The ruddy shades that tint the horizon at break and close of day result from the scattering of sunlight by small particles suspended in dry, dusty air. At sunset, these conditions imply a zone of dry, high pressure between you and the sun. Since weather in the mid-latitudes moves mainly west to east, that means a day of clear sailing. But in the atmosphere, as in life, highs and lows tend to follow hard upon one another. So, if the red, dusty skies occur near sunrise, it suggests that the calm high-pressure zone has already passed and that a stormy low-pressure system could move through soon [source: Library of Congress].
The saying works fairly well in the middle latitudes, which include most of North America, Europe, and Asia, as well as South Africa and the southern halves of South America and Australia. At the poles and in the tropics, where weather tends to progress in the opposite direction, sailors (and shepherds) would do well to take the opposite advice [source: Pann].