Red Sky at Night, Sailors’ Delight. Red Sky in the Morning, Sailors Take Warning!
This popular saying was used in the European Middle Ages to predict the day’s weather forecast. The adage, “red sky at night, sailors’ delight, red sky in the morning, sailors take warning” was used as early as the 14th century as a mnemonic device for a common belief that the color of the sky predicted the forthcoming weather conditions. The adage is rooted in a mixture of hard science and superstitious lore.
Sunlight appears white because it is made up of an evenly mixed spectrum of visible colors that range in wavelength from .75 micrometers red to .45 micrometers blue. The sky appears blue in the afternoon because the wavelengths are traveling through a condensed amount of atmospheric space and interacting with smaller particles. The small particles in the atmosphere cause the short blue wavelengths to scatter at a higher rate than the longer red wavelengths.
When the sun rises or sets, sunlight travels through a larger area of the earth’s atmosphere than at noontime. The blue is filtered from the light and longer red and yellow wavelengths remain visible. A red sunrise or sunset occurs when the sky is clear in the direction of the sun. When the sky is red at night, the sun is setting in the west. This means the light coming from the west is traveling through a clear sky and good weather lies ahead for the next day. When the sky is red in the morning, it indicates clear skies in the east; this then means that the good weather is behind the traveler. The adage lacks accuracy because it assumes that the traveler is always moving along the route of a prevailing westerly wind. A storm or hurricane that creates its own geostrophic path could disrupt this rule of thumb—clouds appearing from the opposite direction of the sunlight can reflect the red light, causing the sky to appear red from all directions. Despite the lack of accuracy in these predictions, the adage was given common credence because rural and maritime economies relied on fair weather.
Here is an early excerpt of the phrase written in Middle English from Matthew XVI in the Wyclif Bible, 1395:
“The eeuenynge maad, ye seien, It shal be cleer, for the heuene is lijk to reed; and the morwe, To day tempest, for heuen shyneth heuy, or sorwful.”
Are there other popular adages that reference color in similar ways?
Fox-Kemper, Baylor. 2002. Look for Signs that Foretell Tomorrow’s Weather. SAIL, September.
Martin, Gary. 2009. Meanings and Origins of Phrases, Sayings and Idioms. Phrases, Sayings and Idioms at The Finder Index. http://www.phrases.org.uk/meanings/red-sky-at-night.html (accessed Sept. 22, 2009).
Image by Flickr Member: Jeffedoe