I’m not sure what caused this effect. If you click on the photo above you will see the horizon line is blue, but the grey line, immediately above it, is a little puzzling. The grey is not fog but a bank of clouds extending just beyond the horizon, but why does the top line seem to be straight? Of course clouds never appear to have straight edges.

In any case, in my experience ‘appearances’ are almost always deceiving. As we all know, straight does not always mean straight, and even the horizon line is in reality curved. BTW, if you google ‘straight lines in nature’ for images, you won’t find many convincing examples.