Pliny (Hist. Nat. ii. 43) writes:  “Si in nube luctetur flatus aut vapor, tonitrua edi:  si erumpat ardens, fulmina; si longiore tractu nitatur, fulgetra.  His findi nubem, illis perrumpi.  Etesse tonitrua impactorum ignium plagas.”  cf. Sen., Quæst. Nat. ii. 12.