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).
This root appeared in The Etymologies of the 1930s as ᴹ√ROS “distil, drip” with derivatives like ᴹQ. rosse “fine rain, dew” and N. rhoss “rain”, the latter an element in N. Celebros “Silver-rain” (Ety/ROS¹). In later versions of The Silmarillion, the name S. Celebros was translated “Foam-silver” (WJ/151), indicating a shift in meaning, though the element still meant “rain” in other later names like S. Silivros “Sparkling Rain” (MR/155) and S. Dimrost “Rainy Stair” (S/220).
Tolkien discussed this root at length in a 1968 essay labeled The Problem of Ros (PM/367-371). He indicated that by this point, the intended meaning of the root √ROS was “spindrift, spray”, but he felt this meaning was problematic due to its conflict in Sindarin with S. ross “red haired” and the similarity of the root to Latin “rōs” = “dew” (PM/368). He then launched into a lengthy discussion formulating a new theory whereby the element -ros was actually Beorian to explain its use in various Beleriandic names, only to remember at the last minute that he had also used this element in the Third Age place name S. Cair Andros “Ship of Long Foam”, rendering his alternate theories unviable (PM/371). Presumably at this point the original meaning of the root was restored.