bluehour

Bluehour refers to the period of twilight after sunset or before sunrise when the sun is between about 4 and 6 degrees below the horizon and the sky takes on a deep, predominantly blue color. This time occurs twice each day and contrasts with the warmer tones of the golden hour. The blue hour arises as direct sunlight is blocked by the Earth's curvature and shorter blue wavelengths remain scattered through the atmosphere, producing a cool, diffuse illumination. The hue can vary with cloud cover, air pollution, and humidity, sometimes shifting toward indigo or slate gray.

The duration of bluehour is variable. At mid-latitudes, it typically lasts 20 to 40 minutes, shorter in the summer and longer in winter, and it can extend near dawn or dusk in high latitudes. In urban areas or coastal settings, artificial lighting and sea or city reflections can influence the perceived color.

In photography and film, the bluehour is prized for its soft light and cool tonal range, which can balance architectural highlights with a tranquil sky. Photographers commonly shoot with long exposures, wide-angle lenses, and a stable tripod, and may adjust white balance toward the cooler end of the spectrum.

See also: golden hour, civil twilight, nautical twilight.