red sky in the morning, sailors warning; red sky at night, sailors delight

Red sky in the morning, sailors warning; red sky at night, sailors delight is a line from an old rhyme that was repeated by mariners during the past centuries. The rhyme was used as a rule of thumb for weather forecasting, which dated back about 2,000 years. It is based on the reddish glow of the morning or evening sky which was caused by haze or clouds related to storms in that region.