Veronique Greenwood, reporting for Discover Magazine:
The clouds in the Amazon, just like everywhere else, consist of water vapor clinging to tiny clumps of carbon compounds. In forested areas, the carbon compounds are byproducts of plants’ metabolism; in populated areas, they are often from human pollution. Most of the time, atmospheric chemists can see the carbon clumping taking place; when the microscopic bits reach a certain size, they are able to attract and hold water. In the Amazon, the clumps seem to appear out of nowhere, nearly fully formed. No one has ever been able to catch them in the act of coming together.
This is a perfect example of why we should keep hammering away at seemingly impossible questions. It would be easy to throw our hands in the air. But these scientists didn’t, and now we know something new.