Blue hour

Blue hour in Singapore
Blue hour in Innsbruck, Austria
Blue hour. Ystad 2018.
Midtown Manhattan during the blue hour
Praia da Ursa, Sintra, Portugal: A wide-angle view of the seascape during the blue hour
Blue hour in Rothenburg ob der Tauber, Germany

The blue hour (from French l'heure bleue;[1][a] pronounced [lœʁ blø]) is the period of twilight (in the morning or evening, around the nautical stage) when the Sun is at a significant depth below the horizon and residual, indirect sunlight takes on a predominantly blue shade, which differs from the one visible during most of a clear day, which is caused by Rayleigh scattering.

The blue hour occurs when the Sun is far enough below the horizon so that the sunlight's blue wavelengths dominate due to the Chappuis absorption caused by ozone.[2] Since the term is colloquial, it lacks an official definition similar to dawn, dusk, and the three stages of twilight. Rather, it refers to a state of natural lighting that usually occurs around the nautical stage of the twilight period (at dawn or dusk).[3]

  1. ^ Verfaillie, Roland (2011). L'heure Bleue. San Francisco: Purple Onion Press. p. 5.
  2. ^ Hoeppe, Götz (2007). Why the Sky Is Blue: Discovering the Color of Life. Princeton: Princeton University Press. pp. 249–53. ISBN 0-691-12453-1.
  3. ^ "The Blue Hour". Retrieved 6 April 2019.

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