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.
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