Signs in the sky have long been used for weather predictions. 2000 years ago the Gospel of Matthew records an ancient version of the rhyme many of us learned as kids: “Red sky at night, sailors delight; Red sky at morning, sailors warning.”
A red sunset is often seen when the western sky is especially clear. The colour occurs because the sun is low in the sky, and its light passes through additional atmosphere. The colour is enhanced if a stable high pressure region is present. The reason for this is that high pressure suppresses cloud formation and holds air pollutants near the Earth. These pollutants further scatter the colours of sunlight, enhancing the reddening effect in the west.
Since high pressure generally brings good weather, red skies in the evening often indicate that fair weather is approaching from the west. On the other hand, if the red appears in the eastern morning sky, then the high pressure region has already passed through. Since lower pressure usually follows a high pressure system, and is often associated with unsettled weather, red sky in the morning can indicate stormy weather.
Evening redness can be caused by sunlight reflecting from a cloud layer retreating in the east and morning redness may be due to reflection from an advancing cloud layer.
So next time you hear the ancient rhyme about the red sky, don’t dismiss it altogether, it may well predict tomorrow’s weather.