1For the sky to become cloudy or overcast.
午後から天気が曇るそうだ。
They say it'll turn cloudy from the afternoon.
空が曇ってきたので早めに帰ろう。
The sky is clouding over, so let's head home early.
今週は曇る日が多い。
There are many cloudy days this week.
2For glass, a lens, etc. to fog up or become misty from moisture.
マスクをすると眼鏡が曇って困る。
When I wear a mask, my glasses fog up and it's a nuisance.
お風呂の湯気で鏡が曇った。
The bathroom mirror fogged up from the steam of the bath.
窓が曇って外がよく見えない。
The window is fogged up and I can't see outside well.
3For one's expression or mood to become gloomy or downcast.
その知らせを聞いて彼女の顔が曇った。
Hearing the news, her expression darkened.
心配事があると表情が曇りがちだ。
When something is worrying you, your expression tends to cloud over.
4In Noh theater, a technique of tilting the mask slightly downward to express sorrow (a specialized term).
悲しい場面では、能面をわずかに曇らせる型が使われる。
In sad scenes, the technique of slightly tilting the mask downward is used.