Blue Hour

The blue hour is the part of twilight, at dawn or dusk, when the sun sits far enough below the horizon that the remaining light is dominated by blue wavelengths. This happens because ozone in the upper atmosphere absorbs the longer, reddish wavelengths of the low sun through a process called Chappuis absorption, letting blue light dominate the sky rather than through the more commonly assumed simple scattering of blue light alone. It typically lasts from about 20 to 96 minutes before sunrise or after sunset, running longer at high latitudes and shorter near the equator, where the path of the sun meets the horizon at a steeper angle.