MadSci Network: Physics

Re: Why does the sunrise look brighter than the sun set in the desert?

Date: Thu Nov 5 19:16:54 1998
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Physics
ID: 907133966.Ph

The first point to establish when you are thinking about an effect like this is just exactly what the phenomenon is. Some points to think about:

  1. Are you speaking of subjective or objective brightness -- that is, is the effect one that you notice using your eyes only, or can it be reproduced with careful light meter readings?
  2. Has the phenomenon been noticed for one place in one desert, a few places in one desert, several places in several deserts. or all places in all deserts?
  3. Both sunrise and sunset are times of rapidly varying light intensity. How are we standardizing times for comparison? First/last appearance of sun's limb? Half sun on horizon? Are we talking about brightness of the sun itself, the nearby sky, or the desert landscape?
I will give you one likely explanation for an objective difference in brightness, and one for a subjective difference.

Although 'there is no smog in the desert', scattering by particulate matter in the air is still an important factor in reducing the intensity of sunlight when the sun is very low in the sky. The very fact that sunrises and sunsets appear reddish is testimony to that (blue light is scattered away much more efficiently than red). In the case of most deserts remote from cities, major highways, and industrial effluents, the predominant particulate matter is wind-borne dust. During the night time the ground cools much more rapidly than the surface layers of the air. In the small hours of the morning, the ground gets to be cooler than the air above it, forming an inversion layer where warmer air overlies cooler. This arrangement often leads to very still air. Around sunrise, there will be still air, and a minimum of wind-borne particulates. At sunset, on the other hand, even though winds often die down a bit at that time of day, the air will still be cooler above and warmer below, so that there will still be plenty of draughts and eddies that keep a lot of dust aloft.

There is a completely different reason why desert sunrises might APPEAR to be brighter than sunsets, and it does not only apply in the desert. If you go out to observe a sunrise, you start out in the dark, and the pupils of your eyes are wide open. As the brightness of the sunrise increases, your eyes will adjust to the higher light levels, but there will be a bit of a delay. On the other hand, at sunset, the light will be getting dimmer, and your tightly contracted pupils will gradually dilate to adjust to the lower light level, but again there will be a delay. So when you observe a particular level of light, it will actually look brighter if your eyes were previously dark accommodated than it will if they were previously accommodated to a brighter light.

So there are two possible and very different explanations. There may well be several others. You would really need to be a lot more precise about the detail of just exactly what phenomenon you were describing, and the circumstances under which it was observed to discover a better answer.

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