Hidden anatomy of clouds reveals how rain may begin without ice crystals
The initiation of rain in clouds without ice remains one of the largest unsolved mysteries in atmospheric science.
Researchers from the Max Planck Institute for Dynamics and Self-Organization (MPI-DS) have uncovered previously invisible structures within shallow cumulus clouds in their study published in PNAS.
Using the unique Max Planck CloudKite platform, the team discovered submeter regions where cloud droplets cluster together. Such areas provide ideal conditions for droplet collisions, which in turn have a high chance of triggering rainfall.
Revealing the reasons for rainfall
Globally, shallow clouds constitute a large proportion of all clouds covering oceans and land. They do not contain ice crystals that could serve as seeds for rainfall. Instead, shallow cumulus clouds are composed solely of tiny liquid water droplets. While they can produce rain within minutes, it remains unclear what causes larger raindrops to form and how researchers can ultimately predict rainfall. Before rain initiation, the droplets face a bottleneck: They must collide with others to form a larger drop that eventually falls. The scientists thus investigated how this initial bottleneck can be overcome.
"We assessed the structure of a warm cloud at high spatial resolution," says Mohsen Bagheri, group leader at MPI-DS and last author of the study. "It is like a 3D microscope in the clouds, investigating particle size and distribution," he continues. When using planes to measure particles, their high speeds allow for only a few measurements at greater intervals. In contrast, drones are limited in flight time and conditions and may also create turbulence, altering cloud composition. Using a helikite balloon, MPI-DS scientists led by Bagheri and director Eberhard Bodenschatz therefore developed a unique measurement platform to study cloud structure in unprecedented detail.
The CloudKite: A unique airborne laboratory
At the heart of this observatory are two custom-built optical imaging systems using powerful lasers and high-speed cameras.
One is capable of reconstructing the three-dimensional positions and sizes of individual cloud droplets at a rate of 75 times per second, making it one of the fastest airborne holographic instruments ever developed. The other measures turbulence within clouds and is the first airborne particle image velocimetry system. Both instruments operate autonomously, allowing them to simultaneously capture cloud microphysics and turbulence from micrometers to kilometers. The CloudKite platform drifts through clouds at only about 10 meters per second (22 mph), significantly slower than research aircraft.
"Together with the instrument's high imaging rate, this enables measurements every 12 centimeters (4.7 inches)—around 250 times more frequently than previous airborne observations," Bodenschatz says. "The CloudKite thus opens a new observational window into clouds," he summarizes.
Hidden hotspots where rain may begin
Using data from the CloudKite, researchers reconstructed the internal anatomy of a 55-meter (180-foot) section of a shallow cumulus cloud. Rather than being evenly distributed, droplets formed highly localized "hotspots" only about a meter (3 feet) across or even less. Within these regions, droplets are much closer together than elsewhere in the cloud, significantly increasing the chances of collisions and the formation of larger drops.
"Because droplets cluster there, collisions become much more likely. These localized hotspots may therefore represent the places where rain starts in shallow cumulus clouds," says Birte Thiede, first author of the study.
These observations challenge the long-standing assumption that droplet clustering is weak and evenly distributed throughout clouds. Instead, clouds possess a hidden internal structure that had remained invisible until now.
Toward better climate predictions
The team is currently investigating how turbulence is linked to these localized hotspots of clustering. Future field campaigns with the CloudKite observatory are planned in Amazonia, the Baltic Sea and northern Finland.
As warm clouds are responsible for much of Earth's rainfall, particularly in the tropics, they play a central role in regulating Earth's energy budget.
Their lifetime and ability to reflect sunlight back into space depend critically on how efficiently cloud droplets grow into raindrops and are among the largest uncertainties in climate projections. "Revealing the hidden structure of warm clouds will lead to better descriptions of rain formation and more accurate weather forecasts," Bagheri concludes.
Publication details
Birte Thiede et al, Highly localized droplet clustering in shallow cumulus clouds, Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2602976123
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Citation: Hidden anatomy of clouds reveals how rain may begin without ice crystals (2026, August 7) retrieved 8 August 2026 from https://phys.org/news/2026-08-hidden-anatomy-clouds-reveals-ice.html
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