Think how a goatherd off on a mountain lookout
spots a storm cloud moving down the sea…
bearing down beneath the rush of the West Wind
and miles away he sees it building black as pitch,
blacker, whipping the whitecaps, full hurricane fury –
the herdsman shudders to see it, drives his flocks to a cave –
so dense the battalions grouped behind the two Aeantes,
packed, massed with hardy fighters dear to the gods,
battalions black and bristling shields and spears,
fighters sweeping into the breaking storm of war.

– Homer

The Iliad, Book 4, lines 315-324. This is a description of the Achaean troops under their captains Great Ajax and Little Ajax as they ready for battle. In this Homeric simile the dense battalions are liked to storm clouds.