Joe Sienkiewicz, chief of the Ocean Applications Branch and a science and operations officer with the NOAA/NWS Ocean Prediction Center, explains.

Indeed, there is scientific validity to the adage, "red sky at night sailors delight; red sky in the morning sailors take warning." This saying has very old roots. In the bible (Matthew 16:2-3), the following quote is attributed to Jesus: "When it is evening, ye say, fair weather: for the heaven is red. And in the morning, foul weather today for the heaven is red and lowering." There are also versions of this saying that refer to shepherds instead of sailors.

Two factors contribute to the cogency of this saying. The first is that weather systems generally travel from west to east in the mid latitudes. Because the sun rises in the east and sets in the west, a rising sun in advance of an approaching weather system would illuminate the approaching mid- and high-level clouds to create a red sky in the morning. Alternatively, if the sun is setting as a weather system exits and high pressure is building, then the departing clouds would be illuminated. This would create a red sky at night with fair weather to follow.

The reddish color results from scattering of sunlight by suspended particles and aerosols in the atmosphere. The suns rays pass through a greater length of atmosphere at sunrise and sunset than at any other time of day. In addition, aerosol, dirt, and dust concentrations are maximized in the lowest layers of the atmosphere when the atmosphere is dominated by sinking air (high pressure). Therefore when under high pressure we can see vivid red sunsets and sunrises.

So this saying is valid in mid latitudes if the timing of weather systems is just right. That is, clearing in the east at sunrise with approaching clouds and clearing prior to sunset in the west as clouds exit to the east. If weather systems and their associated clouds are moving from south to north (as can occasionally occur), however, then the saying does not hold.