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 trapped particles scattering the blue light from the sun in a stable air mass. If the morning skies are of an orange-red glow, it signifies a high-pressure air mass with stable air trapping particles, like dust, which scatters the sun's blue light. This high pressure is moving towards the east, and a low-pressure system moves in from the west. Conversely, in order to see "red sky" in the evening, high-pressure air mass from the west scatters the blue light in the atmospheric particles, leaving the orange-red glow. High-pressure air mass signifies stable weather, while low pressure signifies unstable weather. There are occasions where a storm system might rain itself out before reaching the observer (who had seen 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. The rhyme is generally correct at
middle latitudes where, due to the rotation of the
Earth, prevailing winds travel west to east. ==Other versions==