Blue hour refers to the darker stages of morning and evening twilight, when the Sun is quite far below the horizon, coloring the sky deep blue.
On a clear day, the transition between day and night can be a colorful spectacle, with the indirect sunlight tinting the sky yellow, orange, red, and blue. The blue hour refers to the phase of twilight when the Sun is so far below the horizon that the light's blue wavelengths dominate.
Because the blue hour is a colloquial term, it doesn't have an official definition similar to dawn, dusk, and the 3 phases of twilight. Rather, it refers to a state of natural lighting that usually occurs at daybreak - the pre-peak moment prior to sunrise - in the morning, and during the last stages of twilight in the evening, during the final transition of dark blue sky and total -black sjy - darkness.
The blue color spectrum is most likely to emerge when the Sun is between 4 and 8 degrees below the horizon. By this definition, the blue hour encompasses parts of both nautical twilight and civil twilight.
Why Is the Light Blue?
The different colors of twilight are created by an atmospheric scattering effect. As the solar elevation angle—the angle of the Sun above or below the horizon—changes during the transition between day and night, different wavelengths are filtered out by the Earth's atmosphere, leaving the remaining rays of light to color the sky in ever-changing hues.
During blue hour, the Sun is so far below the horizon that the atmosphere scatters only the short-wave blue light, sending it back to Earth. Meanwhile, the longer red wavelengths pass through it and into space.
How Long Does Blue Hour Last?
The duration of blue hour depends on both the latitude and the season. In locations where the Sun is directly overhead at noon—for example at the equator during the equinoxes—the Sun traverses the horizon at an angle of 90°, making for swift transitions between night and day and a relatively short blue hour. At higher latitudes, the same process takes longer. At around 40 degrees north and south, for example in New York, it takes around 20 minutes in March. In Atlanta, in March about 10 minutes, or less.
Generally, the best blue hour photographs I have created are in a 3 to 12 minute window depending on location and time of year. However, I must be set up waiting for the actual moment well in advance.
How is a Location Selected for Blue Hour Photography
The blue hour is awesome for commercial or industrial clients as it combines saturated colors from the sparkling lighting to the deep rich blue hues of the sky. Since the light is soft and has a very high color temperature during that time of the day, it is best suited to capture artificial light sources, such as commercial buildings, plants and mills, cityscapes, buildings, bridges, or monuments.
To capture the magic of the blue hour I have to plan ahead. Blue hour is short—too short to start when the light is just right. I do preplanning long before arriving on site using a blue hour calculator. The exact time changes each day depending on time of year. I also must calculate when the sun will be at just the right angle. Then, the final determination is made, whether I arrive onsite at 5 AM or 6 PM. This all depends solely on location in relation to the sun. I will never recommend blue hour photography for locations not suitable for it. Doing that is just a waste of time!
The end results of blue hour photography is well worth the effort. Blue hour photographs are commonly used in marketing commercial locations because it makes an otherwise plain photograph come to life with a polished advertising visual that will set your business image apart from others using only a daylight captured image.
Should you have a question, send Mark an email. He will respond within one business day.