Contrail shadows form when the long thin artificial clouds formed behind aircraft cast a shadow on a lower, or occasionally higher, layer of cloud.
Contrails, the linear clouds often seen trailing behind aircraft, are created when hot, damp air expelled from a jet’s engines combines with the frigid, thin air high in the atmosphere. This interaction causes the water vapour in the exhaust to condense and freeze into fine ice crystals . These visible crystals form around minute airborne specks, such as soot from the engine or existing atmospheric particles. For a contrail to last—rather than quickly dissipate—the surrounding air must be sufficiently cold and moist to prevent the ice crystals from rapidly evaporating.
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The Secret World of Weather – The Book
My thanks to Carolyn Coles for the images above.