Chapter 4 of De mundo concludes the first, ‘scientific’ part of the treatise, which contains a description of the universe. Chapter 4 deals with the phenomena that belong to the science of meteorology as the ancients understood it, which included not only all kinds of precipitation, cloud formations, wind and thunder, but also phenomena such as shooting stars, earthquakes, tides and volcanic eruptions. Chapter 4 of De mundo shows similarity with Aristotle’s Meteorology in theory and general structure, but there are notable differences – the construction of the wind rose, omission of the Milky Way, and the inclusion of a lunar tide theory – which seem to indicate another source for this chapter. This is also indicated by the compendium style of Chapter 4: instead of offering causal explanations of the meteorological phenomena, the author proceeds by dividing them into groups and then briefly describing each member of every group. Some scholars have argued that the source of this chapter of De mundo is Posidonius’ treatise on meteorology. Despite some striking similarities, reasons are given against accepting this thesis. It is suggested that an earlier compendium of meteorology, combining Peripatetic and Stoic elements, was the principal source of Chapter 4 of De mundo.