The Blue Hour, or l’heure bleue, is a fleeting period of twilight celebrated for its unique quality of light. It is defined by the position of the sun, creating a window of soft, diffused illumination. The world is washed in deep, cool, and saturated hues of blue, lending a tranquil mood to the scene. This ethereal light appears both in the morning before sunrise and in the evening after sunset, offering a brief transition between day and night.
Defining the Blue Hour by Solar Angle
The precise timing of the Blue Hour is determined by the angle of the sun below the horizon, a measurement consistent regardless of geographic location. This period aligns with the stages of Civil and Nautical Twilight. In the evening, it begins after the sun has dropped approximately four degrees below the horizon and lasts until the sun reaches about eight degrees below. In the morning, the sequence is reversed, starting at eight degrees below the horizon and concluding when it rises to four degrees below.
The phenomenon always occurs twice daily. The window is commonly understood to fall within the Civil Twilight phase, defined as the time when the sun is less than six degrees below the horizon. Though the term suggests a 60-minute duration, the actual length is much shorter, lasting only 20 to 40 minutes in most temperate regions.
The Physics Behind the Distinct Blue Color
The distinct deep blue color results from specific atmospheric processes occurring when the sun is well below the horizon. Unlike the daytime blue sky, which is caused by Rayleigh scattering, the Blue Hour involves a different mechanism. At this low angle, sunlight travels through a significantly greater portion of the atmosphere. This extended path scatters most of the direct light away from the surface.
The remaining light is influenced by Chappuis absorption, which takes place high in the stratosphere. Ozone molecules in this layer preferentially absorb the longer, warmer wavelengths of light, such as yellows, oranges, and reds. This selective absorption allows the shorter-wavelength blue light to dominate the diffuse light scattered down to the Earth’s surface. The result is a pervasive, indirect blue glow that illuminates the landscape without casting harsh shadows.
Factors That Change the Duration and Timing
Although the solar angle definition is fixed, the practical duration of the Blue Hour varies significantly based on geographic factors. The most influential factor is latitude, which dictates the angle at which the sun traverses the horizon. Near the equator, where the sun rises and sets almost perpendicularly, the transition through the twilight phases is rapid, often condensing the Blue Hour to as little as 16 minutes.
Conversely, at higher latitudes (above 60 degrees north or south), the sun moves at a much shallower angle relative to the horizon. This shallow trajectory stretches the twilight periods, causing the Blue Hour to last for 30 minutes or more. In extreme polar regions during the winter, the sun remains just below the horizon all day, allowing the conditions to persist for several hours. Local weather conditions, including cloud cover and atmospheric pollution, also impact the phenomenon by affecting the clarity and intensity of the blue light.