A noun for “fog” in The Etymologies of the 1930s derived from primitive ᴹ✶khithme under the root ᴹ√KHITH “mist, fog” (Ety/KHIS; EtyAC/KHIS).

Neo-Quenya: The phonetic developments thm > þm̌ > þw > sw were consistent with Quenya phonology of the 1930s (PE19/44), but sometime around 1960, or shortly before, Tolkien revised these rules as discussed in the entry on the phonetic development of voiceless stops and aspirates before nasals (PE19/87 plus p. 88 note #88). I personally prefer the pre-1960 rules, and would retain this form for Neo-Quenya. If, however, you prefer the later rules, you should (a) stick to Q. hísë for both “mist” and “fog”, (b) revise this word to ᴺQ. *hitwe to fit later phonology, or (c) assume the primitive form was *khithwē rather than ᴹ✶khithme.