Ten syllables, one sentence, and the weather changes. Shelley's fragment says that the wind carried the gentleness of rain, and that is all it says. But the line makes us feel something specific: the moment when the air turns soft and damp, and a hard, restless wind begins to behave like the rain it might bring. The central idea is that qualities do not stay in their boxes. Gentleness, usually a human virtue, belongs to rain, and rain's gentleness is then found somewhere else, in the wind.
The grammar is worth noticing. The subject of the sentence is not rain or wind but "gentleness", an abstract noun. We meet the feeling before we meet the thing that causes it. Rain arrives only as a possession ("of rain"), and the wind is where all this is held: the gentleness was "in" the wind, as if the wind were a vessel or a room. The line never says rain is falling. Judging by the wording, what is present is rain's character, not rain itself, which leaves us in a pleasantly uncertain place, perhaps just before a shower or just after one. The past tense "was" adds to this. The moment is already being remembered, as if the speaker is looking back at it.
The rhythm does much of the work. The line scans as a regular iambic pentameter: "The GEN-tle-NESS of RAIN was IN the WIND." This is the standard line of English verse, and here it runs without a stumble or a sudden stress. Nothing jolts, which suits a line about gentleness. The long word "gentleness" takes up almost half the syllables and spreads itself out, so that we slow down over it. Then the two nouns that matter, "rain" and "wind", land on strong beats, with "wind" closing the line.
The sounds are quiet too. The soft n turns up again and again, in "gentleness", "rain", "in" and "wind", so the line seems to hum under its breath. The l in "gentleness" and the hushed "-ness" ending add to the softness, and there is no harsh, clipped consonant until the final d of "wind". Even that closing stop is a small one. The words "in" and "wind" also echo each other, so "wind" feels like a natural extension of the preposition before it. The sound seems to settle where the sense does.
There is no rhyme, and there cannot be, since a single line has nothing to rhyme with. Yet a pentameter line usually expects company, and its absence is part of the effect. The title, "Fragment: Rain", tells us that this is a piece of something, and it stops with the ear still waiting for a second line. I can't say what, if anything, was meant to follow. As a reader's experience, though, the stillness works well. The line hangs in the air like the pause before rain begins.
Shelley is counted among the English Romantic poets, for whom the natural world and human feeling are closely tied together, and this line shows that habit in miniature. It is also interesting to set it beside his "Ode to the West Wind", where the wind is addressed as a "destroyer and preserver", a huge and violent force. Here the same element is almost tender. The comparison shows how far a poet's range can stretch, from storm to softness.
The line is significant because it shows how much one sentence can hold. A student can test this by tampering with it. Try "The softness of rain was in the wind", and the line loses the humming n sounds and the faint human warmth of "gentleness", a word that suggests care and touch. Swapping one word shows how exactly the original was chosen. Fragment or not, it is a complete little experience: a change in the air, caught in ten syllables.