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The blue hour (from French language l'heure bleue; ) 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. 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 dawn, 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).
The blue hour is shorter in regions near the equator due to the sun rising and setting at steep angles. In places closer to the poles, the illumination and twilight periods are longer as the sun rises and sets at shallower angles.
When the sky is clear, the blue hour can be a colourful spectacle, with the indirect sunlight tinting the sky yellow, orange, red, and blue. This effect is caused by the relative photon diffusion of shorter wavelengths (bluer rays) of visible light versus the longer wavelengths (redder rays). During the blue "hour", red light passes through space while blue light is scattering in the atmosphere, and thus reaches Earth's surface.
Blue hour usually lasts about 20–96 minutes right after sunset and right before sunrise. Time of year, location, and air quality all have an influence on the exact time of blue hour. For instance in Egypt (every Summer solstice), when sunset is at 7:59 PM: blue hour occurs from 7:59 PM to 9:35 PM. When sunrise is at 5:54 AM: blue hour occurs from 4:17 AM to 5:54 AM. Golden hour occurs from 5:54 AM to 6:28 AM and from 7:25 PM to 7:59 PM.
==Art==
Photographers use blue hour for the tranquil mood it sets. When photographing during blue hour it can be favourable to capture subjects that have artificial light sources, such as buildings, monuments, cityscapes, or bridges.
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