"The State of The World" Number 35
Complete Index Of All Issues
Why "Red Sky At Night, Sailor's Delight"?
"Red sky at night, sailor's delight. Red sky in the morning, sailor's warning" is one of the most well-known sayings of weather lore. It is very ancient, certainly learned the hard and dangerous way by mariners throughout the ages.
Other also-ancient versions of the saying exist. The example below is identical, except that it has shepherds in place of sailors.
Red sky at night, shepherd's delight.
Red sky in the morning, shepherd's warning.
It seems likely that it was the shepherd version that was referred to by Jesus of Nazareth as recorded in the Book of Matthew: "When it is evening, you say, 'It will be fair weather, for the sky is red.' And in the morning, 'It will be stormy today, for the sky is red and threatening."
The saying is generally accurate - but why? And with what conditions?
First, keep in mind that white light is actually all colors together, as demonstrated by a prism in a simple physics experiment. The colors can be separated however, as shown below.
The visual appearance of a red sky is caused by higher pressure air in the upper atmosphere concentrating particles of dust or soot. A relatively clear sky diffuses the shorter wavelengths of sunlight, which are blue, but dust and soot, when in abundance, scatter the longer red wavelength of sunlight in a process called Rayleigh scattering.
At sunrise and sunset, the sun is lower in the sky, thereby causing the sunlight to travel through more of the atmosphere, so scattering more red light (the reason that astronomers try to view stars when they are overhead, which has the least distance of atmosphere, rather than near the horizon where they must attempt to look through "a long air blanket" of a much longer distance of atmosphere). This visual effect is made even more intense when clouds are present to reflect the prevailing light color to the viewer.
The principle is however generally valid based on prevailing wind direction. Where prevailing winds, and the weather conditions that they transport, run from the west to the east, a red sky at night is a sign that the high pressure air, with good weather, is to the west - the poor weather is thereby actually seen moving away.
Conversely, at sunrise the morning light is to the east, so a red sky at that time is a sign that the high pressure, with its good weather, has passed the viewer's location, and that an area of low pressure, with poor or stormy weather, is approaching.
The direction of the prevailing winds, not the color of the sky alone, is the actual basis of the wisdom of the saying. As such, the saying is most accurate in areas of the world where the prevailing winds move, along with the saying, from west to east.