Sometime in the coming weeks, a satellite about the size of a shoebox and roughly four kilograms, the heft of a small housecat, is set to lift off from California to attempt something no weather satellite has managed. It will begin the work of looking inside a cloud.

That sounds modest until you learn what it is in service of. The single largest source of uncertainty in how hot the planet will eventually get is not oil politics or carbon accounting. It is the cloud. The UN climate panel’s own assessment puts the likely warming from a doubling of carbon dioxide somewhere between 2.5 and 4 degrees Celsius, a range wide enough to mean either a manageable century or a brutal one, and the biggest reason it stays that wide is that we still cannot pin down what clouds will do. Clouds both cool the planet by reflecting sunlight and warm it by trapping heat, and which effect wins as the world heats up remains the single greatest open question in climate prediction.

The frustrating part is that the clouds giving us the most trouble are the small ones. The fair-weather cumulus you would have doodled as a child, a few hundred meters across, are too small and too lumpy for today’s satellites and climate models, which mostly treat the sky as smooth, flat layers. These are the small clouds “generally missed by today’s remote-sensing technologies,” in the words of Ilan Koren, the Weizmann Institute atmospheric physicist who leads the science side of the mission. We have decent pictures of giant storm systems and almost nothing on the three-dimensional guts of the ordinary clouds that drift over much of the planet.

A CT scan, but for light that bounces

This is where an Israeli-led project called CloudCT gets clever. The idea is borrowed straight from a........

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