This is a group of phenomena where the lack of light in some way produces an effect. One of the most striking examples I've seen, and never managed to get a photo of because it is so uncommon and short lived, is being in the shadow of a solid sharp contrail. For a moment the sky is split in two by a dark line of the evening sky intruding in the daytime. Then there are more subtle effects, such as the opposition effect, most often seen with object of uniform size evenly distributed over a large surface. The objects can vary in magnitude dramatically from grass, to plow rows, and up to trees in a forest. The lack of light is noticeable because of scattering at the edges of the shadows. But because the dominant mechanism involves the lack of light I group these phenomena together under shadows rather than including them in scattering.

Crepuscular Rays Better known as sun beams or rays of light, when there is lots of moisture or particulates in the air, beams of light, or dristinct shadow appear as radiating or converging crepuscular rays.

Opposition Effect Gradiation of light caused by hiding of shadows.

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