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 has been a rule of thumb 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. Conversely, in order to see red clouds in the evening, sunlight must have a clear path from the west, and therefore the prevailing westerly wind must be bringing clear skies.
There are occasions where a storm system might rain itself out before reaching the observer looking at the morning red sky. For ships at sea however, the wind and rough seas from an approaching storm system could still be a problem, even without rainfall.
Because of different prevailing wind patterns around the globe, the traditional rhyme is generally not correct at lower latitudes of both hemispheres, where prevailing winds are from east to west. But the rhyme is generally correct at mid-latitudes where, due to the rotation of the Earth, prevailing winds travel west to east.
On Vancouver Island, we’re having lovely, sunny days and more than a hint of cool autumn nights. Whether this weather has been predicted by red skies at night, I do not know. My view of the west is blocked by trees and condos. This is not a complaint, merely an observation. I’m enjoying the sunshine along with everyone else!