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.
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.