ᴹQ. hiswe [þ] n. “fog” (Category: Mist, Fog, Haze)
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.
Reference ✧ Ety/KHIS ✧ “fog”
|ᴹ✶khithme > hiswe||[kʰitʰme] > [kʰitʰwe] > [xitʰwe] > [xiθwe] > [hiθwe] > [hiswe]||✧ Ety/KHIS|