Ama-mori is a native Japanese word, a compound consisting of ame 'rain' (here modified to ama) and mori, from the verb moru meaning 'to leak'. The meaning is 'a leak' of the kind that occurs in a building or shelter when it rains.
Ama-mori is written in Chinese characters. This is how it is written in this temporary sign at a railway station:
At another station, mori had been simplified to hiragana as , another example of a native Japanese verb being written in hiragana in preference to kanji. One reason for this is the relative complexity of the character . This is the sort of character that people readily recognise but may occasionally be unable to recall if they have to write it on the spur of the moment.
A Google search in September 2003 revealed the following pattern for ama-mori:
Note that the preferred usage is kanji first (), followed by kanji or hiragana ( or ). This top-heavy arrangement is preferred. The opposite arrangement, the bottom-heavy , is virtually non-existent. See also Akasuri.