Variations of this weather lore abound, involving both shepherds and sailors or a combination of the two, and even appear in the Bible and Shakespeare:
Yet all heed the same – a red sky at night foretells calm weather the following day, whereas a red sky in the morning warns of storms.
Originating most likely in England sometime before the 14th century, the phrase incorporates the path of weather systems and the sun.
Weather fronts typically travel west to east across England (and the U.S., for that matter), in opposition to the rise and fall of the sun.
As the sun sets in a clear westerly sky, its light reflects off the clouds to the east – clouds that have already passed over the region. When the sun rises in a clear easterly sky, its light reflects off the clouds to the west – clouds that have yet to affect the region.
The “red sky” comes from the sunlight’s refraction through the atmosphere before reaching the clouds on the opposing horizon. The shorter wavelengths of the visible light spectrum (the blues and violets) are diverted, leaving the longer wavelengths (the reds and oranges) to reflect off the clouds.
Wavelengths aside, the phrase merely suggests you’ll experience clear weather when clouds have already passed and none appear to be incoming, and the opposite when clouds are fresh on the horizon.