The painting below shows two shadowbeams, which are slightly darker than the background sky, slanting down to the left, where they intersect a cast shadow on the floor of the Hudson Valley.
The shadowbeams above were cast by natural clouds, but they most often occur when a jet contrail aligns with the line of sight. The dark beam below is cast by a contrail that's not visible in the photo; it's to the left offscreen.
The shadow is a bar of unilluminated vapor seen edge-on. The adjoining illuminated air is a notch lighter in value. The darker beam is usually only visible when there is a light hazy sky behind it.
Contrail shadow photo courtesy Atmoptical.