  Many testimonia credit Xenophanes with an interest in meteorological and astronomical phenomena. Not only are these comments of interest in their own right, they also present us what was arguably his single most important scientific contribution--his contention that clouds or cloud-like substances play a basic role in a great many natural phenomena. The term nephos (“cloud”) appears only twice in the fragments of his work (in B30 and 32) but many testimonia either bear directly on the nature of clouds or make use of clouds in order to explain the nature of other phenomena. To cite an example of the first type, according to Diogenes Laertius “he says…the clouds are formed by the sun’s vapor [i.e. vapor caused by the heat from the sun’s rays] raising and lifting them to the surrounding air” (A1.24–5). Aëtius (A46) provides a similar account: