In Quaestiones naturales 4b.4.2 Seneca states that in early spring the weather drastically changes: in the warmer sky larger water droplets are formed and cause rain. The description of this ‘greater change’ (maior inclinatio) is linked in the manuscript tradition to two different controversial readings, temporis and aeris, which are irregularly distributed. Most recent editors have printed the first reading, but H.M. Hine is probably right to accept aeris. A careful linguistic, stemmatic and stylistic examination shows that temporis is likely to be a Medieval Latin gloss of aeris: the equivalence of both words would be difficult to justify in Classical Latin, but in Late Latin and in Medieval Latin tempus developed a climatological meaning which is explicitly found in medieval writers and glossaries and is also very widespread in Romance languages. The presence of this gloss in the hyparchetype Ψ, which is ultimately the source for most medieval copies, accounts for the irregular distribution of both readings in the manuscript tradition; this hypothesis is particularly consistent with Hine's suggestion that Ψ probably had interlinear or marginal readings. This historical investigation on the meaning of tempus is also relevant to the end of the same passage, where stylistic and linguistic evidence supports the reading tepore rather than tempore.