White light contains many colours
Sunlight contains a range of wavelengths. As it enters the atmosphere, it interacts with molecules and particles. Molecules scatter shorter wavelengths, associated with blue light, more efficiently than longer wavelengths, associated with red light. This process, called Rayleigh scattering, helps explain why a clear daytime sky looks blue. The sky is not a painted surface: it is light reaching our eyes from different directions.
A longer journey near the horizon
When the Sun is low, its light travels through more atmosphere before reaching an observer. Much of the blue light is scattered out of that direct path. The remaining light can contain a larger proportion of yellow, orange and red. The Sun itself has not suddenly changed colour. Instead, the conditions along the path between the star and our eyes have changed. This also explains why the surrounding sky can look different from the direction of the setting Sun.
No two sunsets are quite the same
Clouds, moisture, dust and other particles affect the appearance of the horizon. Depending on their height and position, clouds can remain in sunlight after the ground below is in shadow. Larger particles also interact with light differently from molecules. A red sunset alone therefore cannot provide a reliable diagnosis of air quality or a dependable forecast for tomorrow. A photograph records a combination of conditions rather than a single atmospheric measurement.
Try a simple observation exercise
Watch the same horizon on several evenings and note the clouds and clarity of the air. Compare the sky towards the sunset with the sky behind you. If you take photographs, remember that white balance and exposure can strengthen or soften the colours. Never look directly at the Sun, including through binoculars or a camera without appropriate protection. You can enjoy the colours by observing the illuminated sky around it. Comparing several evenings can make the changing role of clouds much easier to notice.
