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Nighttime cloud comparison by Christopher Kyba and Ray Stinson is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. Based on this work.

Comparison of nighttime clouds under pristine and light polluted conditions

The image at left was taken by Ray Stinson, and shows clouds passing over the unpolluted sky of Glacier National Park. In natural environments clouds appear as dark silhouettes against the brilliant starlit sky. Near cities, however, instead of darkening the sky clouds brighten it, by reflecting light back towards the ground, as in the image at right. The extent to which the clouds brighten the sky depends on the brightness of the city, the observer's position relative to it, and cloud properties. In the summer in Berlin, fully overcast skies are ten times brighter than clear skies. When considering an animal's exposure to light at night from skyglow, this effect should always be taken into account.