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Atmospheric light scattering: Difference between revisions
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→Predawn light scattering
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[[File:Predawn-scatter1.jpg|400px|Usual predawn light]] | [[File:Predawn-scatter1.jpg|400px|Usual predawn light]] | ||
[[File:Predawn-scatter2.jpg|400px|Predawn light with high haze layer]] | [[File:Predawn-scatter2.jpg|400px|Predawn light with high haze layer]] | ||
== Light scattering on clouds == | |||
Despite being rendered technically in a completely different way from haze and fog (texture stacks vs. fog function in terrain shaders), the physics of light scattering on clouds is no different from the physics of light scattering in haze. Mie and diffuse scattering are the main mechanisms on water droplets. The thickness of the cloud makes all the difference here. | |||
We have already discussed the mean effect of light attenuation on a cloud layer, let's look now at some specifics. | |||
=== Mie scattering on thin clouds === | |||
When a thick cloud is before the sun, it absorbs the sunlight and the cloud appears dark, which is the effect of diffuse scattering. However, when a thin cloud is illuminated from behind, it glows brilliant white during the day or red-golden during sunrise. This is the effect of forward scattering - the light scattering is strong as long as the scattering angle is small and sun, cloud and eye are approximately lined up. As with haze, Mie scattering creates a bright halo in thin cloud cover. | |||
At daytime, this creates a whiteout of a cloud when the sun is right behind it. If other, more dense clouds are also in the scene, this creates an impressive play of light and shadow: | |||
[[File:Cloud_mie03.jpg|600px|Mie scattering during the day]] | |||
Interestingly enough, cloud dominated by Mie scattering appear a relatively dark grey (as if they were in shadow) when seen from the side, even when they are fully illuminated. This is caused by the low probability of light scattered to large angles - since all light is focused forward, the side of the cloud becomes dark. | |||
At sunrise and sunset, the effect of Mie scattering is even more prominent - since the direct sunlight coming through the upper atmosphere is red-golden due to the effect of Rayleigh scattering, the Mie-halo created by light scattering in the clouds is also coloured. Combined with diffuse shading on denser clouds, this generates very vivid colors which change quickly as the sun rises higher and both light intensity and hue change. | |||
[[File:Cloud_mie01.jpg|400px|Mie scattering on thin clouds during sunrise]] | |||
[[File:Cloud_mie02.jpg|400px|Mie scattering on thin clouds during sunrise, a bit later]] | |||
Mie scattering is essentially absent whenever a cloud becomes opaque, i.e. when it is larger than the light attenuation length in the cloud. | |||