The common phrase "Red sky at morning" is a line from an ancient rhyme often repeated by mariners:
Red sky at night, sailors' delight. Red sky 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 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 rhyme 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.
Because of different prevailing wind patterns around the globe, this rhyme is generally not correct at lower latitudes of both hemispheres, where prevailing winds are from east to west.
The rhyme is generally correct at mid-latitudes where, due to the rotation of the Earth, prevailing winds travel west to east.
Reference: Wikipedia