A cloud arose...in likeness and form resembling a pine tree; for it was elevated to a good height, with a long trunk, and distributed in several branches. The reason, I suppose, was that it was raised aloft by a sudden wind, and then relinquished by it, as it decayed, or else overpowered by its own weight, it spread it self into a large breadth; appearing sometimes white, sometimes shadowy, and variously coloured, as it was loaded with ashes or with earth. It struck him with surprise, and seemed to merit a nearer examination.
He's describing the eruption of Vesuvius, but if you're writing an alternative history about aliens nuking the Eternal City, it'd work there too.
[The same letter contains the most simultaneously silly and foreboding image I've seen in ancient literature: "They debated all together, whether they should stay in the house, or walk in the open field: For the buildings were shocked by violent and repeated earthquakes [but] the fall of the pumice stones, though light and eaten through, alarmed them. A comparison of the two dangers fixed their choice on the field...and to guard against the fall of the stones, they tied each of them a pillow about their heads with handkerchiefs or napkins. It was now day in other places, but there it was still night, more black and dismal than ever was known..."]