fulmotondro

Both halves already have pages, and this is the compound to celebrate. Fulmo is lightning, at rank 1,559. Tondro is thunder, at 1,658. Put them together with the linking -o- and you get the storm that has both, fulmotondro, at 2,297. Nothing is lost, nothing is added, and a learner who knows the two parts can read the whole the first time they see it, without a dictionary and without being told. That is the entire argument for the language in one word, and the fact that the frequency list handed us all three within eight hundred ranks of each other is a small gift.

The linking -o- is doing real work. Fulmtondro would be legal and horrible: three consonants in a row across the seam, and a reader's eye stumbling to find it. Esperanto lets you insert an -o- between the roots of a compound whenever it helps pronunciation, and this is exactly that case. Compare vaporŝipo (steamship), which keeps it, with dormĉambro (bedroom), which does not need it. The rule is your ear, and it is one of the few places in the language where you are trusted with a judgement call. The compounding article sets out when the vowel earns its place.

Weather vocabulary, gathered. Fulmi is to flash with lightning and tondri is to thunder, both used impersonally: fulmas, tondras. Ventego is a gale, ŝtormo a storm at sea, pluvego a downpour with the -eg- augmentative, and hajlo is hail. A thunderstorm is fulmotondro; a storm without the lightning is just ŝtormo or ventego, and Esperanto keeps that distinction more carefully than English does.