There are several words which have vowel variations. This seems like it could be a fossilization of morphological processes in an older version of Japanese. How much do we know about these processes? Can we reconstruct a grammatical or phonological explanation for these vowel variations?
雨: あめ
雨水: あまみず
月：つき
月読：つくよみ
高：たか
丈：たけ
