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Contrail Shadow by Ágúst Guðmundsson. Imaged during the 10th Paragliding World Championship 500 km north of Sydney, Australia March 4,'07. ©�g�st Gu�mundsson, shown with permission.

Aircraft contrails are occasionally seen paired with a dark line, their shadow cast onto on a lower layer of very thin cirrus cloud or atmospheric haze.

Here the shadow is cast deep through the hazy atmosphere. The shadow is a 3D volume of unlit air that extends down to ground level.

The shadow might also be cast onto the landscape. Alternatively, the distant hills could appear darker because the intervening shadowed air cannot scatter (predominantly blue) sunlight.