What do we know about vowel variations such as 雨 read as あめ/あま


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?

[code]

    雨: あめ
    雨水: あまみず
    
    月：つき
    月読：つくよみ
    
    高：たか
    丈：たけ
    
[/code]

