Are Sunrise and Sunset Colors Different?

A listener to the BBC podcast The Naked Scientists asks: ""Can you tell from a painting, or a photo, whether it's sunrise or sunset?"

My answer: No, there's nothing fundamentally different about the light effects at sunset or sunrise, and there's no way to tell which you're looking at from the light and color effects alone.

The cause of those light effects are the same. Sunlight travels through more atmosphere as the rays approach the horizontal. Passing through more air scatters out more blue wavelengths from the light rays, making the light that remains appear increasingly orange or red. As the sun passes below the horizon, beneath the curvature of the earth, it may briefly illuminate the bottoms of the clouds. Of course this effect happens both at sunrise and at sunset when colors are at their richest.

A single photo or a painting can tell you something about the position of the sun and about the height and distribution of cloud layers. And some art historians have argued that paintings of sunsets after the eruption of Krakatoa in 1883 reveal colors that were more pronounced worldwide. But it can't tell you whether it's morning or evening.

If there are differences between sunrise and sunset, they're qualitative and subjective. In some environments, humidity and dust may be stirred up at the end of the day because of evaporation and turbulence, and these effects can increase the saturation of the colors. But you wouldn't be able to guess that from a single image. Emotional subjectivity also plays a part in our human perception of sunsets and sunrises as we experience them in time. While a sunset builds gradually to a dramatic crescendo before quickly transitioning to twilight, a sunrise starts off with a blast of color and, as Wordsworth says, the "vision splendid" fades "into the light of common day."
M. Minnaert, author of the book "The Nature of Light and Color in the Open Air," addressed this topic and here's how he puts it: "Are there any differences between dawn and dusk? If any they are so small that it is not possible to mention any really typical differences. One important thing, however, is that the eye is completely rested in the morning and sees the light-intensity increase continuously, so that it is more sensitive to dawn phenomena than to dusk phenomena. The latter have generally a greater richness of colour on account of the greater humidity of the air, and because the air is a little more turbulent, and contains more particles of dust than in the morning." (page 280, paragraph 193)

Listen to the Q and A on The Naked Scientists Podcast: Can you tell if it's sunrise or sunset?