Red Skies at Night...

April 25, 2014

There's an old nautical adage,

"Red skies at night, sailors' delight.
 Red skies in morning, sailors take warning."

The basis of this adage is the fact that airborne dust and aerosols concentrated in weather fronts will make the sky appear to be red.

Weather generally travels from west to east, and red at sunset indicates that the air mass moving towards you from the west is a stable air mass, full of dust, a portent of good weather. Likewise, a red sky at sunrise indicates that your good weather has passed you.

Sunset at Lake Starnberg On the evidence of this photograph, the following day must have been a good day for water sports on Lake Starnberg. (Photograph by Christian Thiergan, via Wikimedia Commons.)

Shakespeare, who included much folklore in his works, referred to the red morning portion of this adage in his 1593 poem, Venus and Adonis (ll. 453-456):

Like a red morn, that ever yet betoken'd
Wrack to the seaman, tempest to the field,
Sorrow to shepherds, woe unto the birds,
Gusts and foul flaws to herdmen and to herds.[1]

Red skies are caused by Rayleigh scattering, a wavelength-dependent scattering of light from airborne particles. The scattering is more pronounced at short wavelengths, so red light is preferentially transmitted, and the effect is more pronounced at sunrise and sunset when the Sun's rays travel through more of the atmosphere.

This optical effect was explained by Lord Rayleigh (1842-1919), also known as John William Strutt, Third Baron Rayleigh, an English physicist who co-discovered the element, argon, for which he was awarded the 1904 Nobel Prize in Physics. Rayleigh is best known for the Rayleigh–Jeans law. This law explained radiance at long wavelength, but it had a problem at short wavelengths. This "ultraviolet catastrophe" was eventually solved by quantum mechanics.

Lord Rayleigh in a Vanity Fair caricature from its December 21, 1899, issue. The public equates science with laboratory glassware, so even a physicist is shown doing chemistry. (Via Wikimedia Commons.)

Although there are various remote sensing techniques to better measure atmospheric particulates, it would be possible to estimate these using the red sky phenomenon. You would essentially measure excess red at the rising or setting Sun by ratio this color to a shorter wavelength color, such as green. The ratio of red to green radiance should correlate with particulate concentration.

That's the approach used by an international team of atmospheric scientists in a study of historical aerosol optical depth (AOD) around the time of major volcanic eruptions.[2] The team was comprised of scientists from the Academy of Athens (Athens, Greece), the Navarino Environmental Observatory (N.E.O., Messinia, Greece), the University of Patras (Greece), the National Observatory of Athens (Greece), the Hellenic Open University (Patras, Greece), the Justus Liebig University of Giessen (Giessen, Germany), the University of Athens (Greece), the National Observatory of Athens (Greece), and the National Technical University of Athens (Greece).

The most interesting aspect of this research, which is a continuation of an earlier study,[3] is that the red/green ratios were taken from paintings by master artists in the period 1500-2000. This was necessary, since color photography wasn't invented until just recently; and, even today, color rendering by photochemical means is still less accurate than what's perceived by an artist.

Since bringing spectrometric equipment to numerous museums would be tiresome, but many images of paintings are available online, a first step was to determine whether online representations are accurate enough to be used. As the figure shows, there was reasonable correlation between the red/green ratios of online images and a high resolution color profile protocol for 124 images at the Tate Gallery.[2]

Comparison of web image color rendition versus a more accurate color profile technique. A perfect correlation is shown in red, while the actual correlation is shown in green. (Fig. 1 of ref. 2, Creative Commons Licensed.)

As can be expected, this method produces suggestive, although not completely accurate, results, as can be seen in the comparison of the optical depth of aerosols using the red/green method and data derived from another source (see figure). The study also allowed an estimate of the affect of the industrial revolution on air quality.[2]

Comparison of the optical depth of aerosols using the red/green method and data derived from another source. The major features are evident. (A portion of fig. 4 of ref. 2, Creative Commons Licensed.)

These are observations, but nothing is more satisfying to a scientist than an actual experiment. To that end, watercolor artist, Panayiotis Tetsis, was commissioned to paint four successive sunsets during the passage of a Saharan dust outbreak over the island of Hydra, Greece, in June 2010. This was a "blind" experiment, since Tetsis wasn't told about the dust storm. The red/green ratios of the painting and photometric measurements matched quite well.[2]

As the authors state in their conclusions,

"Regardless of the school, red-to-green ratios from great masters can provide independent proxy AODs that correlate with widely accepted proxies and with independent measurements."

References:

1. William Shakespeare, "Venus and Adonis," Project Gutenberg.
2. C. S. Zerefos, P. Tetsis, A. Kazantzidis, V. Amiridis, S. C. Zerefos, J. Luterbacher, K. Eleftheratos, E. Gerasopoulos, S. Kazadzis and A. Papayannis, "Further evidence of important environmental information content in red-to-green ratios as depicted in paintings by great masters," Atmos. Chem. Phys., vol. 14, no. 6 (March 25, 2014), pp. 2987-3015.
3. C.S. Zerefos, V.T. Gerogiannis, D. Balis, S.C. Zerefos and A. Kazantzidis, "Atmospheric effects of volcanic eruptions as seen by famous artists and depicted in their paintings," Atmos. Chem. Phys., vol.7, no. 15 (August 2, 2007), pp. 4027-4042.