A sign for a change in weather.
It may be a portion of a rainbow that does not (seem to) touch the ground anywhere, a colored circle around the sun or moon, a certain formation of clouds. All are taken as signs for rain or a dry spell, depending on color and shape. Often they are also taken as a sign for a war to come.
Perhaps it will soon rain because there is a sign of rain in the east.
Perhaps there will soon be fighting because I have seen a sign in the sky.