Clouds have always bedeviled those trying to model Earth's climate, because they are such complex systems, involving parameters whose size and time scales range over many orders of magnitudes. (See the article by Jeffrey Kiehl inPHYSICS TODAY, November 1994, page 36.) While struggling to represent clouds realistically in their models, atmospheric researchers have at least felt that they understood the basic physics of clouds. But now they are not so sure.