The best known Sindarin word for “rain” (MR/155; Ety/ROS¹), also used for “spindrift, spray” (PM/368) and “foam” (PE17/121), derived from the root √ROS (PM/368).
Conceptual Development: The earliest precursor to this word may be G. {nôs >>} G. noss or noth “rain” in the Gnomish Lexicon of the 1910s (GL/60), likely derived from the early root ᴱ√NOSO or ᴱ√NOTO which had Qenya derivatives of similar meaning (QL/67). The Etymologies of the 1930s instead had N. rhoss “rain” derived from the root ᴹ√ROS “distil, drip” (Ety/ROS¹), as seen in names from this period such as N. Celebros “Silver-rain” (Ety/ROS¹; LR/140), N. Silivros “Glimmering Rain” (Ety/ROS¹; LR/210), and N. Rauros “Rush-rain, Roar-rain” (TI/285).
In later writings Tolkien began to translate S. ross as “foam”, in names like S. Cair Andros “Ship of Long Foam” (LotR/1115; PM/371), S. Elros “Star-foam” (PM/349; Let/448) and S. Celebros “Foam-silver” (WJ/151). This word and its root gave Tolkien considerable difficulty, and in a 1968 essay labeled The Problem of Ros (PM/367-371), Tolkien first gave their meaning as “spray, spindrift”, but then explored the possibility that they were instead loan words from Bëorian. However, he was forced to abandon this line of reasoning when he remembered that S. Andros “Long-foam” has appeared in The Lord of the Rings appendices as a Sindarin word.
Neo-Sindarin: For purposes of Neo-Sindarin, I would use the noun ross mainly with the sense “rain”. I would use gwing for “foam, spindrift”. However, to preserve words like Cair Andros, I would also allow the use of ross for any more or less continuous “spray of water”, such as with waterfalls as indicated by the name Rauros “Roaring Spray” (RC/327).
n. foam. >> hross