Atmospheric light scattering: Difference between revisions

Atmospheric light scattering (view source)

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 == Light scattering at dawn / dusk ==
+For low sun, one can no longer assume that the scene is illuminated in homogeneous light or that terrain altitude does not matter, because both the path of light in the atmosphere and the shading due to the spherical shape of earth play a role. To see this, compare the sun just at the horizon with the sun one degree below the horizon - in the first case there is direct sunlight, in the second case there is not, and so a small change in sun angle makes an enormous difference in lighting.
+In fact, the apparent sun position relative to the horizon (and thus the light) is much dependent on altitude - mountaintops can be observed in sunlight while the rest of the terrain is still dark (a phenomenon known in German as [http://de.wikipedia.org/wiki/Alpenglühen Alpenglühen]). This is even more apparent when looking at high-altitude clouds. The following screenshots show the effect in Flightgear:
+[[File:Alpine glow.jpg|400 px|While the valleys are still dark, the high mountain slopes already see the first direct sunlight]]
+[[File:Predawn-cloud-glow.jpg|400px|Predawn high altitude cloud glow]]
+Similarly, from high altitude the curvature of earth is apparent by the terrain (or haze in the direction towards the sun being more brightly illuminated than the terrain or haze away from the sun.