the half hour time limit is an abstraction for gameplay. the battles last longer than 30 min. (it's actually rated for a daytime to dusk time period), but for playability you only have a half hour - imagine how slow and boring this game would be if playing a battle really took all day long. you need an abstract system for games - it's the same reasoning on why it takes 6 months (1 campaign turn) to move a fleet from Italy to Sicily.

anyway, so the weather changes occur at intervals (abstractly) over a day or half-day.