Luftwaffe and Allied Air Forces Discussion Forum - US bombs producing a very long smoke trail

These strange smoke trails have been making me curious for a long time. First time I saw a photograph with such a smoke trail I thought a 210 mm rocket fired by a German fighter had narrowly missed but recently seen footage shows clearly that a few of the bombs dropped by US bombers (at least in Europe) produced such smoke trails (not black like in the case of a fire),obviously billowing from the rear of these bombs.

I guess this was sort of an aiming aid for the bombers coming behind, showing how and where the bombs were falling until they hit the ground or some structure. In fact I have no idea, I don't know.

Who knows and is able to explain? Many thanks in advance.