The Greek noun eudia (εὐδία) means fair weather, a clear and calm sky, fine weather. It combines eu (good, well) and dios (of Zeus/sky, sky-god), and refers to pleasant atmospheric conditions — clear skies, good sailing weather. It appears once in the NT, on the lips of Jesus in Matthew 16:2.
In Matthew 16:2-3 Jesus rebukes the Pharisees and Sadducees for their spiritual blindness in a meteorological metaphor. They can read the sky — red sky at evening means eudia tomorrow; threatening morning sky means storm — but they cannot read the signs of the times. Their request for a "sign from heaven" is ironic: the greatest sign (the Son of God Himself) stands before them, and they see nothing. The contrast between reading weather and reading revelation exposes the willful nature of unbelief. God's signs are plain; spiritual perception requires a heart aligned with God.