Derek Lilley grabbed his 13th goal of the season to secure all three points, but those behind the goal were the only ones who had a clear view of the action as the "pea souper" cast real doubts whether the game would finish.
Lilley struck from close range after Peter Weatherson had fired in a shot following Chris Millar's corner.
Until then play had been very even with both teams working hard and both keepers earning their pay with a number of excellent stops.
Morton's Andy McLaren showed his frustration early on after he tumbled twice in the box, but his penalty claims were waved aside.
At the other end Barry Sellars tested Stefan Gonet with a shot at the end of a neat attack.
Sellars then sent a powerful header just past the post before Darren Gribben shot wide after breaking clear. Morton hit back through Millar then Scott McLaughlin, who stung the fingers of the keeper with a 30-yard shot.
After the break Gonet pulled off two great stops before Lilley broke the deadlock.
Forfar almost levelled when Gribben smashed a spectacular volley from 25 yards only for the ball to come back off the post with the keeper well beaten.