Jisho

I believe this is a popular phenomenon in Japanese that is called "ateji", where a word in Japanese has a Kanji that has nothing to do with its reading. This can be explained by the theory that, Japanese people invented the word verbally first, thus the word exists in Hiragana first (as it has a sound). Then for some reason they decided it should have a Kanji too, but the word itself doesn't originate from Chinese nor any other Japanese word, so they forcefully made up (当てる＝ateru) a Kanji for it and its Kanji form was born. This is confusing and even confused other Japanese people as it seemed like other people who did not invent the kanji for this word - つゆ, saw it somewhere as 梅雨, did not know how to read, and read it as "ばいう" that is the combination of Kun-yomi's from each of the kanji characters. This resulted in the fact that the word now has (at least) 2 correct-and-common ways to read, つゆ and ばいう. The latter phenomenon is popular too in my observation.