A Sindarin word for “dew” given as míð in the Outline of Phonology (OP2) from the 1950s as a derivative of ✶mizdē “drizzle” (PE19/101), illustrating how [[os|[z] vanished before [d] lengthening preceding vowel]] in (Old) Sindarin.
Conceptual Development: N. mîdh “dew” also appeared in The Etymologies of the 1930s, already with the same derivation as given above (Ety/MIZD), though Tolkien first wrote its gloss as “fine rain” (EtyAC/MIZD). This deleted gloss seems to be a remnant of ᴱN. midh “mist, drizzle” from Early Noldorin Word-lists, but there its primitive form was ᴱ✶míye (PE13/150).
Neo-Sindarin: For purposes of Neo-Sindarin, I would extend the meaning of this word to include “✱moisture, damp(ness)” in general, especially as the result of a previous rain.
dew