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A new study shows that natural dust particles swirling in from faraway deserts can trigger freezing of clouds in Earth's Northern Hemisphere. This subtle mechanism influences how much sunlight clouds reflect and how they produce rain and snow—with major implications for climate projections.

The study is in the journal Science.

Drawing on 35 years of satellite observations, an international research team led by ETH Zurich found that mineral dust—tiny particles swept up by the wind and carried into the upper atmosphere—can trigger the freezing of cloud droplets. This process is particularly important in northern regions, where clouds often form in a temperature range just below freezing.

"We found that where there's more dust, clouds are much more likely to freeze at the top," explains Diego Villanueva, a Post-doctoral researcher for Atmospheric Â鶹ÒùÔºics at ETH Zurich and lead author of the study. "This has a direct impact on how much sunlight is reflected back into space and how much precipitation is generated."

Dust turns clouds to ice

The researchers focused on mixed-phase clouds, which contain both supercooled water and ice, forming between −39 °C and zero °C. These clouds are common in mid- and high-latitudes, especially over the North Atlantic, Siberia, and Canada. They are known to be extremely sensitive to changes in their environment—particularly to the presence of ice-nucleating particles which stem primarily from desert dust aerosols.

By comparing the frequency of ice-topped clouds with dust levels, the researchers observed a remarkably consistent pattern: the more dust and the cooler the clouds, the more frequent the ice clouds. What is more, according to the researchers, this pattern aligned almost perfectly with what laboratory experiments had predicted about how dust triggers droplet freezing.

"This is one of the first studies to show that satellite measurements of cloud composition match what we've known from lab work," says Ulrike Lohmann, senior co-author, and Professor of Atmospheric Â鶹ÒùÔºics at ETH Zurich.

A new benchmark for climate models

The way clouds freeze directly affects how much sunlight they bounce back into space and how much water they release as precipitation. These factors are vital for climate models, but until now, many of these models lacked a solid reference point for how cloud freezing really works on a global scale.

The new findings establish a measurable link between airborne dust and cloud-top ice frequency, providing a critical benchmark for improving climate projections. "It helps identify one of the most uncertain pieces of the climate puzzle," says Villanueva.

A complex picture—with a clear signal

For decades, atmospheric scientists have studied droplet freezing at the microscale. This study shows, for the first time, that cloud ice formation (or glaciation) follows the same behavior as droplet freezing—but on a much larger scale.

This finding expands the scope of atmospheric research in this area—from nanometer-scale structures of dust surfaces that form atmospheric ice crystals to kilometer-scale cloud systems in which ice formation can be observed from space.

Still, the dust–ice link does not play out equally across the globe. In desert regions like the Sahara, cloud formation is sparse, and the strong movement of hotter air may suppress the freezing process.

Also, in the Southern Hemisphere, marine aerosols may take over dust's role. The research team emphasizes the need for further studies to better understand how other factors such as updraft strength or atmospheric humidity, for example, influence cloud freezing.

For now, however, one thing is clear: tiny dust grains from distant deserts help shape the clouds above our heads—and with them, the future of our climate.