Understanding Blue Hour
It’s the moment when the sky has a predominantly deep blue hue and the light is soft enough to emphasise the most of the dark areas of the scene, without requiring any additional light source. This is due to Rayleigh scattering, which is the scattering of light by particles smaller than the wavelength of visible light, the same process that makes the sky blue.
When the sun reaches six degrees below the horizon, it is no longer directly illuminating the ground, but it is illuminating the upper atmosphere. This is known as civil twilight. During this period of time, red light, which has a longer wavelength than blue light, passes through the atmosphere into space, while the blue light gets scattered and diffused.
In the morning, blue hour occurs just before golden hour at the beginning of civil twilight, which is around 30 minutes before the sun rises. In the evening, it occurs at the end of the civil twilight, just after the golden hour and starts roughly 10 to 15 minutes after the sun has set.