It IS straight. The curved effect here is because you can't render a 180 degree scene with straight lines. You should try this: Sit on the floor about two feet from the middle of a wall (the longer the better), and look up at the corners of the room. See how the ceiling line goes UP from each corner when you look at that corner. Trace the line by pointing at it - you can see after a while how the line visually seems to go up and then curve down, just like these contrails do. Go give it at go.