Blue Hour
The twilight period before sunrise and after sunset when the sun is below the horizon and the sky glows an even, saturated blue.
Formula / rule
Sun elevation ≈ −4° to −6°
Measured in
degrees (sun elevation)
In depth
With the sun below the horizon, all illumination is scattered skylight, which is strongly blue and almost perfectly diffuse — a source with no direction and no shadows. Its real value to city photographers is balance: for roughly twenty minutes the sky's brightness matches artificial lighting, so both record properly in one frame.
Examples in practice
A cityscape shot in blue hour holds detail in a deep blue sky while the building lights read as warm and correctly exposed.