After centuries of serving as the metaphor for mutability, clouds began to be classified by genera and species in the nineteenth century, on the model of Linnaean taxonomy. In order to standardize nomenclature, cloud watchers had to learn to see in unison, recognizing cloud types as one would recognize human faces. The analogy between cloud and facial recognition runs deep: in both cases, a few salient features (that aquiline nose, those long wispy streaks) are foregrounded at the expense of a great many others. What the art of caricature is to faces, condensed description was to clouds: a few bold strokes that focused attention on the essential and screened out everything else. Cloud classification depended crucially on description by omission.