The rhyme:

"Red sky at night, shepherd's delight, Red sky in the morning, shepherds take warning"

The reason:

This is the traditional rhyme for the UK but North Americans altered it to be a warning for sailors instead of shepherds. Either way, this fable warns if rain and storms are approaching. Weather systems typically move from west to east, and red clouds result when the sun shines on their undersides at either morning or night.

At these two times of day, the sun's light is heavy at the red end of the spectrum. If the morning skies are red, it is because clear skies to the east permit the sun to light the undersides of moisture-bearing clouds coming in from the west - therefore it will actually rain later in the day and vice versa.