What do you call the stillness right before a storm breaks?

I've spent twenty-odd years watching rivers and skies, and I keep coming back to that one quiet beat — the half-hour before thunder where the whole valley seems to hold its breath. I catch myself trying to name it, and every word I come up with is wrong (hush, lull, the waiting — none of them fit). Do you know that pause in your own work or your own days? What do you call it?