Clouds and Light PollutionCompare the effect of light pollution with and without clouds.
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Clouds were approaching from the south-west to cover the starry sky over
the Swabian Alb in southern Germany. Under a cloudless sky, light pollution
seems to be not very dramatic for an unexperienced observer. However,
clouds amplify the effect of artificial light in the landscape through
effective scattering of light pollution. Light from towns
between 7 and 20 kilometers away (Burladingen, Albstadt, Balingen, and
Hechingen) reach this part of the Swabian Alb. Both images were exposed identical
but with 74 minutes time difference. The amount of visual light on the ground
rises by a mean factor of about 3.4 with the cloud cover. The blue channel
gives an amplification factor of 2.0, in green 4.0, and the red channel rises
most with a factor of 4.9.
Red is favored because of the wide spread use of reddish sodium vapour lamps
and also due to the effect of Rayleigh scattering: the light of distant city
lights is reddened because the bluish part of the spectrum is scattered away
on its long path through the air.
Only when the clouds are
very low (or even fog) they can not transfer the light pollution over
a large distance (see example).
The darkness of the sky is much better in the central and southern part
of the Swabian Alb with a projected
Dark Sky Park.
In the late night many
smaller towns turn off the street lights and other public lighting, further
enhancing the night sky, saving energy and protecting the environment.
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Clouds in unsettled regions appear dark in front of the moonless sky. Here it is shown in the vast mountains of Iceland (with some minor Northern Lights). Therefore clouds dim the natural lighting from the night sky to the landscape even further down. In settled regions, however, clouds light up the landscape through effective scattering of light pollution. This makes the impact of light pollution on ecology much worse, even in more rural areas like the Swabian Alb. See also:
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