Imaged by Andrew Baker while sailing off the coast of Maine.
Contrails are ice crystals and supercooled water droplets condensed during the airflow over aircraft wings and from water generated by burning fuel.
Paradoxically, modern higher efficiency airplanes and engines produce contrails over a wider range of air conditions than do older craft. For this reason, and the greater number of aircraft flying, we see more of them.
This dark shadow is a three-dimensional volume of unlit air like those of cloud shadows. The surrounding sunlit air is brighter because haze scatters the light. Other contrail shadows are two dimensional ~ cast downwards onto the screen of a lower cloud layer.
Image ©Andrew Baker, shown with permission.