Blue hour

Blue Hour is now mainly a poetic term for the twilight period between sunset and darkness of the night and for the time just before sunrise. During this time, the deep blue sky has roughly the same brightness as the artificial light from buildings and street lighting. However, the blue of the sky has a different physical origin than by day and thus has a different spectral composition.

Physical explanation of the coloring

They got their name from the deep blue color of the clear sky at this time. The deep blue is caused not by the Rayleigh scattering, which is responsible for the sky blue during the day. On the contrary: During a sunset the short-wave blue portion of sunlight is scattered out on the long path through the atmosphere, the sky would actually appear gray to black.

This phenomenon was first discovered in 1952 by the American geophysicist Edward Hulburt ( 1890-1982 ). Götz Hoeppe calls it " the blue hour of ozone ." In fact, the blue sky at sunrise and sunset is mainly caused by the presence of the ozone layer 20 to 30 km altitude above the ground. Edward Hulburt was able to prove that the sky blue at the zenith during the sunset is based on only one-third of the Rayleigh scattering, but two thirds based on the specific absorption behavior of the ozone. At dawn, however, the blue light of the sky is completely caused by the ozone layer.

Color temperature

Physically describes the color temperature, the modified lighting conditions. While the sun has a color temperature of about 5500 K, is the blue sky between 9000 K and 12000 K. On the day these two light sources for typical daylight mix with a color temperature of 6500 K. In the blue hour of direct sunlight does not apply remains the sky blue.

The higher blue component of light from the sky can also be observed during the day in the shade.

Duration

The duration is between 20 minutes in the tropics, 30 minutes ( day and night are equally long ) to 50 minutes ( solstice) in Central Europe, up to 5 hours in the White Nights and theoretically two weeks at the poles.

The Blue Hour in photography

In photography, the blue hour for so-called available-light shots, and night photography is used. Compared to shooting in complete darkness by also illuminates the surroundings easily and thus more visible, the contrasts between light and dark are mitigated and the pictures have an interesting mood. The fact that the illumination puts inside buildings in the photographically same contrast range is not artificially illuminated facade and surroundings, and the color contrast for street lighting and building illumination provides photographic incentives. The different color temperatures ( blue of the sky, the orange light bulbs, fluorescent tubes Turquoise ) make such photos unusually colorful. This effect is enhanced by the required long exposure time.

The blue hour in the literature

• A poem by Gottfried Benn is titled Blue hour.
• A poem by Ingeborg Bachmann is titled The Blue Hour.
• A novel by William Boyd from 1994 bears in the German translation of the title The Blue Hour ( Original: The Blue Afternoon).
• A novel by Joan Didion from 2012 bears in the translation the title Blue Hour ( Original: Blue Nights ).