The term 'RED SKY, TO MARINERS' is an OMEN that sailors have used for centuries to predict the weather. It is derived from the proverb 'Red sky at night, sailor's delight; red sky in morning, sailors take warning'. This proverb is based on the idea that a red sky in the evening indicates that the following day will be fair and sunny, while a red sky in the morning indicates that a storm is likely to come.