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to become clouded; to darken; to get dark; to become hidden (behind clouds)
to darken (of a face)
to worsen; to deteriorate; to become gloomy
空が次第に蔭ってきた。
The sky gradually became clouded.
夕方になると空が蔭る。
The sky darkens in the evening.
山の向こうで空が急に蔭った。
The sky suddenly darkened beyond the mountains.