Red Sky At Night... The origin of the saying is unknown, although a form of it appears in the bible (Matthew 16:2-3). It has some basis in science and is a fairly good predictor of-though no guarantee-of weather at the mid latitudes in the Northern Hemisphere, where storm systems generally follow the jet stream from west to east. A red sky in the morning indicates a sun rising in clear eastern skies casting its rays on storm clouds approaching from the west. At night the clear sight of the red setting sun would tell a sailor that no storms are to the west.