blue hour

Blue hour at the Old Cathedral of the Holy Name of Jesus, Bragança in Portugal The blue hour (from French l'heure bleue; pronounced [lœʁ blø]) is the period of twilight (in the dawn or dusk, around the nautical stage) when the Sun is at a significant depth below the horizon. During this time, the remaining sunlight takes on a mostly blue shade. This shade differs from the colour of the sky on 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 in the ozone layer. Since the term is colloquial, it lacks an official definition such as nautical dawn, nautical dusk, or the three stages of twilight. Rather, blue hour refers to the state of natural lighting that usually occurs around the nautical stage of the twilight period (at dawn or dusk).