By Andrew Price

Clouds seem pretty straightforward, right? They're just droplets of water hanging in the atmosphere. But scientists still don't have a good understanding of how clouds form or what role they play in the Earth's climate.

Figuring that out is the goal of the CLOUD project at CERN (that's the same European research center where scientists are looking for the "God particle"). The project, led by British physicist Jasper Kirkby, is trying to understand how clouds form in the sky by recreating those conditions on Earth. To do that, Kirkby and his colleagues are using "a very, very high-tech metal bin" called the CLOUD chamber.

Scientists know that cloud formation happens when tiny particles in the atmosphere grow large enough (through condensation or coagulation) to become "seeds" for cloud droplets. But that seed-formation process isn't well understood. Scientists don't know what kinds of particles can become cloud seeds and, more importantly, they don't know how cosmic rays streaming in from space affect the process. The CLOUD chamber helps them recreate high-altitude conditions and then add particles and cosmic rays to create clouds themsevles.

But particles are so sparse at high altitudes that doing controlled experiments requires a very sterile space. "That's why doing the CLOUD experiment is very tough," Kirkby explained in a recent interview with Yale e360. "It means that you have to suppress any contaminant vapors at the level of one in a million million molecules. It's right at the limit of technology, but I think we've gotten there with CLOUD."

Like most work at CERN, the CLOUD project is long (three years), interdisciplinary, and international. Physicists working on the project come from nine countries and represent many different subfields.

So who cares about clouds? Well, clouds cool the Earth by reflecting sunlight (when a cloud goes overhead, you get cooler). So changes in how much cloud cover we have can have big effects on our climate. Some climate change skeptics have suggested that global warming has more to do with changes in cosmic radiation, and thereby cloud cover, than human activity.

Are they right? That's what Kirkby and his colleagues are trying to find out.

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