An element meaning “foam” (or perhaps “✱spray”) in the name Q. Elerossë “Star Foam” cognate of S. Elros in notes from the late 1960s (PM/349), or “rain” in the name of the waterfall Q. Raurossë “Roaring-rain” cognate of S. Rauros in notes probably from the 1950s (PE19/99). In The Etymologies of the 1930s, Tolkien gave ᴹQ. rosse “fine rain, dew” as a derivative of the root ᴹ√ROS “distil, drip” (Ety/ROS¹).
Neo-Quenya: For purposes of Neo-Quenya, I would assume this word means a light rain or a continuous spray of water, or the remnants of such water in the form of dew, in other words any persistent collection of small droplets of water either in the air or on the ground, for example from light rain, morning dew or falling mist off a waterfall. This seems to be in keeping with most of its attested glosses.
For more ordinary or heavier rain, I’d use ulo instead, or for a drizzle I’d use mistë, and would reserve rossë only for a light mist-like rain. For a floating mist or fog, I’d use hísë or (if thicker) hiswe. Thus of precipitation, I’d have ulo > mistë > rossë > hísë in decreasing ranks of density, where hísë “mist” is light and/or cold enough not to longer fall. But I think rossë can refer to “already fallen” droplets in the form of dew as well.
rossë noun "fine rain, dew" (ROS1, PM:371)