pastel shaded morning light in October colours view from
  Victoria, BC looking eastward
Credit: E. Wiebe

This photo (above) was taken on 2011-09-02 at 06:25. The next photo (below) was taken 12 minutes earlier. What a difference sunrise makes. Though the camera wasn't positioned identically the same distant peak is visible in both photos.

Rayleigh Scattering in the atmosphere causes diffuse sky radiation due to the scattering of light off of molecules in the air. This light is always present in our atmosphere from early dawn to late dusk but is much weaker than direct sunlight and it is this sky-glow that is illuminating the objects in these photos. In both of these images the objects in the scene are darker than the intervening atmosphere. We see the glow against the contrasting hills and ridges. Each ridge in view has a much greater separation from the other ridges than the roughness scale of the ridge itself so in this weak illumination they seem featureless. Also, the ridges are approximately equidistant from the camera along their lengths so the brightness of the atmosphere is constant along their lengths. This gives the images a distinctly layered effect, much like a Ted Harrison painting. If there were more ridges we'd see more tones, getting proportionally lighter, as the ridges get farther away. If we were looking across a flat plane we'd see the distance fade into light haze.

The image taken earlier (below), when the sun was farther below the horizon, shows bluer tones because those wavelengths of light were more dominant in the sky-glow. As the sun gets closer to the horizon and finally rises above it more red tones and ultimately the entire spectrum of sunlight, illuminate the scene. At this point, the clear sky becomes bright blue since that is dominant colour scattered out of sunlight by clean air.

Even though the diffuse sky radiation is weaker than sunlight it is easy to convince yourself it is present. Take a good look at shadows on a bright day you can see that they are really blue, not black, since they are lit by the diffuse sky radiation. This becomes more obvious when the shadow lies on a white object such as snow but even a piece of paper will do. Artists know about this and use a lot blues when they paint a picture with shadows.

pastel shaded morning scene looking eastward
     from Victoria BC
Credit: E. Wiebe

UVic / SEOS / Climate Group / About Front Page Picture / Pastel Shaded Morning Light Last updated: Monday, 03-Oct-2011 09:47:18 PDT
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