Red Sky at Morning

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 sky, caused by haze or clouds related to storms in the region.  If the morning skies are 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.  Source