The "blue hour" is a term used in photography and art to describe a period of twilight when the sun is far enough below the horizon to produce a diffuse blue light in the sky.
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. 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).
My other blogs:
Roadscape Photography
Chevrolet El Camino 1970
Life @ Köyliö, mostly Finnish
