雲 is one of the most commonly used Japanese kanji, and it also has a lot of special readings in addition to the standard onyomi and kunyomi readings.
ONYOMI 音読み
くも kumo -ぐも -gumo
KUNYOMI 訓読み
ウン un
Examples
雲がだんだん黒くなっています。The clouds are getting darker.空には雲がない。There are no clouds in the sky.雲が流れていく。The clouds are rolling away.空には雲一つ無い。There isn’t a single cloud in the sky.山の上に雲が見えた。We saw clouds above the mountain.昨日より雲が多いな。There are more clouds today than there were yesterday.今日は雲がなかった。There were no clouds today.雲一つ見えなかった。Not a cloud was to be seen.雲が切れてきている。The clouds are breaking.暗い雲は雨の前兆だ。Dark clouds are a sign of rain.