in Thoreau’s Journal:

Within a day or two has begun that season of summer when you see afternoon showers, maybe with thunder, or the threat of them, dark in the horizon, and are uncertain whether to venture far away or without an umbrella. I noticed the very first such cloud on the 25th of May, —the dark iris of June. When you go forth to walk at 2 p. m. you see perhaps, in the southwest or west or maybe east horizon, a dark and threatening mass of cloud showing itself just over the woods, its base horizontal and dark, with lighter edges where it is rolled up to the light, while all beneath is the kind of dark slate of falling rain. These are summer showers, come with the heats of summer.