Not sure what line your referring to. The light line on the horizon at the ocean sky intersection is present on the original sooc, also the line just to the left of the rising sun order the thunderhead is just another distant cloud layer also present in the original. Nothing has been added. The underbelly of the thunderheads are naturally quite a bit darker as is the left edge of most the clouds. In processing I did via dark layer masks use the curve in multiply blending mode that would have further darkened any dark tones leaving the lighter tones as is..