Sailor’s Delight

Red sky in the morning, sailors take warning;

Red sky at night, sailor’s delight.

Why do mariners repeat this lore? Blame William Shakespeare, the playright, or Matthew, the disciple.

In Venus and Adonis, Shakespeare wrote of red skies in the morning:

Like a red morn that ever yet betokened,
Wreck to the seaman, tempest to the field,
Sorrow to the shepherds, woe unto the birds,
Gusts and foul flaws to herdmen and to herds.

In the new testament of the Bible, Matthew (16:2-3), attributed the following red sky at night reference to Jesus, “When it is evening, ye say, fair weather: for the heaven is red.”

These help explain the popularity of the rhyme, but not how it came to be believed or whether it’s true.

The federal National Oceanic Atmospheric Administration Earth System Research Laboratory informs that because weather generally moves from west to east, when the horizon is clear of storms at dusk, sunlight is able to pass through more of the atmosphere.

The longer sunlight passes through air molecules and particulates the more red and orange light is intensified.

That’s because blue light – because of its shorter wavelength – is scattered easiest by nitrogen and oxygen air molecules, whereas reds and oranges – with longer wavelengths – are not scattered as much over the same distance.

And, when there’s no storm approaching from the west, sunlight travels a longer distance through the atmosphere. That phenomenon is even greater in autumn, due to the low angle of sunlight at sunrise and sunset in the Northern Hemisphere.

As for red skies in the morning, the same idea applies, but in reverse. Redness in the sky occurs when warm morning light reflects off high storm clouds approaching from the west.

The reason sailors should take warning when red clouds are overhead at sunrise, is that they are likely the leading edge of an approaching storm, unseen in the morning darkness to the west.