This is probably the most trivial question I've ever asked here, and relates to something mentioned in passing. The passage occurs in The Two Towers, Book IV, "The Taming of Smeagol". While attempting to climb down from a mountain, Frodo and Sam are caught in a fast-moving storm caused by Sauron. Tolkien writes:
With that [Frodo] stood up and went down to the bottom of the gully again. He looked out. Clear sky was growing in the East once more. The skirts of the storm were lifting, ragged and wet, and the main battle had passed to spread its great wings over the Emyn Muil; upon which the dark thought of Sauron brooded for a while. Thence it turned, smiting the Vale of Anduin with hail and lightning, and casting its shadow upon Minas Tirith with threat of war. Then, lowering in the mountains, and gathering its great spires, it rolled on slowly over Gondor and the skirts of Rohan, until far away the Riders on the plain saw its black towers moving behind the sun, as they rode into the West. But here, over the desert and the reeking marshes the deep blue sky of evening opened once more, and a few pallid stars appeared, like small white holes in the canopy above the crescent moon.
Here are the relative positions of the cloud, Frodo, the Riders, the setting sun, and Mordor, as I understand them (obviously not to scale, but that doesn't matter). Note that the cloud never moves anywhere near the Riders - it is always east of them, and the sun is always west of them.
Unless I don't understand weather in Middle-earth, this doesn't make any sense. How can a storm cloud "move behind the sun"? Every storm cloud I have ever seen has been decidedly closer to me than to the sun. Storm clouds are in front of the sun, never behind it. What is Tolkien trying to describe here?