That is a very thin layer of cloud.  I suspect the dark line is the shadow of a contrail higher up.  The contrail itself is probably the white line parallel to it, crossing the photograpg about a quarter of the photograph's height lower.

I have seen this phenomenon twice. On the second occasion the thin white cloud layer was clearly being "divided" by a large four jet passenger aircraft as it progressed in level flight. Clear blue sky was visible only through the lengthening cut beginning immediately behind the aircraft, and the width of the cut was similar to its wingspan. Quirkily, the aft passengers may have had a view while those up front remained in cloud. The effect was undoubtedly caused by exhaust heat raising the air temperature in the wake to above dew point. As a former glider pilot I can attest that cumulus, lenticular & rotor clouds, and cold-frontal clouds mark areas of vertical air motion (though the converse is not always true). This cloud is none of those and in my view rules out natural vertical motion as the cause. The cloud in the photo looks icy, but I will leave identification to a professional. The (discontinued?) pracice of large jets taxiing up and down fog-bound runways in order to raise visibilty to above take-off minima is further evidence of the ability of jet efflux to dissolve cloud.

John Redfern, Stanmore Bay, New Zealand