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to be clear (of a sight, sound, colour, etc.); to be bright; to be vivid; to be crisp
to be clear-headed; to be alert; to be on the ball; to be wide awake
to look upbeat; to be perky; to be cheerful
to master (a skill); to excel at; to cleanly execute
to be satisfying
to become frigid; to become intensely cold
今日は頭が冴えていて仕事がはかどる。
My mind is sharp today and work is progressing.
昨晩は空気が冷えて星がよく冴えた。
Last night the air was cold and the stars were vivid and clear.
冬の朝の空気が冴えている。
The winter morning air is crisp and clear.