies a rare set of satellite-verified numbers to test how future models handle heterogeneous ice nucleation in ash-rich layers.

The LLNL team is already turning to the Arctic, where mineral dust and soot mingle above the pack ice. If northern cirrus respond similarly, changes in heat trapping could accelerate or slow polar warming.

Waiting for the next eruption

Large stratospheric eruptions are infrequent, but each one offers a fresh laboratory in the sky. With improved sensors now flying and more sophisticated retrieval techniques, the next event will allow scientists to verify whether the ash-cirrus link holds in different meteorological settings.

Until then, the decade-long record examined here provides the clearest demonstration that volcanic ash “slices” into the atmosphere in ways that reverberate through cloud physics and, ultimately, Earth’s climate.

The study shows eruptions spread giant ice crystals, linking geology, weather, and Earth’s energy balance in a surprising way.

As Lin puts it, the work reminds us that unexpected discoveries often “overturn our original expectations” – and that science advances when researchers are willing to follow the evidence, even when it clouds their initial view.

The study is published in the journal Science Advances.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–