Red sky at night, shepherds delight Red sky in the morning, shepherds warning
This admonition to shepherds is the original version of the modern saying: Red sky at night, sailors delight; red sky in the morning, sailors take warning.
There are references to the red sky and its relationship to fair and foul weather in the Bible and Shakespeare. But is there any truth to these sayings?
Clear Sky Is Blue
A cloudless sky appears blue. Until the 19th century, the reason for this was unknown. Lord Rayleigh (more properly known as John William Strutt, Third Baron Rayleigh) unravelled the mystery by calculating the effect of individual molecules on a ray of sunlight.
The amount of scattering of the colors that comprise white light is inversely proportional to the fourth power of the wavelength; the shorter the wavelength, the greater the scattering. Therefore, blue light, with a shorter wavelength than the other colors, is most strongly scattered. It’s this scattered blue light that we see as the blue sky.
Correspondingly, the red, yellow and green come through to our eyes and the sun appears yellowish. At sunrise and sunset, when the sun’s rays travel through a longer path of atmosphere, even the greens can be scattered out and the sun may appear orange.
Scattering off Particles
The full explanation of how light scatters off objects of any size is covered by Mie Theory (more properly known as the Mie solution to Maxwell’s equations). Rayleigh’s explanation of light scattering off molecules covers objects that are small relative to the wavelength of light. The Mie solution for pollutants, particles that are large relative to the wavelength of light, predicts that the longer wavelengths (red) would be most strongly scattered in a forward direction, and this has been widely used as an explanation for the Red Sky maxim. But considering the origin of the saying, this seems unlikely to be correct.
It would have been a prescient shepherd in biblical times who could imagine 21st century pollution, let alone observe it, and the saying’s first application to sailors predates the industrial revolution. Though there is natural dust in the atmosphere, it is not in sufficient amounts to cause red sunrises.
Red Sky: Clouds are the Answer
Except for the region very near the sun, the ‘sky’ is never really red. It is likely that ‘red sky’ refers primarily to the color of the clouds. All spectacular sunrises and sunsets are enhanced by the reflection of sunlight off the water droplets in clouds. The path of light from a sun on or just below the horizon, through the atmosphere to the cloud and then to the observer, has the longest possible path through the molecules of the atmosphere. As Lord Rayleigh discovered, the red light is least scattered by molecules — so the clouds appear red.
Moving Weather Systems
Since high and low pressure systems move from west to east in the temperate zone, when one looks east and sees the sun rising in a clear sky but there is red light scattered off clouds overhead and to the west, the clear skies have passed by and clouds are on the increase. The situation when there are red clouds at sunset is the reverse: the clouds are moving away and clear skies are coming.
Weather Warning: Modern Sailors Should Use a Better System
Though the Bible may be a good guide for many things, and may have been useful to shepherds of the day, twenty-first century sailors are better advised to prefer advanced technology to ancient lore before venturing into the open sea. Modern forecasts, aided by computers, satellites, and sensitive measuring devices, are a better guide to the coming weather than the color of the sky.