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An aircraft chases its own contrail shadow. The contrail crossed the sun and its needle-like shadow extrended well across the sky towards the antisolar point. The shadow was cast onto a smooth layer of cirrostratus cloud beneath the aircraft. November 2nd 2002, sun 20° high. Photo c2002 Les Cowley]/ex]
www.weatherscapes.com...
Contrails are clouds with relatively sharp boundaries and hence may cast a sharp shadow on lower-altitude clouds. This shadow is then visible from the underside of the cloud, if the optical thickess of the cloud (the opacity) is not too large. If the lower-altitude cloud is cirrostratus, which is translucent, a three-dimensional shadow will form. This shadow is a plane defined by the sun and the (line-shaped) contrail. As a result, such a shadow is usually only visible if the contrail is in front of the sun for the observer. It is remarkable to see that a contrail shadow usually appears in an odd direction with respect to the sun and realizing that it is being cast on a lower-level cloud: perspective can be really deceptive.
contrailscience.com...
In reality these “dark lines” are simply the visual effects of shadows. There are actually three main kinds of dark line that are formed by contrail shadows.