The common phrase “Red sun at morning” is a line from an ancient rhyme often repeated by mariners:
- Red sun at night, sailors’ delight.
- Red sun at morning, sailors take warning;
The rhyme is a rule of thumb used for weather forecasting during the past two millennia. It is based on the reddish glow of the morning or evening sun, caused by haze or clouds related to storms in the region. If the morning sun is red, it is because clear skies over the horizon to the east permit the sun to light the undersides of moisture-bearing clouds. The saying assumes that more such clouds are coming in from the west. Conversely, in order to see red clouds in the evening, sunlight must have a clear path from the west, so therefore the prevailing westerly wind must be bringing clear skies.