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Clouds and Light Pollution

Compare the effect of light pollution with and without clouds.

with/without clouds

Date:10.2.2013 Time:22:27, 23:41 UT
Exposure:both 15s Field:about 140o diagonal
Camera:Canon EOS1D Optics:15mm, f/2.8
Place:Kornbühl, Salmendingen,
Swabian Alb, Germany
Observer:Till Credner

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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.

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.

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