The Physics of Sunsets

I've long been interested in why sunsets look the way they do - I've discovered the topic by trying to re-create them in real-time 3d rendering in Flightgear and have been admiring, photographing and filming sunsets ever since, trying to figure out more and more of the intricate details one sees.

Of course it is easy to find the standard set of explanations - the sky is blue because short light wavelength are dominantly scattered, therefore the blue is removed from the sunlight when the sun is low and the light has a long path through the atmosphere, so the low sun appears red.

Except when it really doesn't. And in fact we can see all sorts of complicated hues in the sky - red, orange, golden, yellow, salmon, pink - or even (rarely) green.

So where's all that rich variety coming from? Evidently it's quite a bit more complicated, clouds and haze play a role, multiple scattering processes are important - many different things can happen. So let's take the tour together and find out!

Contents

Basics of Light Scattering
Optical Thickness
Different Altitude, different Light
Relief Shading
Cloud Shadows
Indirect Illumination
Scattering on the View Ray
The Question of Green
Haze and Humidity
Looking back
Icy hazes
Appendix A: Color Perception

Note on capturing sunsets on photo or video

With the Sun close to the horizon, illumination can vary rapidly, especially when one is looking into the direction of the Sun disc. The eye is very good in adapting to that, so while we generally notice that it all gets darker as the sun goes down, it doesn't appear to be a dramatic loss in intensity.

Trying to adjust exposure for the camera, one notices that this has to change easily a factor 10 within a span of a few minutes, and it is quite impossible to point the camera into the sun, capture colors and still see foreground objects other than black (which the human eye can do).

On the other hand, a good camera has a much larger aperture than the eye and gathers more light - in addition to light amplifying technology that means a camera can still 'see' something in low light that the eye may not.

An added complication is that modern video cameras try to be smart and aim to correct the white balance as if the scene would be illuminated by white light - which really takes the magic out of a sunset if one doesn't dissuade the camera.

Generally I've been aiming to adjust camera settings and post-process the images to what I remember having seen when standing at the spot, i.e. to capture the general impression well. This is not a hard physics criterion that would allow to measure actual color variations or lighting of the scene in an objective way, but probably what most readers are interested in.

Unless indicated otherwise, the pictures are my own and have been taken in Finland.


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