Praia da Ursa, Sintra
: A blue hour seascape seen in wide angle.
The blue hour (from French l'heure bleue)[a] is a period of twilight in the morning and in the evening, during the civil and nautical twilight phases, when the sun is at a significant depth below the horizon and when the residual, indirect sunlight takes on a predominantly blue shade. On a clear day, blue hour can be a colorful spectacle, with the indirect sunlight tinting the sky yellow, orange, red, and blue. This effect is caused by the relative diffusibility of short blue wavelengths of light versus the longer red wavelengths. During the blue "hour" (typically a period about 20 minutes in length), red light passes straight into space, while blue light is scattered in the atmosphere, so reaches Earth's surface. Many artists treasure this period because of the quality of the light. Although the blue hour does not have an official definition, the blue color spectrum is most prominent when the sun is between 4 and 8° below the horizon.