Twilight spectral sequence - explanation of "The Blue Hour"

A spectral time-sequence taken of the western sky during sunset on the evening of 2010-04-03 from Weilheim i. OB (Lat: 47 50 28 N; Long: 11 08 34 E). The fibre-fed spectrometer accepted illumination from a cone with a half-angle of about 25 degrees. The spectra are plotted as ratios with a reference spectrum taken with a solar altitude of 11 degrees (18:38 local time): the first spectrum, at 18:49, therefore deviates only slightly from a horizontal unit line. The range in solar altitude covered by these observation is from +9.1 to -3.4 degrees and the spectra are separated in time by an average of six minutes (although they are not exactly uniformly spaced). The spectra are all normalised to the integrated spectrometer counts between 720 and 740nm.

The lower plot shows the rapid development of the strong, broad Chappuis absorption band of ozone as the Sun approaches the horizon. At high altitude angles, the ozone band is very weak and has little or no influence on the colour of the sky, but with the long pathlength of light through the atmosphere at twilight, the optical depth of the absorption becomes large enough to have a dramatic influence on the skylight spectrum. The visual effect of this removal of yellow/orange light is to make the average skylight appear extremely blue. Due to the position and width of the absorption, it also has the effect of making the Sun appear much redder than it would if influenced by Rayleigh extinction alone. The blue lines represent spectra before sunset and the red, following sunset. The first, last and last spectra are distinguished by thicker lines. The spectrum at the time of sunset is a thick, black line.

The upper plot shows the corresponding changes of sky brightness (measured at 710nm) and the colour temperature (measured with a Gossen 'COLORMASTER' meter).

Apart from the original discussion of ozone and skylight by E. O. Hulbert (1953, JOSA, 43, 115), there is a nice account of the phenomenon in a delightful new book by Sönke Johnsen called "The Optics of Life" (Princeton University Press, 2012, pp.45-49).

In addition to its strong influence on the apparent colour of a wide range of of 'long-pathlength' atmospheric phenomena, the very blue ambient light (away from direct Sun- or Moonlight), i.e., in forest understory, can exert a powerful influence on the visual adaptation of animals that forage in twilight. An example of this is the Madagascan lemur, the Aye-aye, that has a significantly bueshifted blue opsin pigment in its visual system ( onlinelibrary.wiley.com/doi/10.1002/ajp.21996/abstract;js... ). A different kind of adaptation is seen in the reindeer in the winter which amplifies the sensitivity of the cones to blue light in response to the dim, ozone-blue, prevailing light in the arctic winter ( news.nationalgeographic.com/news/2007/03/070313-reindeer-... ).

The rotated black-line overlay at the bottom of the image shows what is thought to be the first ever photographic observation of the ozone Chappuis band absorption in the twilit sky made by Wulf, Moore and Melvin in 1933 (Astrophysical Journal, 1934, 79, 270, note). The lower line for comparison is their observation of the daytime blue sky with the same equipment: a small direct-vision spectroscope coupled to a simple box camera.

The period around twilight is often referred to by artists as "The Blue Hour". The cause of this phenomenon is often attributed to Rayleigh scattering which is, of course, responsible for the blue of the daytime clear sky. In fact, Rayleigh scattering has little to do with the twilight blue which results predominantly from the effect of ozone absorption.


Flickr page for this photo.

Visit the Eye for Science Flickr group.

Get the "Eye for Science" widget here and install it on your own blog. Spread the scienticity!

Like this image? Want it for your smartphone wallpaper?
Use this QR Code to load it on your smartphone and enjoy!